When asymmetry mimics zygomorphy: flower development in Chamaelirium japonicum (Melanthiaceae, Liliales)

Abstract

Pendent sessile flowers of Chamaelirium japonicum (Willd.) N. Tanaka appear zygomorpic, but they do not possess a bilateral symmetry. The flowers are subtended by a vestigial bract and lack a bracteole. The perianth consists of two small tepals on the abaxial side of the flower and four large tepals, none of which is median. Because the short tepals belong to different whorls, there is no symmetry plane. Despite the absence of a bracteole, the shape of the floral meristem before peri anth inception resembles that of bracteolate monocot flowers. At early stages, all six tepals are equal in shape and size, and the flower is actinomorphic. The differ ence between the dorsal and ventral sides and the pendent nature of the flower become expressed during the gynoecium development. The absence of median organs allows to avoid collision of floral organs with the flower-subtending bract during flower curvature. Species of Chamaelirium reveal a set of different flower groundplans, which makes the genus a perfect model to investigate evolutionary changes in flower symmetry accompanied by differential tepal reduction.

Full text

1
© The Author(s). 2023 Open Access (CC) BY-NC license: hps://creavecommons.org/licenses/by-nc/4.0/
Botanica Pacica. A journal of plant science and conservaon. 2023.
DOI: 10.17581/bp.2023.12s01
ABSTRACT
Pendent sessile owers of Chamaelirium japonicum (Willd.) N. Tanaka appear zygo-
morpic, but they do not possess a bilateral symmetry. The owers are subtended
by a vestigial bract and lack a bracteole. The perianth consists of two small tepals
on the abaxial side of the ower and four large tepals, none of which is median.
Because the short tepals belong to different whorls, there is no symmetry plane.
Despite the absence of a bracteole, the shape of the oral meristem before peri-
anth inception resembles that of bracteolate monocot owers. At early stages, all
six tepals are equal in shape and size, and the ower is actinomorphic. The differ-
ence between the dorsal and ventral sides and the pendent nature of the ower
become expressed during the gynoecium development. The absence of median
organs allows to avoid collision of oral organs with the ower-subtending bract
during ower curvature. Species of Chamaelirium reveal a set of different ower
groundplans, which makes the genus a perfect model to investigate evolutionary
changes in ower symmetry accompanied by differential tepal reduction.
Keywords: Chamaelirium, ower development, asymmetry, zygomorphy, ower symmetry,
tepal reduction
РЕЗЮМЕ
Ремизова М.В., Шипунов А.Б., Соколов Д.Д. Асимметрия имити ру
ет зигоморфию: развитие цветка у Chamaelirium japonicum (Melan
thia ceae, Liliales). Поникшие сидячие цветки Chamaelirium japonicum (Willd.)
N. Tanaka вы гля дят как зигоморфные, но в отличие от типичных зигоморф-
ных цветков не имеют билатеральной симметрии. Цветки с почти неразли-
чимой брак теей, но без брактеолы. Околоцветник состоит из двух малень-
ких и че ты рех крупных листочков околоцветника, ни один из которых не
расположен медианно. Так как маленькие листочки околоцветника принад-
лежат разным кругам, через цветок невозможно провести плоскость симме-
трии. Несмотря на отсутствие брактеолы флоральная меристема перед за-
ложением околоцветника имеет треугольную форму, как это характерно для
цветков однодольных растений с брактеолой. На ранних стадиях развития
все шесть листочков околоцветника одинаковы по форме и размерам, цвет-
ки актиноморфные. Различия в размере верхних и нижних листочков око-
лоцветника и поникание цветка начинают проявляться во время заложения
гинецея. Отсутствие органов в медианной плоскости позволяет облегчить
поникание цветка и избежать их наложения на брактею. Цветки предста-
вителей Chamaelirium весьма разнооб раз ны, что делает этот небольшой род
прекрасным объектом для изучения смены типа симметрии цветка, сопрово-
ждающегося редукцией листочков околоцветника.
Ключевые слова: Chamaelirium, развитие цветка, асимметрия, зигоморфия, симме-
трия цветка, редукция листочков околоцветника
Margarita V. Remizowa 1*
e-mail: margarita.remizowa@gmail.com
Alexey B. Shipunov 2†
Dmitry D. Sokoloff 1
e-mail: sokoloff-v@yandex.ru
1 Lomonosov Moscow State University,
Moscow, Russia
2 Kyoto University, University Museum,
Kyoto, Japan
† deceased
* corresponding author
Manuscript received: 28.04.2023
Review completed: 07.06.2023
Accepted for publication: 09.06.2023
Published online: 11.06.2023
Margarita V. Remizowa1*, Alexey B. Shipunov2† & Dmitry D. Sokoloff1
When asymmetry mimics zygomorphy:
flower development in Chamaelirium
japonicum (Melanthiaceae, Liliales)
Chamaelirium Willd. is a small genus of predominantly
Asia tic species occurring from Japan to Vietnam, though the
type species, Ch. luteum (L.) A. Gray, is restricted to eastern
North America (Tanaka 2017a). Earlier classications
inter pre ted Chamaelirium as monospecic and recognized
Chionographis to accommodate all Asian species.
Spider-like owers of Chamaelirium are small with slender
li form or spatulate tepals. The most striking feature of
the genus is the highly variable oral morphology (Tamura
1998, Tanaka 2003, 2017a, b). The owers contain up to six
tepals and are either actinomorphic or zygomorphic. Cha mae-
lirium japonicum (Willd.) N. Tanaka (Fig. 1) is a spe cies with
zygomorphic owers (Tanaka 2003). Flowers of Сh.japonicum
are reportedly ebracteate and possess four pro mi nent upper
tepals (Tamura 1998, Tanaka 2003, 2017a). The lower two
tepals are vestigial. One or both lower tepals are often absent,
at least to a naked eye. The owers are atta ched horizontally
or are facing downwards. Pendent zygo mor phic owers are
extremely rare in angiosperms (Endress 1994, 1999).
Zygomorphy (or monosymmetry) is one of mani fes ta-
tions of symmetry in owers and one of the most pro-
mi nent homoplastic traits in angiosperms (Endress 1999,
2001, 2012, Rudall & Bateman 2004, Jabbour et al. 2009,
Ci ter ne et al. 2010, Reyes et al. 2016, Bukhari et al. 2017).
Zygo morphic owers have only one symmetry plane and
thus show a bilateral symmetry along so-called plane of
zygomorphy. Zygomorphy can be classied into two types
– positional and constutional (Endress 1999, 2012). The
positional zygomorphy is mainly led by gravity and occurs
in taxa with predominantly polysymmetric owers. It arises
late in development and is often expressed by sig moi dal
curvature of stamens, style and a perianth tube if the latter
is present. The constitutional (or elaborate) zygo morphy
is much more complex; it occurs in owers with highly

