Twenty-seven modes of reproduction in the obligate lichen symbiosis

Abstract

Fungi exhibit some of the greatest reproductive diversity across Eukaryotes. In addition to sexual and asexual reproduction, fungi engage in parasexual (mitotic recombinatorial) processes to acquire new genetic variation. Reproduction has been studied extensively in numerous free-living fungi but comparatively less knowledge exists for lichenized fungi, which are assumed to reproduce only through sexual spores, asexual conidia, and specialized asexual lichen propagules. We present a new conceptual framework describing reproductive modes in lichens that includes sexual and asexual processes as well as accommodating the possibility of parasexual reproduction. To support the plausibility of some of these modes of reproduction, we reviewed data spanning more than 200 years of anatomical investigation. We recovered evidence supporting the possibility of 22 of 27 possible modes of reproduction and found no counter-evidence to suggest the remaining five do not occur in nature. This conceptual framework allows for a greater plurality of reproductive processes than previously acknowledged in lichens, exceeding that of their non-lichenized relatives.

Full text

Twenty-seven modes of reproduction in the obligate lichen symbiosis
ERIN A. TRIPP
1
AND JAMES C. LENDEMER
2
1
Department of Ecology and Evolutionary Biology and Museum of Natural History, University of Colorado,
UCB 350, Boulder, CO 80309, USA; e-mail: erin.tripp@colorado.edu
2
The New York Botanical Garden, 2900 Southern Boulevard., Bronx, NY, USA; e-mail:
jlendemer@nybg.org
Abstract. Fungi exhibit some of the greatest reproductive diversity across Eukaryotes. In
addition to sexual and asexual reproduction, fungi engage in parasexual (mitotic
recombinatorial) processes to acquire new genetic variation. Reproduction has been studied
extensively in numerous free-living fungi but comparatively less knowledge exists for
lichenized fungi, which are assumed to reproduce only through sexual spores, asexual conidia,
and specialized asexual lichen propagules. We present a new conceptual framework describ-
ing reproductive modes in lichens that includes sexual and asexual processes as well as
accommodating the possibility of parasexual reproduction. To support the plausibilityof some
of these modes of reproduction, we reviewed data spanning more than 200 years of anatomical
investigation. We recovered evidence supporting the possibility of 22 of 27 possible modes of
reproduction and found no counter-evidence to suggest the remaining five do not occur in
nature. This conceptual framework allows for a greater plurality of reproductive processes
than previously acknowledged in lichens, exceeding that of their non-lichenized relatives.
Keywords: Genetic, haploid, meiosis, mitosis, parasexual, ploidy, recombination, symbiosis.
Traits that characterize the reproductive biolo-
gy of organisms are fundamental to understanding
evolutionary mechanisms that scale to explain
origins and fates of lineages across diverse do-
mains of life (Darwin & Seward, 1903;Tinkle,
1969; Packard & Seymour, 1997; Barrett, 2008;
Melián et al., 2012;Frohlich,2003). For example,
floral traits directly impact plant fitness and con-
tribute ultimately to diversification and persistence
of clades (Grant, 1949; Stebbins, 1974; van der
Niet et al., 2013). Similarly, animal behavioral
strategies such as avian courting determine repro-
ductive success and have fostered evolution of
remarkable morphologies in vertebrates (Ralls,
1977; Kodric-Brown & Brown, 1984;Moller&
Pomiankowski, 1993;Amundsen,2000).
Theoretical and empirical evidence has empha-
sized the paramount importance of sexual
reproduction as the primary source of recombina-
tion for most lineages (Fisher, 1941; Lynch et al.,
1993; Barton & Charlesworth, 1998;Carranza&
Polo, 2015). Accordingly, while asexuality is ram-
pant across the tree of life (Billiard et al., 2012;
Tripp, 2016), many such lineages are known to
have rare sexual phases or other mechanisms
through which they acquire genetic variation
(Fautin, 2002; Fontaneto et al., 2007;Honegger
& Zippler, 2007;Masel&Lyttle,2011).
Some lineages that are primarily asexual have
evolved mixed modes of reproduction involving
rare sexual phases or alternative,non-sexual strat-
egies to acquire new genetic variation such as
mitotic recombination or Bparasexuality^
(Pontecorvo et al., 1953a,1953b; Peacock &
Brock, 1968). Among Eukaryotes, fungi exhibit
some of the most elaborate reproductive strate-
gies, which include sexual and asexual reproduc-
tion, these oftentimes manifested concomitantly
in a given organism. Fungi additionally engage in
mitotic recombinatorial processes to generate ge-
netic diversity (Taylor et al., 1999; Yun et al.,
1999; Read & Roca, 2006; Billiard et al., 2012).
Electronic supplementary material The online version
of this article (https://doi.org/10.1007/s12228-017-9500-6)
contains supplementary material, which is available to
authorized users.
Brittonia, DOI 10.1007/s12228-017-9500-6
ISSN: 0007-196X (print) ISSN: 1938-436X (electronic)
© 2017, by The New York Botanical Garden Press, Bronx, NY 10458-5126 U.S.A.

Such processes, which are fundamentally differ-
ent from but functionally similar to other non-
meiotic modes of genetic exchange such as hori-
zontal gene transfer, rely on crossing over events
between homologous chromosomes in non-
meiotic cells or gene conversion mechanisms dur-
ing DNA repair and occur either within an indi-
vidual or between individuals (Stern, 1936;
McGuire et al., 2005). The extent to which para-
sexual processes contribute to the evolution of
genetic diversity in fungi remains to be answered
by additional research across a broader diversity
of fungal lineages.
Evolution of reproduction in this important
group of organisms may be even greater than fully
appreciated as a result of obligate symbioses be-
tween fungi and algae. Lichenized fungi (or simply
"lichens"; ~18,500 to 20,000 species globally),
comprise one fifth of all described fungi on Earth
and approximately half of all Ascomycetes
(Honegger, 1997; Jaklitsch et al., 2016). First, sex-
ual reproduction in Ascomycete fungi, which in-
cludes the vast majority of lichenized fungi, begins
typically with fusion of a spermatium (i.e., a type
of conidium) to an ascogonial filament. The nuclei
contained within these two types of tissues are
believed to be haploid, in which case this fusion
(i.e., plasmogamy) yields a brief phase of
dikaryotic (n+n) hyphae. These dikaryotic hyphae
give rise to asci and within asci, karyogamy occurs
and is followed immediately by meiosis. Thus, as
far as is known, diploid nuclei exist only momen-
tarily in most Ascomycetes, and fusion of ordinary
vegetative hyphae (i.e., somatogamy) is not char-
acteristic of most species. However, higher ploidy
strains of Ascomycetes are known and exist in
stable states in nature (Zhu et al., 2016) and para-
sexual recombination has been shown to occur in
some Ascomycetes (Taylor et al., 1999;Yunetal.,
1999; Read & Roca, 2006; Billiard et al., 2012).
This suggests that diploid or higher ploidy states in
lichen fungal mycelia is plausible if not to be
expected, and may arise as a product of parasexual
processes. While this sentiment is outside of cur-
rent orthodox views in lichen reproductive biology,
there are no published data documenting ploidy of
lichen mycelia. On the contrary, mycelia of several
lineages of early diverging non-lichenized
Ascomycetes have been documented to be
dikaryotic or diploid (Yamazaki & Oshima,
1979;Sugiyamaetal.2006), and a recent study
using bioinformatic methods–but not direct visual-
ization of lichen fungal chromosomes–presented
data in support of a hypothesis of either haploid
or diploid/dikaryotic lichen fungal mycelia (Tripp
et al., 2017).
Regardless of the above, it is clear is that li-
chens exhibit the full diversity of reproductive
modes present in non-lichenized Ascomycetes
but have additionally evolved other strategies as
a means of maintaining and enriching the obligate
symbiosis (Fünstück, 1926; Poelt, 1993; Nash,
2008; see below). Typical sexual forms of repro-
duction in lichens resemble those of their non-
lichenized relatives and involve fruiting bodies
termed ascomata, which are the sole structures
that produce meiotic ascospores (Letrouit-
Galinou, 1973). Ascomata usually disperse only
the mycobiont but more rarely also co-disperse
the photobiont (Vines, 1878; Thomson, 1991;
Gueidan et al., 2007). Haploid ascospores germi-
nate, become re-lichenized, yield a new thallus,
and must then go through karyogamy (i.e,
diploidization) prior to future meiotic events.
Mitotic processes, however, give rise to a much
greater diversity of reproductive units (i.e., prop-
agules), and these can be either lichenized or non-
lichenized (Pyatt, 1973). Remarkably, some of
these propagules function sexually or asexually
whereas others are thought to function only asex-
ually (Moreau & Moreau, 1928; Mitchell, 2006).
For example, conidia are mitotic, fungal-only re-
productive units that are produced on the thalli of
thousands of lichens in specialized structures
termed pycnidia (Vobis, 1980). Some authors
have demonstrated that conidia in some cases
function as sexual gametes that fertilize a recep-
tive hypha on the thallus of a compatible mating
type (Stahl, 1877;Baur,1898; Johnson, 1954). In
these instances, one maternal thallus may be fer-
tilized on multiple occasions by different paternal
conidia (Kroken & Taylor, 2001). Other authors
however have demonstrated that conidia disperse,
establish contact with new photobionts, continue
to divide mitotically, then yield new lichen thalli
that are assumed to be haploid, at least initially
(Möller, 1887; Sanders, 2014; Sanders & de los
Rios, 2015). In these latter instances, it is un-
known whether a given new thallus derives from
one or multiple such re-lichenization events.
Second, mitotic processes also give rise to a
marked diversity of lichenized propagules long
known to function asexually (Fünstück, 1926;
Poelt, 1993). These reproductive units consist of
variations on a theme of packages of photobiont
cells surrounded by differentially arranged hyphal
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tissue and include soredia (sphere-shaped propa-
gules), isidia (columnar propagules), phyllidia
(miniature lobes), and schizidia (pustules). Clear
evidence exists that these lichenized propagules
disperse to new locations, continue to divide mi-
totically, and ultimately yield new thalli (Schuster,
1985;Ott,1987c; Sanders, 2005).
The above suggests a complex yet understand-
able roadmap that describes the full set of mech-
anisms through which lichens can reproduce.
Nonetheless a comprehensive map, one that spe-
cifically incorporates sexual, asexual, and poten-
tially parasexual processes across different poten-
tial ploidy states of a given thallus, has not previ-
ously been proposed. This can in large part be
attributed to the fact that lichen ploidy has never
been explicitly investigated (but see Kroken &
Taylor, 2001)—that is, we still have no convinc-
ing data to suggest that lichens are exclusively
haploid, as commonly assumed, or whether dip-
loid states (see Tripp et al. 2017) and/or higher
replicates of chromosome numbers made possible
through polyploidization events are possible in
lichens. More generally, we lack detailed under-
standing of the full diversity of recombinatorial
processes in lichens. We do not have extensive
evidence regarding whether, how, when, how of-
ten, and where precisely the transition from hap-
loidy to diploidy (or dikaryotization) occurs on the
lichen thallus (but see Kroken & Taylor, 2001).
We also lack knowledge as to whether mitotic
propagules can function parasexually in lichens,
as they do in non-lichenized Ascomycetes.
Additionally, lichens that reproduce via asexual
propagules, which comprise at least a fifth of all
species in North America (Tripp & Lendemer,
unpub. data), have long been regarded as the most
difficult to study (Hodkinson & Lendemer, 2012;
Lendemer & Hodkinson, 2013). Finally, lichen
biology as a whole is marked by insufficient evi-
dence otherwise needed to understand whether
reproductive ecology and evolution is driven by
biotic factors, abiotic factors, shared evolutionary
history, or some combination of these elements.
The overall lack of modern inquiry with respect
to reproductive modes and ploidy in lichens is
noteworthy given the interest that these areas
attained historically (Bachman, 1913). It is even
more surprising when one considers that more
than a century of extensive study of reproduction
and genetics in non-lichenized fungi has
established a rich and diverse body of research
documenting reproductive biology of these
organisms (e.g., Raper, 1966; Jennings &
Rayner, 1986; Heller et al., 2016). Thus paradox-
ically, while study of the mechanisms and pro-
cesses underlying non-lichenized fungal repro-
duction has flourished, the converse is true of
lichenized-fungi despite their possessing even
more elaborate reproductive modes resulting from
the obligate symbiosis (but see numerous exam-
ples of careful developmental work of R.
Honegger, W. Sanders, and colleagues cited
throughout this contribution). Nonetheless, this
rich body of research on genetics and reproductive
biology of non-lichenized fungi provides numer-
ous opportunities to understand reproductive pro-
cesses in lichenized fungi.
Here we establish a first conceptual framework
outlining the full diversity of reproductive modes
possible in lichenized fungi assuming both haploid
and higher levels of ploidy are possible, as they are
for most other Eukaryotes (especially non-animal
Eukaryotes). Among these modes, some have pre-
viously been documented whereas others are new-
ly proposed. Our primary goal is to present these
in a single venue that unifies the various possibil-
ities into a single framework. This framework
furthermore emphasizes modes of reproduction
long held to be strictly asexual under modern
dogma but that are here reconsidered to serve as
potential sources of inter-individual recombination
through parasexual mechanisms. Several of these
newly proposed modes represent evolutionary in-
novations made possible by the obligate symbiosis
such as soredia and isidia. Because lichens have
long been largely excluded from the work of clas-
sical geneticists owing to slow growth rates, in-
ability to observe complete sexual cycles in the lab
(but see Sanders, 2014), and rare expression of the
symbiotic phenotype in culture (Honegger, 1993),
development of this framework draws on critical
evaluation of a body of literature that spans two
centuries of investigation at cellular and anatomi-
cal levels. We use this literature to more fully
support plausibility of most of 27 different modes
of reproduction possible in lichens. We conclude
that mitotic propagules are likely to have both a
greater diversity and wider function than currently
acknowledged in lichenology.
Materials and methods
Conceptual framework: definitions.––We here
defined reproduction as an event initiated by
TRIPP &LENDEMER:LICHEN REPRODUCTION2017]

dispersal of one or more reproductive units, these
deriving from either mitosis or meiosis, and cul-
minating with establishment of a spatially discrete
new lichen thallus, regardless of its subsequent
development. Thus, reproduction includes asexu-
al, sexual, and parasexual processes, and may or
may not involve recombination. For the purposes
of this study, we assumed that reproductive units
that yield new individuals are only haploid or
diploid, but acknowledge the possibility that
higher ploidy levels of some reproductive units
(e.g., hyphae) may occur; however, no such in-
stances have ever been documented in the litera-
ture for lichens to our knowledge. We considered
only first generation dispersal of reproductive units
that give rise to an immediately subsequent gener-
ation of thalli, and ploidy stasis or change occur-
ring therein (i.e., we do not explore third and
subsequent reproductive generations). We defined
modes of reproduction as unique mechanisms of
thallus establishment resulting from either trans-
mission of a single reproductive unit (i.e., propa-
gule) or a union of two (and only two) reproductive
units derived from these propagules (e.g.,twohy-
pha that develop from two, genetically distinct,
germinating ascospores). Recombination is here
defined as any set of processes that break down
existing and yield new combinations of alleles.
This framework was built around reproduction of
the mycobiont because fungal taxonomy defines
the lichen symbiosis at present (Brodo et al., 2001).