2Botanica Pacica. A journal of plant science and conservaon. 2023. 12(S)
Remizowa et al.
synorganized organs (Endress 2006, 2011). It does not
depend on gravity. The owers are conspicuously mono-
sym metric with clear differences between the upper (dorsal)
and lower (ventral) sides (Bukhari et al. 2017). The stamens
and the pistil are often hidden in the keel on the lower
side of in the lip on the upper side (Endress 1994, 2012,
Westerkamp & Claßen-Bockhoff 2007). Zygomorphy by
reduction or simplicity as well as the elaborate zygomorphy
represents a structural type. Here, the plane of zygomorphy
is often due to reduction of inner organs (stamens or
carpels), but visually owers with perianth do not differ
much from actinomorphic ones (Endress 1999, 2011).
Figure 1 Chamaelirion japonicum (Willd.) N. Tanaka. A – part of anthetic inorescence, photo by Alexey Shipunov, taken on 15.05.2022 at
Tachiki. B – unusual inorescence with two lower owers subtended by well-developed leaves, photo by Alexey Shipunov, taken on 15.05.2022
at Tachiki. C – ower with gynoecium having two styles, photo by Margarita Remizowa. D-E – preanthetic plants, photo by Alexey Shipunov,
taken on 13.03.2022 at Tachiki. F – oral diagrams with left and right transversal outer tepal. G – oral diagram of a monocot bracteate ower
without oral prophyll (e.g. Veratrum). H – oral diagram of a monocot bracteate with a single lateral oral prophyll (e.g. Lilium)