Development of conceptual schematic.—First,
we delimited all components of lichenized and
non-lichenized fungi that can actually or poten-
tially serve as reproductive units. We categorized
these as lichenized or non-lichenized reproductive
units, the latter thus requiring acquisition of a
compatible photobiont for new thallus develop-
ment. We distinguished reproductive units by
their ploidy, thus assuming ascospores and co-
nidia to be haploid. While it is thought that fungal
mycelia are primarily haploid in nature, this has
neither been studied extensively outside of model
organisms (Jennings & Rayner, 1986; but see
Kroken & Taylor, 2001) nor has it been demon-
stratedinlichens(seeTrippetal.,2017).
Reflecting this uncertainty, our conceptual frame-
work allows other reproductive units (e.g.,
soredia, isidia, phyllidia, hyphae including pa-
raphyses) to be either haploid or diploid, consis-
tent with experimental data (Pontecorvo et al.,
1953a,1953b;Crawfordetal.,1986; Schoustra
et al., 2007; Chien et al., 2015; Tripp et al., 2017).
Second, we enumerated all possible unique
modes of reproduction via transmission or union
of reproductive units. A given unique mode can
be characterized by transmission of haploid units
or diploid units, or unions of haploid + haploid
units, haploid + diploid units, or diploid + diploid
units. These starting states comprise both
lichenized and non-lichenized reproductive units
but always yield a new, spatially discrete
lichenized thallus. Consideration of all possible
transmissions and unions permits disentangle-
ment of modes of reproduction that do not allow
for recombination (e.g., transmission of haploid
or diploid soredium into the next generation) from
those that do (e.g., transmission of ascospore or
union of conidium with hypha from different
genetic individual).
Evaluation of evidence relating to conceptual
schematic.––To evaluate evidence for reproduc-
tive modes delimited in our study, we surveyed
relevant literature spanning the year 1800 to the
present (Suppl. Material 1). Specifically, we
attempted to find examples of empirical work that
supported each of the modes herein conceptual-
ized. Most of this literature was experimental or
developmental, with only latter decades yielding
genetic evidence that could be used to evaluate
reproductive modes. Our investigation was not
limited to reviewing outcomes specifically stated
/ discussed by authors but rather also included
new interpretations of data (e.g., derived from
our study of micrographs, line drawings, or results
reported in the text) not inferred in original pub-
lications because of different focuses of such
studies. That most studies explored only the time
span between first and second generations pre-
cluded analysis of outcomes of advanced genera-
tion reproduction.
Results
Our development of a conceptual framework
that describes lichen reproduction resulted in de-
limitation of 27 potential modes (see below). Of
these, we found evidence that supported the pos-
sibility of 22 modes; evidence was lacking for the
remaining five, but we did not uncover any
counter-evidence to suggest that these five cannot
and do not occur in nature (Suppl. Material 1).
Overall support for the hypothesis that mitotic
reproductive units have both a greater diversi-
ty and wider function than previously recog-
nized was found.
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Six classes of reproductive units including hap-
loid, diploid, mitotic, meiotic, lichenized, and non-
lichenized units were identified in this study: hy-
phae from ascospore germination, conidia, undif-
ferentiated as well as differentiated hyphae such as
paraphyses (haploid, diploid), and lichenized prop-
agules such as soredia, isidia, and phyllidia (hap-
loid, diploid). The predicted transmission or union
of these six reproductive units yielded 27 unique
reproductive modes (Fig. 1; see also Figs. 2&3).
Of these 27, 13 co-dispersed the mycobiont and
photobiont (modes 1–13) and 14 dispersed only
the mycobiont thus assuming subsequent acquisi-
tion of a photobiont (modes 14–27). Of the 27
modes, six involved no change in ploidy (trans-
mission of fungal-only reproductive units = modes
14–17; transmission of reproductive units that co-
dispersed the mycobiont and photobiont = modes
1 & 6) whereas 21 involved a ploidy change
resulting from the union of two reproductive units:
11 co-dispersing the mycobiont and photobiont
(modes 2–5, 7–13) and 10 dispersing fungal-only
reproductive units thus assuming subsequent ac-
quisition of a photobiont (modes 17–27).
Following traditional definitions of reproduc-
tion in lichens, seven modes would be classified
as sexual (modes 3, 7, 14, 18, 19, 22, 24) because
they implicate the sole meioticpropagule (i.e., the
ascospore) whereas the remaining 20 (all others)
would be classified as asexual because they donot
involve transmission or union of the meiotic prop-
agule. Modes 21 and 25 however may serve to
initiate the sexual process, that is, with conidia
and hyphal trichogynes assuming male and fe-
male functions, respectively.
We found evidence for numerous modes of
reproduction that transcend traditional concepts of
reproduction in lichens (i.e., modes 1, 14, and 16).
Specifically, data from the literature yielded sup-
port for plausibility of all but 5 of the 27 modes of
reproduction (modes 3, 4, 7, 10, and 19; these
discussed in detail, below). Of the 22 modes for
which support was found, 17 allow for the possi-
bility of recombination: modes 2, 5, 8, 9, 11–14,
18, and 20–27. Only mode 14 reflects the tradition-
al view of recombination in lichens. Four of these
modes, modes 3, 4, 7, and 10, involved union of a
lichenized reproductive units such as a soredium
with either a conidium or an ascospore. In total, the
majority of studies we surveyed addressed repro-
duction via transmission of single reproductive
units (modes 1, 6, 14–17) rather than unions of
reproductive units (modes 2–5, 7–13, 18–27).
Discussion
The overarching objective of the present study
was to devise a framework within which the full
complement of potential reproductive processes
in lichens can be identified and further investigat-
ed. Our intention was to build a framework in a
manner that (1) took sexual, asexual, and parasex-
ual mechanisms into consideration and allowed
for both meiotic and mitotic recombination, and
(2) was not constrained by a lack of data or a lack
of interpretation of prior data. While we fully
recognize that the concepts herein presented are
outside the established paradigm in lichen biolo-
gy, our intention is to highlight the panoply of
possibilities that could occur in lichens outside
of the small fraction of species that have been
studied. Below, we use newly analyzed data in
combination with earlier presented data to ar-
gue that the reproductive diversity present in
Ascomycota—involving both meiotic and mitotic
recombination—is likely also to be present in
lichenized Ascomycetes but with even further
embellishment of asexual and potentially parasex-
ual strategies owing to the plausibility of haloid
and diploid strains as well as opportunities to co-
disperse obligate symbionts. The conceptual
framework presented in Fig. 1and elaborated on
below highlights this diversity of reproductive
processes that exceeds what has been previously
recognized in past research. We found data in
support of 22 of 27 possible modes of reproduc-
tion. While evidence was lacking for the remain-
ing five modes, there similarly exists no evidence
refuting their possibilities. As such, we speculate
these may eventually be documented in nature.
Twenty-seven modes of reproduction.––Evidence
consistent with our delimited 27 modes is or-
ganized into and discussed in three sections:
(1) transmissions of mitotic reproductive units
that are asexual, (2) transmissions or unions
involving meiotic propagules that are sexual,
(3) and unions of mitotic reproductive units
that are asexual but may participate in para-
sexual processes. Although chromosome
counts have been reported for a handful of
lichens (Altman & Dittmer, 1972), we discuss
functionally similar modes that differ only in
ploidy (e.g., haploid vs. diploid soredia, or
modes 1 and 6) in concert because we know
of no lichen research in which ploidy transi-
tions (e.g., from haploid to diploid) have been
specifically investigated or documented. In the
TRIPP &LENDEMER:LICHEN REPRODUCTION2017]

discussion that follows, such cases are denoted
with the two functionally similar modes sepa-
ratedbyaslash(e.g.,modes1/6).
Asexual modes.––In Fig. 1, only modes 1, 6,
15–17 represent unambiguously asexual (i.e.,
clonal) reproduction in lichens because they
represent transmissions, not unions, of mitotic
reproductive units. However, an additional 15
modes (modes 2, 4, 5, 8–13, 20–22, 25–27) rep-
resent other opportunities for asexual reproduc-
tion when unions of these mitotic reproductive
units do not involve parasexual recombination.
FIG.1. Hypothesized 27 modes of reproduction possible in the obligate lichen symbiosis. See text for definitions of
reproduction. Reproduction is defined as the dispersal of one or more reproductive units (e.g., propagules), these deriving from
either mitosis or meiosis, to yield establishment of a new, spatially discrete thallus; see text for further details. Reproduction
involves either transmission of a single reproductive unit (modes 1, 6, and 14–17; no change in ploidy) or union or two
reproductive units (modes 2–5, 7–13, and 18–27; change in ploidy). Modes 1–13 involve transmission or unions of lichenized
reproductive units whereas modes 14–27 involve transmission or union of fungal-only reproductive units. These modes reflect
processes known or hypothesized in non-lichenized fungi but further embellished as a result of evolution of the obligate symbiosis
in lichens. Additionally, the modes allow for traditionally circumscribed recombination through meiotic means as well as mitotic
recombination, which is documented in non-lichenized fungi but not yet reported in lichens. Evidence in support of most of these
27 modes is describedin the text. Shown at bottomof figure are types of reproductive units present in lichenized fungi (note that
for ascospore, the reproductive unit is a hypha that arises from a young, germinating ascospore [vs. direct fusion of the
spore itself with another reproductive unit] but for simplicity, a spore is shown); blue depicts haploid and red depicts
diploid reproductive units.
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FIG.2. Examples of lichen reproduction that corroborate modes delimited in Fig. 1.ExamplesA–F represent modes
encountered in our own research. A. Example of mode 14: meiotic ascospore of Rhizocarpon disporum producing a hypha
surrounding a newly captured photobiont (source: lichen development experiment in Boulder, Colorado, E. A. Tripp, unpubl.
data). B. Variation on mode 16 wherein ascospore of Anthracothecium nanum (mode 14) undergoes endosporic mitosis to yield
haploid conidia (source: R. C. Harris, unpubl. data). C. Variation on mode 16 wherein ascospore of Architrypethelium sp. (mode
14) undergoes mitosis to yield a presumably haploid pycnidium, in which conidiaare then manufactured via mitosis (source: R. C.
Harris, unpubl. data). D. Example of modes 5/12: mitotic granules (soredia) of Lepraria pacifica showing connections between
hyphae, these possibly originating from genetically different individuals (source: J. C. Lendemer, unpubl. data). E. Example of
modes 2/9/11/13: dispersed mitotic granules (soredia) of Lepraria squamatica merging with larger primary hyphae of the same
taxon but possibly a different genetic individual (source: J. C. Lendemer, unpubl. data). F. Example of modes 5/12: individual
aggregations of dispersed mitotic granules (soredia) with hyphae that have connected and begun to merge into a single thallus
(source: J. C. Lendemer, unpubl. data). Arrows indicate the following labeled structures: A = ascospore (meiotic), C = conidia
(mitotic), G = Granules functionally equivalent to soredia (mitotic), H = hyphae (mitotic), Ph = photobiont, Py = pycnidium, X =
connection between hyphae. Scale bars = 20 μminA,50μm in B and C, as indicated in D–F.
TRIPP &LENDEMER:LICHEN REPRODUCTION2017]

FIG.3. Examples of lichen reproduction that corroborate modes delimited in Fig. 1.ExamplesA–F represent modes
encountered in our review of the literature; all figures are reproduced from the original publications and are unaltered except
for scale/cropping to facilitate formatting and additional call out letters in black that follow the descriptive terms outlined in Fig. 2.
A. Example of modes 1/6: soredia of Parmelia sulcata germinated and produced hyphae (source: Anstett et al., 2014:Fig.2b). B.
Example of mode 18: two ascospores of Calopadia puiggarii germinated to produce hyphae that have begun to merge (source:
Sanders, 2014:fig.38).C. Example of modes 5/8/12: soredia of Peltigera didactyla germinated and began to merge to form a
single thallus (source Stocker-Wörgötter & Türk, 1989:fig.4).D. Example of mode 14: ascospore of Rhizocarpon lecanorinum
germinated, formed hyphae that captured an alga and produced a new thallus (source Clayden, 1997:fig.10).E. Example of mode
22: conidia of Calopadia puiggarii germinated to produce hyphae that have captured algae and begun to intermingle, presumably
they will merge to form a single thallus (source: Sanders, 2014:fig.22).F. Example of mode 18: ascospores of Endocarpon
pusillum germinated to produce hyphae that merged to produce a thallus (source: Vines, 1878: pl. 9, fig. 4). Arrows indicate the
following labeled structures: A = ascospore (meiotic), C = conidia (mitotic), T = thallus (mitotic), H = hyphae (mitotic), S =
soredia, X = connection between hyphae.
BRITTONIA [VOL

Modes 1/6 are extremely well documented as
primary strategies for asexual reproduction in li-
chens (Kershaw & Millbank, 1970;Jahns,1979;
Jahns et al., 1979;Schuster,1985; Honegger,
1987; Ott, 1987a; Yoshimura et al., 1993;
Scheidegger, 1995;Zolleretal.,2000;Sanders
& Lücking, 2002;Ott&Jahns,2002;Sanders,
2005; Buldakov, 2010; Hilmo et al., 2011;Anstett
et al., 2014)asismode16(Möller,1887;
Hedlund, 1895; Vobis, 1977;Sanders&
Lücking, 2002;Sanders,2014; Sanders & de los
Rios, 2015). Modes 1 and 6 describe transmis-
sions of lichenized reproductive units such as
soredia whereas mode 16 describes transmission
of fungal conidia. These modes are thought to
represent the single largest source for new lichen
establishment via asexual means in nature and are
also commonly employed in culturing experi-
ments (Stocker-Wörgötter & Türk, 1989;
Kranner et al., 2002). Modes 15/17 are presum-
ably more common modes of asexual reproduc-
tion in nature than have been documented and
likely occur through fragmentation (Armaleo,
1991;Sanders,2005).
Sexual modes.––In Fig. 1, all transmissions or
unions of ascospores (modes 3/7, 14, 18, 19,
23/24) unambiguously implicate meiotic recom-
bination. The simplest and best documented
among these is mode 14, which involves trans-
mission of a single ascospore (Werner, 1931;
Stevens, 1941;Garrett,1968; Ahmadjian et al.,
1980; Ostrofsky & Denison, 1980;Ott,1987a;
Garty&Delarea,1987; Stocker-Wörgötter &
Türk, 1988; Crittenden et al., 1995 [>1,000 spe-
cies therein included]; Sanders & Lücking, 2002;
Sweetwood et al., 2012; Meeßen & Ott, 2013;
Sanders, 2014). Mode 18 describes the union of
two ascospores and has only been depicted in a
few studies (Stahl, 1877; Vines, 1878; Sanders &
Lücking, 2002;Denison,2003; Sanders, 2014)
but may be more common and important during
colony formation than previously understood
(Heller et al., 2016). The remaining modes
(modes 3/7, 19, 23/24) depict unions between a
germinating ascospore and a mitotic reproductive
unit (i.e., a conidium, hypha, or lichenized unit).
These modes have largely not been documented,
with the exception of modes 23/24 that involve
union of an ascospore with a haploid or diploid
hypha (Honegger, 1993;Sanders,2014). This
lack of documentation is not particularly surpris-
ing considering it does not serve as a source of
new lichenization and most studies do not
approach development from this perspective.
For example, the focus of Sanders (2014)was
not to discover spore-hyphal unions but rather to
document the complete life-cycle of a crustose
lichen, with emphasis on re-lichenization.