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Botanica Pacica. A journal of plant science and conservaon. 2023. 12(S)
Floral development in Chamaelirium japonicum
In owers with elaborate zygomorphy, the plane of
mono symmetry is usually median (Endress 1999, 2012,
Bu kha ri et al. 2017). When oral zygomorphy develops
along the median plane, there is an odd (central) median
perianth member (petal in eudicots or tepal in monocots).
This odd perianth member is either median-adaxial (dorsal)
or median-abaxial (ventral). The plane of zygomorphy can
de velop not only along the median (vertical) plane, but also
along an oblique or transversal plane, but the owers become
resupinate and adopt a vertical position at anthesis (Bukhari
et al. 2017). In these cases, the odd perianth element is not
median morphologically but becomes topologically ventral
or dorsal. Both ventral and dorsal perianth members are
distinctive in size and shape, possess showy marks with
distinctive colors and are thus essential for pollinators
serving as visual signals or/and a landing platform.
The positional zygomorphy represents a ‘light’ type and
appears to be easily reversible to polysymmetry (Endress
1999, 2001, 2012). Indeed, species of Chamaelirion demon-
strate different types of ower symmetry and the occurrence
of zygomorphy is sometimes unstable even within a species
(Tanaka 2003, 2017a). Apart from Chamaelirion, among
Liliales, only some Alstroemeriaceae, some Colchicaceae
and Corsiaceae develop zygomorphic owers (Dahlgren
et al. 1985, Takhtajan 2009, Rudall & Bateman 2004,
Endress 2012). Gloriosa (Colchicaceae) and Alstroemeria
(Alstroemeriaceae) develop owers with positional mono-
sym metry (Endress 1994, 2012, Hoshino et al. 2014). Both
genera of non-photosynthetic Corsiaceae possess highly
specialized constitutional zygomorphy (Neinhuis & Ibisch
1998, Rudall & Eastman 2002). Little is known on the
nature of zygomorphy in Chamaelirion. At least it is unique
in positions of small and large tepals. Apparently, there is
no ventral or dorsal odd tepal, all tepals are of uniform in
colour and do not act as a landing platform. It is not clear
whether a resupination is involved in ower positioning and
which type of zygomorphy is present here.
MATERIAL AND METHODS
The morphology and anatomy of the inorescences
and owers were examined in the following specimens
– Japan, Hon shu, Hiroshima Prefecture, near Higashi-
Hiro shi ma, cam pus of Hiroshima University, 04.05.2005
(voucher: Soko loff and Remizowa s.n. MW0734664) and
Japan, Kyoto, Tachiki, 35.2693056ºN 135.3313669ºE,
Crypto meria forest, stony slope, 13.03.2022, 06.08.2022,
22.09.2022, 14.10.2022, 05.11.2022 (material collected by
Alexey Shipunov).
The plant material was xed and stored in 70 % ethanol.
For scanning electron microscopy (SEM), inorescences at
dif ferent developmental stages were dissected in 96 % etha-
nol under an Olympus SZX7 stereomicroscope, dehydrated
through absolute acetone, critical-point dried using a Hi ta chi
HCP-2 critical-point drier, then coated with gold and pal la-
dium using an Eiko IB-3 ion-coater. Observations were made
using a CAMSCAN S2 SEM at Moscow State University.
For light microscope observations, owers were sec tio-
ned using standard methods of Paraplast embedding and se-
rial sectioning at 15 mm thickness using a Thermo Scien ti c
Microm HM 355s rotary microtome at Moscow State Uni-
ver sity. Sections were stained in picroindigocarmine and car-
bolic fuchsine and mounted in Euparal mounting medium.
Digital photomicrographs were made using an Olympus
BX53 microscope tted with a digital camera. Images
were processed and assembled using Adobe Photoshop
Elements and Adobe Illustrator.
RESULTS
Organography
The owers are spirally arranged in terminal, dense,
many-owered spikes (Fig. 1A–B). The owers are pendent
on very short pedicels. The inorescence stalk and the
axis are ridged (Figs 1C, 2A–C). The inorescence appears
ebracteate but each ower is supported by a vertical ridge
of the main axis that is suddenly abrupted just below the
ower. The rounded tip of the ridge should be interpreted
as a reduced ower-subtending bract.
Flowers are bisexual, trimerous and pentacyclic
(Fig. 1A–C). The perianth is biseriate and consists of six free
tepals, two of them are minute and sometimes indiscernible
without a special examination and four are strip-like and
spa tulate (Fig. 1A–F, 2, 3). The minute tepals are usually of
unequal size. Irrespective of their length, all tepals have the
same width as stamen laments.
None of the tepals and consequently the other organs
occupy a strictly abaxial or adaxial position (Fig. 1F). Two
tepals are abaxial, two are transversal and two are adaxial.
The ower-subtending bract is located between the two
small tepals. Each perianth whorl consists of two long and
a short tepal. Thus, the short tepals (as well the transversal
and adaxial ones) belong to different whorls. If unequal,
the longer minute tepal (belonging to the inner whorl) is
inserted closer to the transversal outer tepal (Figs 2A–C,
3). Because the owers are pendent, the shorter (abaxial,
lower) tepals occur closer to the inorescence axis while
longer upper tepals are hanging over the rest of the ower.
The free six stamens possess thick and rounded in cross
section stamen laments, outer stamens are slightly longer
than the inner ones in open owers (Figs 1A–C, 2). Anthers
are basixed. Both tepals and stamens are white whereas
the gynoecium is greenish.
The gynoecium consists of three united carpels.
There is a carpel in a transversal position, the two others
are transversal-adaxial and transversal-abaxial. The ovary
is superior. The carpels are congenitally united along
the ovary; the styles are free (Figs 1C, 2D–L). The ovary
consists of synascidiate and symplicate zones, and the styles
form an asymplicate zone (Fig. 2). The synascidiate zone
(the basal part of the ovary) is relatively short (up to 1/3
of the ovary length) and sterile (Fig. 2D–G). The rest of
the ovary is formed by fertile symplicate zone (Fig. 2H–K).
The symplicate zone is unilocular with incomplete septa
deeply protruding into the ovary locule. The ovules (two
per carpel) are inserted at the base of the symplicate zone,
they are anatropous and bitegmic.
The styles are solid with a narrow furrow on the adaxial
(ventral) side. The stigmatic papillae occupy the entire vent-
ral side of the style (Figs 2L, 3H). In some owers, the