Finally, modes 21/25 may serve in a sexual or a
parasexual manner (Bitter, 1901; see below for
parasexual function). This involves the union of a
conidium with a fungal hypha. This union is
considered to be sexual when the former func-
tions in a microconidial manner, i.e., as a ‘male’
gametangium that makes contact with a sexual
hypha (termed a ‘trichogyne’) that functions as a
‘female’gametangium (Stevens, 1941;Sanders,
2014). This union represents an important source
of dikaryotization in Ascomycete fungi and is
well documented in lichenized Ascomycetes
(Hale, 1983;Honegger,1984).
Parasexual modes?––In Fig. 1, all unions be-
tween mitotic reproductive units may be involved
in asexual reproduction (see above) but also po-
tentially undergo mitotic recombination (modes
2/9/11/13, 4/10, 5/8/12, 20/26/27, 22, 21/25). We
found varying degrees of evidence consistent with
these modes, which is likely explainable by the
nature of study involved. For example, we have
not yet located studies consistent with modes 4/10
that describe unions between conidia and
lichenized reproductive units, but this is likely
attributable to a dearth of microscopists having
searched for this condition as well as the bias
towards study of sexual species that tend to be
more frequent in nature (Murtagh et al., 2000).
Modes 2/9/11/13, which describe unions between
fungal hyphae and lichenized reproductive units,
have also been rarely documented (but see
evidence in Lendemer, 2011), presumably for
similar reasons. By far, the best-documented
unions of mitotic reproductive units are those
depicted in modes 5/8/12 (Armstrong, 1984;
Schuster, 1985; Stocker-Wörgötter & Türk,
1989; Honegger, 1993; Lendemer, 2011). This
extensive documentation stems from the ex-
clusive use of lichenized reproductive units
in experiments (e.g., Kershaw & Millbank,
1970;Schuster,1985; Hilmo et al., 2011).
Modes 20/26/27 involve unions of fungal hy-
phae from different individuals and have been
documented with some frequency for more
than a century (Sturgis, 1890;Jahns,1972,
1987; Yarranton, 1975; Laundon, 1978;
Hawksworth & Chater, 1979; Clayden, 1997;
Sanders & Lücking, 2002). These interactions
TRIPP &LENDEMER:LICHEN REPRODUCTION2017]

are readily observed in nature in the context of
thallus-thallus interactions wherein zones of
inhibition do or do not form as a function of
compatibility of genotypes (e.g., Jahns, 1972;
Hawksworth & Chater, 1979; Clayden, 1997).
Similarly, mode 22 involves the union of co-
nidia from different genetic progenitors. This
interaction is not commonly sought by anatomists
but has been documented at least twice in recent
years in lichens (Wieczorek, 2009; Sanders,
2014), and more frequently in non-lichenized
Ascomycetes (Roca et al., 2003,2004,2005).
Finally, modes 21/25 can represent either sex-
ual or parasexual unions (the former context
discussed above). When asexual, these in-
volve fusion of conidia with hyphae, which
has been documented in Honegger (1984)and
Wieczorek (2009).
Conclusion
How reasonable are current assumptions about
lichen reproduction given (1) presentation of data
that are at least consistent with hypotheses of both
haploid and diploid hyphae in lichens as well as
(2) the remarkable diversity in reproductive
modes well established in their non-lichenized
relatives? In particular, there are exceptions to
the maxim that asexual reproduction is unsuccess-
ful in nature, for example, demonstration that
mitotic recombination accelerates adaptation in
fungi (Schoustra et al., 2007; see also Tripp,
2016). Fungi have elaborate and versatile repro-
ductive strategies, some of which facilitate their
escape of detrimental effects of asexuality and
involve mitotic recombination. Examples include
cryptic sex, other variations on mixed mating
systems, and parasexuality (Hansen, 1938;
Beadle & Coonradt, 1944; Pontecorvo, 1946;
Buxton, 1960; Caten & Jinks, 1966;
Clutterbuck, 1996, Geiser et al., 1998; Taylor
et al., 1999; Kroken & Taylor, 2001; Cornejo
et al., 2009; Billiard et al., 2012). That parasexu-
ality occurs throughout fungi has been well un-
derstood since the development of early concep-
tual models in the 1950’s and 1960’s(Pontecorvo,
1946; Pontecorvo et al., 1953a,1953b; Peacock
& Brock, 1968). Given this, it seems likely that
similar processes exist in lichenized fungi but
have not yet been documented.
The evidence we have presented is consistent
with a hypothesis that lichens have a greater di-
versity of reproductive modes, facilitated by the
potential of diverse ploidy levels as well as their
obligate symbioses with photobionts, than their
non-lichenized relatives and that these modes
may involve both meiotic and mitotic recombina-
tion. While it may be likely that one or two of the
27 modes dominate reproductive processes in
nature, less frequent occurrence of the other
modes does not negate their potential importance
in generating and maintaining genetic diversity in
lichens. Although it seems prudent to accept the
possibility of a greater diversity of lichen hyphae
ploidy than previously appreciated, we still lack
direct documentation of this phenomenon and
others such as parasexuality in lichens, which
representsone of the most obvious areas for future
experimental work, microscopy, and genetic re-
search. Although direct experimental evidence
is lacking, genetic and phylogenetic studies
have begun to provide increased evidence that
support an increasingly complex map of lichen
reproduction wherein fungi use diverse modes
to maintain and acquire genetic diversity
(Murtagh et al., 2000; Kroken & Taylor, 2001;
Seymour et al., 2005; Nelsen & Gargas, 2008;
Cornejo et al., 2009;Molinaetal.,2013;Otálora
et al., 2013; Rolstad et al., 2013; Singh et al.,
2015). The implications of a far greater di-
versity of reproductive strategies are substan-
tial, as has been documented for other line-
ages presumed to be largely asexual. Across
all 27 possible modes herein described, mei-
otic or mitotic recombination is possible in
22 (81.5%) of them. Yet, less than a third of
these (7 of 22, or 31.8%) involve a meiotic
ascospore. The presented framework thus
conveys a much greater potential for mitotic
recombination in lichens than has been ap-
preciated and explored.
Acknowledgements
We thank Rosmarie Honegger, William
Sanders, and their colleagues whose developmen-
tal studies in lichenology have helped to inspire
the present contribution. We are especially grate-
ful to William Sanders whose comments substan-
tially improved an earlier version of this manu-
script. We additionally thank Robbin Moran for
his time and editorial contributions. Funding for
this work was provided by the U.S. National
Science Foundation, Dimensions of Biodiversity
Award #s 1542629 (University of Colorado) and
#1542639 (New York Botanical Garden).
BRITTONIA [VOL

Literature cited
Ahmadjian,V.,L.A.Russell&K.C.Hildreth.1980.
Artificial reestablishment of lichens. I. Morphological in-
teractions between the phycobionts of different lichens and
the mycobionts of Cladonia cristatella and Lecanora
chrysoleuca. Mycologia 72: 73–89.
Altman, P. L. & D. S. Dittmer. 1972. Biology Data Book, 2
nd
ed. Vol. 1. Federation of American Societies for
Experimental Biology, Bethesda.
Amundsen, T. 2000. Why are female birds ornamented?
Trends in Ecology and Evolution 4: 149–155.
Armaleo, D. 1991. Experimental microbiology of lichens:
mycelia fragmentation, a novel growth chamber, and the
origins of thallus differentiation. Symbiosis 11: 163–177.
Armstrong, R. A. 1984. Dispersal of soredia from individual
soralia of the lichen Hypogymnia physodes (L.) Nyl. in a
simple wind tunnel. Environmental and Experimental
Botany 34: 39–45.
Anstett, D. N., A. Salcedo & E. W. Larsen. 2014. Growing
foliose lichens on cover slips: A method for asexual prop-
agation and observing development. Bryologist 117: 179–
186.
Bachman, F. M. 1913. The origin and development of the
apothecium in Collema puplosum (Bernh.) Ach. Wilhelm
Engelmann, Leipzig.
Beadle, G. W. & V. L. Coonradt. 1944. Heterocaryosis in
Neurospora crassa. Genetics 29: 291–308.
Barrett, S. C. H. 2008. Major evolutionary transitions in
flowering plant reproduction: an overview. International
Journal of Plant Science 169: 1–5.
Barton, N. H. & B. Charlesworth. 1998. Why sex and
recombination? Science 281: 1986–1990.
Baur, E. 1898. Zur Frage nach der Sexualitat der Collemaceen.
Berlin Deutsche Botanische Gesellschaft 16: 363–367.
Billiard, S., M. López-Villavicencio, M. E. Hood & T.
Giraud. 2012. Sex, outcrossing and mating types: Un-
solved questions in fungi and beyond. Journal of Evolu-
tionary Biology 25: 1020–1038.
Bitter, G. 1901. Ueber die Variabilität einiger Laubflechten
und über den Einfluß äusserer Bedingungen auf ihr
Wachstum. Jahrbuch für wissenschaftliche Botanik 36:
421–492.
Brodo, I. M, S. D. Sharnoff & S. Sharnoff. 2001. Lichens of
North America. Yale University Press, New Haven.
Buldakov, M. S. 2010. Intraspecific variation in the
viability of soredia in Hypogymnia physodes (L.)
Nyl. (Ascomycota: Lecanorales). Russian Journal of
Ecology 41: 211–217.
Buxton, E. W. 1960. Heterokaryosis, saltation and adaptation.
Pp. 359–405. In: J. G. Horsfall & A. E. Dimond (eds.).
Plant pathology, an advanced treatise, vol. 2. Academic
Press, London and New York.
Carranza, J. & V. Polo. 2015. Sexual recombination with
variable mating systems can resist asexuality in a rock-
paper-scissors dynamic. Royal Society Open Science 2:
1403893.
Caten, C.E. & J. L. Jinks. 1966. Heterokaryosis: its signifi-
cance in wild homothallic ascomycetes and fungi
imperfecti. Transactions of the British Mycological Society
49: 81–93.
Chien, T., T. L. Tseng, J. Y. Wang, Y. T. Shen, T. H. Lin & J.
C. Shieh. 2015. Candida albicans DBF4 gene inducibly
duplicated by the mini-Ura-blaster is involved in hypha-
suppression. Mutation Research 779: 78–85.
Clayden, S. R. 1997. Intraspecific interactions and parasitism
in an association of Rhizocarpon lecanorinum and
R. geograpicum. Lichenologist 29: 533–545.
Clutterbuck, A. J. 1996. Parasexual recombination in fungi.
Journal of Genetics 75: 281–286.
Cornejo, C., S. Chabanenko & C. Scheidegger. 2009. Phy-
logenetic analysis indicates transitions from vegetative to
sexual reproduction in the Lobaria retigera group
(Lecanoromycetes, Ascomycota). Lichenologist 41: 275–
284.
Crawford, M. S., F. G. Chumley, C. G. Weaver & B.Valent.
1986. Characterization of the heterokaryotic and vegetative
diploid phases of Magnaporthe grisea. Genetics 114:
1111–1129.
Crittenden, P. D., J. C. David, D. L. Hawksworth & F. S.
Campbell. 1995. Attempted isolation and success in the
culturing of a broad spectrum of lichen-forming and
lichenicolous fungi. New Phytologist 130: 267–297.
Darwin, F. & A. C. Seward. 1903. More letters of Charles
Darwin. John Murray, London.
Denison, W. C. 2003. Apothecia and ascospores of Lobaria
oregana and Lobaria pulmonaria investigated. Mycologia
95: 513–518.
Fautin, D. G. 2002. Reproduction of Cnidaria.Canadian
Journal of Zoology. 80: 1735–1754.
Fisher, R. A. 1941. Average excess and average effect of a
gene substitution. Annals of Eugenics 11: 53–63.
Fontaneto, D., E. A. Herniou, C. Boschetti, M. Caprioli &
G. Melone. 2007. Independently evolving species in
asexual Bdelloid rotifers. PLoS Biology 5.4: e87.
Frohlich, M. W. 2003. Opinion: An evolutionary scenario for
the origin of flowers. Nature Reviews Genetics 4.7: 559–
566.
Fünstück, M. 1926. Lichenes (Flechten). Pp. 1–60. In: A.
Engler & K. Prantl (eds.). Die Natürlichen
Pflanzenfamilien nebst ihren Gattungen und wichtigeren
Arten, insbesondere den Nutzpflanzen. Wilhelm
Engelmann, Leipzig.
Garrett, R. M. 1968. Observations on the germination of
lichen ascospores. Revue Bryologique et Lichenologique.
36: 330–332.
Garty, J. & J. Delarea. 1987. Some initial stages in the
formation of epilithic crustose lichens in nature: a SEM
study. Symbiosis 3: 49–56.
Geiser, D. M., J. I. Pitt & J. W. Taylor. 1998. Cryptic
speciation and recombination in the aflatoxin-producing
fungus Aspergillus flavus. Proceedings of the National
Academy of Sciences of the U.S.A. 95: 388–393.
Grant, V. 1949. Pollination systems as isolating mechanisms
in flowering plants. Evolution 3: 82–97.
Gueidan, C., C. Roux & F. Lutzoni. 2007. Using a multigene
phylogenetic analysis to assess generic delineation and
character evolution in Verrucariaceae (Verrucariales, As-
comycota). Mycological Research 111: 1145–1168.
Hale,M.E.Jr.1983. The biology of lichens. Edward Arnold,
London.
Hansen, H. N. 1938. The dual phenomenon in imperfect
fungi. Mycologia 30: 442–455.
Hawksworth, D. L. & A. O. Chater. 1979. Dynamism and
equilibrium in a saxicolous lichen mosaic. Lichenologist
11: 75–80.
TRIPP &LENDEMER:LICHEN REPRODUCTION2017]

Hedlund, T. 1895. Ueber Thallusbildung durch
Pyknokonidien bei Catillaria denigrata (Fr.) und
C. prasina (Fr.). Botanisches Centralblatt 63: 9–16.
Heller, J., J. Zhao, G. Rosenfield, D. J. Kowbel, P. Gladieux
& N. L. Glass. 2016. Characterization of Greenbeard
Genes involved in long-distance kind discrimination in a
microbial eukaryote. PLoS Biology 14: e1002431.
Hilmo, O., L. Rocha, H. Holien & Y. Gauslaa. 2011. Es-
tablishment success of lichen diaspores in young and old
boreal rainforests: a comparison between Lobaria
pulmonaria and L. scrobiculata. Lichenologist 43: 241–
255.
Hodkinson, B. P. & J. C. Lendemer. 2012. Phylogeny and
taxonomy of an enigmatic sterile lichen. Systematic Bota-
ny 37: 835–844.
Honegger, R. 1984. Scanning electron microscopy of the
contact site of conidia and trichogynes in Cladonia furcata.
Lichenologist 16: 11–19.
———.1987. Isidium formation and the development of
juvenile thalli in Parmelia pastillifera (Lecanorales,
lichenized ascomycetes). Botanica Helvetica 97: 147–152.
———.1993. Tansley Review No. 60. Developmental biolo-
gy of lichens. New Phytologist 125. 659–677.
———.1997. Metabolic interactions at the mycobiont-
photobiont interface in lichens. Pages 209–221. In: G. C.
Carroll & P. Tudzynski (eds.). The Mycota V. Plant Rela-
tionships. Spring, Berlin.
——— &———.2007. Mating systems in representatives of
Parmeliaceae, Ramalinaceae and Physciaceae
(Lecanoromycetes, lichen-forming ascomycetes). Myco-
logical Research 111: 424–432.
Jahns, H. M. 1972. Individualitat und Variabilitat in der
Flechtengattung Cladina Nyl. Herzogia 2: 277–290.
———.1979. De Neubesiedling von Baumrinde durch
Flechten II. –Natur und Museum 109: 88–94.