4Botanica Pacica. A journal of plant science and conservaon. 2023. 12(S)
Remizowa et al.
lower (transversal-abaxial) carpel lacks the style or/and the
ovule/ovules (Figs 1C, 2, 3G–H).
The ovary has six grooves opposite the stamens – three
along the dorsal carpel sides and three between the carpels.
Inside the ovary, there are three ribs along the dorsal veins;
the fruit dehiscence occurs along those ribs (Fig. 2). Septal
nectaries are absent, but septae contain conspicuous air
cavities in synascidiate zone (Fig. 2F–G).
Organogenesis
The inorescence development starts in August. By No-
vem ber, all oral organs are initiated. The anthesis occurs
in early May. The owers are initiated acropetally along the
inorescence axis and simultaneously with their ower-
subtending bracts (Fig. 4A–B). The primordia of the ower
and the ower subtending bract form a comp lex, which is
Figure 2 Floral anatomy of Chamaelirium japonicum, serial cross-sections. A – a section through inorescence node and receptacle base. B–C
– ower base. D – gynoecium base. E-G – ovary, synascidiate zone with air cavities in septa. H–I – ovary, symplicate zone with placentae and
ovules. J – upper part of ovary above placentae, symplicate zone. K–L – ovary roof and style bases, asymplicate zone. ia – inorescence axis,
it – large inner tepal, it* – small inner tepal, ist – inner stamen, ist* – inner stamen on the radius of small inner tepal, ot – large outer tepal,
ot* – small outer tepal, ost – outer stamen, ost* – outer stamen on the radius of small outer tepal. Arrowhead indicates lower carpel containing
single ovule and lacking style. Scale bars – 200 µm (A–C) and 100 µm (D–1L)