———.1987. New trends in developmental morphology of
the thallus. Bibliotheca Lichenologica 25: 17–33.
———, D. Mollenhauer, M. Jenniger & D. Schonborn.
1979. Die Neubesiedlung von Baumrinde durch Flechten I.
Natur und Museum 109: 40–51.
Jaklitsch, W., H. O. Baral, R. Lücking, H. T. Lumbsch &
W. Frey. 2016. Syllabus of plant families –A. Engler's
Syllabus der Pflanzenfamilia Part 1/2: Ascomycota. W.
Frey (ed.). Borntraeger, Stuttgart, Germany.
Jennings, D. H. & A. D. M. Rayner. 1986. The ecology and
physiology of the fungal mycelium. Cambridge University
Press, Cambridge.
Johnson, G. T. 1954. Ascogonia and spermatia of
Stereocaulon. Mycologia 46: 337–345.
Kershaw, K. A. & J. W. Millbank. 1970. Isidia as vegetative
propagules in Peltigera aphthosa var. variolosa (Massal.)
Thoms. Lichenologist 4: 214–217.
Kodric-Brown, A. & A. J. H. Brown. 1984. Truth in adver-
tising: the kinds of traits favored by sexual selection.
American Naturalist 124: 309–323.
Kranner, I., R. Beckett & A. Varma. 2002. Protocols in
lichenology: culturing, biochemistry, ecophysiology and
use in biomonitoring. Springer-Verlag, Berlin.
Kroken, S. & J. W. Taylor. 2001. Outcrossing and recombi-
nation in the lichenized fungus Letharia. Fungal Genetics
and Biology 34: 83–92.
Laundon, J. R. 1978. Haematomma chemotypes form fused
thalli. Lichenologist 10: 221–225.
Lendemer,J.C.2011. A standardized morphological termi-
nology and descriptive scheme for Lepraria
(Stereocaulaceae). Lichenologist 43: 379–399.
——— &B.P.Hodkinson.2013. A radical shift in the
taxonomy of Lepraria s.l.: molecular and morphological
studies shed new light on the evolution of asexuality and
lichen growth form diversification. Mycologia 105: 994–
1018.
Letrouit-Galinou, M. A. 1973. Sexual reproduction. Pp. 59–
90. In: V. Ahmadjian & M. E. Hale (eds.). The lichens.
Academic Press Inc., New York.
Lynch, M., R. Butcher, D. Buerger & W. Gabriel. 1993. The
mutational meltdown in asexual populations. Journal of
Heredity 84: 339–344.
Masel, J. & D. N. Lyttle. 2011. The consequences of rare
sexual reproduction by means of selfing in an otherwise
clonally reproducing species. Theoretical Population Biol-
ogy 80: 317–322.
McGuire, C., J. E. Davis, M. L. Double, W. L. MacDonald,
J. T. Rauscher, S. McCawley & Milgroom, M. G. 2005.
Heterokaryon formation and parasexual recombination
between vegetatively incompatible lineagesin a population
of the chestnut blight fungus, Cryphonectria parasitica.
Molecular Ecology 14: 3657–3669.
Meeßen, J. & S. Ott. 2013. Recognition mechanisms during
the pre-contact state of lichens: I. Mycobiont-photobiont
interactions of the mycobiont of Fulgensia bracteata.
Symbiosis 59: 121–130.
Melián, C. J., D. Alonso, S. Allesina, R. S. Condit & R. S.
Etienne. 2012. Does sex speed up evolutionary rate and
increase biodiversity? PLoS Computational Biology 8:
e1002414.
Mitchell, M. E. 2006. BFunction is smother’dinsurmise^:a
survey of observations on the role of lichen conidia, 1850-
2000. Huntia 12: 149–167.
Molina, M. C., P. K. Divakar, N. Zhang, N. Gonzalez & L.
Struwe. 2013. Non-developing ascospores in apothecia of
asexually reproducing lichen-forming fungi. International
Microbiology 16: 145–155.
Möller, A. 1887. Über die Cultur flechtenbildender
Ascomyceten ohne Algen. Unterschungen aus dem
botanishen Institut der Königliche. Akademie zu Münster
i.w. Germany.
Moller, A. P. & A. Pomiankowski. 1993. Why have birds got
multiple sexual ornaments? Behavioral Ecology and So-
ciobiology 32: 167–176.
Moreau, F. & M. F. Moreau. 1928. Les phénomenes
cytologiques de la reproduction chez les Champignons de
Lichens. Botaniste 20: 1–67.
Murtagh,G. J., P. S. Dyer& P. D. Crittenden. 2000. Sexand
the single lichen. Nature 404: 564.
Nash, T.H. 2008. Lichen biology, 2
nd
Edition. Cambridge
University Press, Cambridge.
Nelsen,M.P.&A.Gargas.2008. Dissociation and horizontal
transmission of codispersing lichen symbionts in the genus
Lepraria (Lecanorales: Stereocaulaceae). New Phytologist
177: 264–275.
Ostrofsky, A. & W. C. Denison. 1980. Ascospore discharge
and germination in Xanthoria polycarpa. Mycologia 72:
1171–11 79.
Otálora, M. A. G., C. Salvador, I. Martínez & G. Aragón.
2013. Does the reproductive strategy affect the transmis-
sion and genetic diversity of bionts in cyanolichens? A case
BRITTONIA [VOL

study using two closely related species. Microbial Ecology
65: 517–530.
Ott, S. 1987a. Differences in the developmental rates of li-
chens. Annales Botanici Fennici 24: 385–393.
———.1987c. The juvenile development of lichen thalli from
vegetative diaspores. Symbiosis 3: 57–74.
——— & H. M. Jahns. 2002. Differentiation processes in
lichens –in vivo cultivation and transplantation methods.
Pp. 65–74. In:I. Kranner, R. P. Beckett & A. K. Varma
(eds.). Protocols in lichenology. Culturing, biochemistry,
ecophysiology and use in biomonitoring. Springer-Verlag,
Berlin.
Packard, G. C. & R. S. Seymour. 1997. Evolution of the
amniote egg. Pp. 265–290. In: S. S. Sumida (ed.), Amniote
origins: completing the transition to land. Academic Press,
San Diego.
Peacock, W. J. & R. D. Brock. 1968. Replication and re-
combination of genetic material. Australian Academy of
Science, Canberra.
Poelt, J. 1993. La riproduzione asessuale nei licheni.
Notiziario della Società Lichenologica Italiana 6: 9–28.
Pontecorvo, G. 1946. The parasexual cycle in fungi. Annual
Review of Microbiology 10: 393–400.
———, J. A. Roper, L. M. Hemmons, K. D. Macdonald &
A. W. J Bufton. 1953a. The genetics of Aspergillus
nidulans. Advances in Genetics 5: 141–238.
———,——— &E.Forbes.1953b. Genetic recombination
without sexual reproduction Aspergillus niger. Journal of
General Microbiology 8: 198–210.
Pyatt, F. B. 1973. Lichen propagules. Pp. 117–145. In:V.
Ahmadjian & M. E. Hale (eds.). The Lichens. Academic
Press, New York.
Ralls, K. 1977. Sexual dimorphism in mammals: avian models
and unanswered questions. American Naturalist 111: 917–
938.
Raper, J. R. 1966. Genetics of sexuality in higher fungi. The
Ronald Press Company, New York.
Read, N. D. & M. G. Roca. 2006. Vegetative hyphal fusion in
filamentous fungi. Pp. 87–98. In: F. Baluska, D. Volkm ann,
P. W. Barlow (eds.).Cell-cell channels. Landes Bioscience,
Georgetown.
Roca, L. C. Davide, M. C. Mendes-Costa & A. Wheals.
2003. Conidial anastomosis tubes in Colletotrichum.Fun-
gal Genetics and Biology 40: 138–145.
———,——
—, L. M. C. Davide, R. F. Schwan & A. E.
Wheals. 2004. Conidial anastornosis fusion between
Colletotrichum species. Mycological Research 108: 1320–
1326.
———, J. Arlt, C. E. Jeffree & N. D. Read. 2005. Cell
biology of conidial anastomosis tubes in Neurospora
crassa.EukaryoticCell4:911–919.
Rolstad, J., S. Ekman, H. L. Andersen & E. Rolstad. 2013.
Genetic variation and reproductive mode in two epiphytic
lichens of conservation concern: A transatlantic study of
Evernia divaricata and Usnea longissima. Botany 91(2):
69–81.
Sanders, W. B. 2002. In situ development of the foliicolous
lichen Phyllophiale (Trichotheliaceae) from propagule
germination to propagule production. American Journal of
Botany 89: 1741–1746.
———.2005. Observing microscopic phases of lichen life
cycles on transparent substrata placed in situ. Lichenologist
37(5): 373–382.
———.2014. Complete life cycle of the lichen fungus
Calopadia puiggarii (Pilocarpaceae, Ascomycetes) docu-
mented in situ: Propagule dispersal, establishment of
symbiosis, thallus development, and formation of sexual
and asexual reproductive structures. American Journal of
Botany 101: 1836–1848.
——— &R.Lücking.2002. Reproductive strategies,
relichenization and thallus development observed in situ in
leaf-dwelling lichen communities. New Phytologist 155:
425–435.
——— & A. de los Rios. 2015. Structure and in situ devel-
opment of the microlichen Gyalectidium paolae
(Gomphillaceae, Ascomycota), an overlooked colonist on
palm leaves in southwest Florida. American Journal of
Botany 102: 1403–1412.
Scheidegger, C. 1995. Early development of transplanted
isidioid soredia of Lobaria pulmonaria in an endangered
population. Lichenologist 27: 361–374.
Schoustra, S. E., A. J. M. Debets, M. Slakhorst & R. F.
Hoekstra. 2007. Mitotic recombination accelerates adap-
tation in the fungus Aspergillus nidulans. PLoS Genetics 3:
e68.
Schuster, G. 1985. Die Jugendentwicklung von Flechten ein
Indikator fur Klimabedingungen und Umweltbelastung.
Bibliotheca Lichenologica 20: 1–206.
Seymour, F. A., P. D. Crittenden, M. J. Dickinson, M.
Paoletti, D. Montiel,L. Cho & P. S. Dyer. 2005. Breeding
systems in the lichen-forming fungal genus Cladonia.
Fungal Genetics and Biology 42: 554–563.
Singh, G., F. dal Grande, S. Werth & C. Schei degger. 2015.
Long-term consequences of disturbances on reproductive
strategies of the rare epiphytic lichen Lobaria pulmonaria:
Clonality a gift and a curse. FEMS Microbiology Ecology
91(1): https://doi.org/10.1093/femsec/fiu009.
Stahl, E. 1877. Beitrage zur Entwicklungsgeschicte des
Flechten. Felix, Leipzig.
Stebbins, G. L. 1974. Plant species. Evolution above the
species level. Harvard University Press, Cambridge.
Stern, C. 1936. Somatic crossing over and segregation in
Drosophila melanogaster. Genetics 21: 625–730.
Stevens, R. B. 1941. Morphology and anatomy of
Dermatocarpon aquaticum. American Journal Botany 28:
59–69.
Stocker-Wörgötter, E. & R. Türk. 1988. Licht- und
elektronenmikroskopische Untersuchungen von
Entwicklungsstadien der Flechte Endocarpon pusillum
unter Kulturbedingungen. Plant Systematicsand Evolution
158: 313–328.
——— &———.1989. Artificial cultures of the
cyanobacterial lichen Peltigera didactyla (Peltigeraceae) in
the natural environment. Plant Systematics and Evolution
165: 39–48.
Sturgis, W. C. 1890. On the carpologic structure and devel-
opment of the Collemaceae and allied groups. Proceedings
of the American Academy of Arts and Sciences 25: 15–52.
Sugiyama, J., K. Hosaka & S. O. Suh. 2006. Early diverging
Ascomycota: phylogenetic divergence and related evolu-
tionary enigma. Mycologia 98: 996–1005.
Sweetwood, G., R. Lücking, M. P. Nelsen & A. Aptroot.
2012. Ascospore ontogeny and discharge in
megalosporous Trypetheliaceae and Graphidaceae (Asco-
mycota: Dothideomycetes and Lecanoromycetes) suggest
phylogenetic relationships and ecological constraints. Li-
chenologist 44: 277–296.
TRIPP &LENDEMER:LICHEN REPRODUCTION2017]

Taylor, J. W., D. J. Jacobson & M. C. Fisher. 1999. The
evolution of asexual fungi: reproduction, speciation and
classification. Annual Reviews of Phytopathology 37:
197–246.
Thomson, J. W. 1991. The lichen genus Staurothele in North
America. Bryologist 94: 351–367.
Tinkle, D. W. 1969. The concept of reproductive effort and its
relation to the evolution of life histories of lizards. Amer-
ican Naturalist 103: 501–516.
Tripp, E. A. 2016. Is asexual reproduction an evolutionary
dead-end in lichens? The Lichenologist 48: 559–580.
———, Y. Zhuang & J. C. Lendemer. 2017.Areviewof
existing whole genome data suggests lichen mycelia may
be haploid or diploid. The Bryologist 120: 302–310.
Van der Niet, T., R. Peakall & S. D. Johnson. 2013. Pollinator-
driven ecological speciation in plants: new evidence and
future perspectives. Annals of Botany 113: 199–212.
Vines, S.H. 1878. Recent researches into the nature of lichens.
The reproduction of lichens and the sexuality of the asco-
mycetes. Quarterly Microscopical Journal 18: 438–445.
Vobis, G. 1977. Studies on the germination of lichen conidia.
Lichenologist 9: 131–136.
———.1980. Bau und Entwicklung der Flechten-Pycnidien
und ihrer Conidien. Bibliotheca Lichenologica 14: 1–141.
Werner, R. G. 1931. Histoire de la synthèse lichénique.
Mémoires de la Société des Sciences Naturelles du Maroc
27: 1–45.
Wieczorek, A. 2009. Ultrastructural examination of the
pycnidia and conidia of the genus Opegrapha (Arthoniales,
Ascomycota). Acta Mycologica 44: 165–171.
Yamazaki, T. & Y. Oshima. 1979. Direct diploidization and
occurrence of polyploidy in Saccharomycodes ludwigii.
Journal of General Microbiology 111: 271–281.
Yarranton, G. A. 1975. Population growth in Cladonia
stellaris (Opiz) Pouz. and Vezda. New Phytologist 75: 99–
110.
Yun, S.-H., M. L. Berbee, O. C. Yoder & B. G.
Turgeon. 1999. Evolution of the fungal self-fertile
reproductive life style from self-sterile ancestors.
Proceedings of the National Academy of Sciences of
the U.S.A. 95: 5592–5597.
Yoshimura, I., T. Kurokawa, Y. Yamamoto & Y. Kinoshita.
1993. Development of lichen thalli in vitro. Bryologist 96:
412–421.
Zhu, Y. O., G. J. Sherlock & D. A. Petrov. 2016. Whole
genome analysis of 132 clinical Saccharomyces cerevisiae
strains reveals extensive ploidy variation. G3 (Bethesda) 6:
2421–2434.
Zoller, S., B. Frey & C. Scheidegger. 2000. Juvenile
development and diaspore survival in the threatened
epiphytic lichen species Sticta fuliginosa, Leptogium
saturninum and Menegazzia terebrata: conclusions
for in-situ conservation. Pp. 1–127. In: The Fourth
IAL Symposium, Progress and Problems in Lichen-
ology at the Turn of the Millennium. Universitat de
Barcelona, Barcelona.
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