5
Botanica Pacica. A journal of plant science and conservaon. 2023. 12(S)
Floral development in Chamaelirium japonicum
elongate in the median plane (along the inorescence axis).
The primordium of the ower-sub tending bract takes up
about one third on this complex. Soon after initiation, the
growth of ower-subtending bract ceases and it remains as
a rounded bulge below the ower. Further, as the internodes
elongate the ridges arise below the ower-subtending bracts.
The oral meristem enlarges in the transversal plane and
then becomes bean-shaped with the ower-subtending bract
adjacent to the concave side (Fig. 4A–E). At the next step, the
oral meristem takes a shape of a scalene triangle (Fig. 4F–G).
The most protruding corner of this meristematic triangle is
transversal. Then, the outer tepals are initiated simultaneously
but by primordia of slightly unequal size (Fig. 4H–I). The
largest one is the primordium of transversal outer tepal
(located at the protruding corner of the oral meristem).
The outer transversal tepal is either right or left (Fig. 4F–I),
both positions co-occur within the same inorescence but
Figure 3 Late ower development in Chamaelirium japonicum. A – part of an inorescence, side view, zygomorphic oral buds. B–C – late oral
bud with small equal tepals, abaxial view, ower-subtending bract removed. D–E – late oral bud with unequal small tepals, abaxial and side
views. F – gynoecium at stage shown in A–E. G – anthetic ower with dehisced anthers. H – gynoecium of ower shown in G, lower carpel
(*) has no style. br – ower-subtending bract, it – large inner tepal, it* – small inner tepal, ist – inner stamen, ist* – inner stamen on the radius
of small inner tepal, ot – large outer tepal, ot* – small outer tepal, ost – outer stamen, ost* – outer stamen on the radius of small outer tepal.
Scale bars – 300 µm in A–B, D–E and H, 100 µm in C and F, 1 mm in G

6Botanica Pacica. A journal of plant science and conservaon. 2023. 12(S)
Remizowa et al.
in our material the right morph was prevailing. The inner
tepals appear shortly after the outer ones and follow the
same pattern: despite simultaneous initiation the primordium
of inner transversal tepal is larger than two other primordia
of the inner whorl tepals (Fig. 5A–D). The outer and inner
stamens are initiated whorl by whorl (Fig. 5E–I).
The carpels appear simultaneously as three free horse-
shoe shaped primordia (Fig. 6A–B). Soon after initiation,
a continuous rim uniting the carpels develops at the gyno-
ecium base below the free carpel tips (Fig. 6C–D). Starting
from this point, the gynoecial growth commences in two
directions. The free carpel tips elongate to produce styles
(asymplicate zone) and the congenitally united region
produces the ovary by zonal growth (Figs 3F, 6E–F). Within
the ovary, the short synascidiate zone is the last to appear.
The orice at the ovary top is postgenitally closed.
Figure 4 Early ower development in Chamaelirium japonicum. A – inorescence tip showing ower initiation. B – simultaneous initiation of
ower and its subtending bract. C – bean-shaped oral meristem. D–E – oral meristem larger than the ower-subtending bract, front and
side views. F–G – triangular oral meristem, left and right forms. H–I – initiation of outer tepals, left and right forms. br – ower-subtending
bract,  – oral meristem, L^ – left transversal protruding corner of oral meristem, R^ – right transversal protruding corner of oral
meristem, ot – primordium of outer tepal, ot^ – transversal outer tepal. Scale bars – 30 µm

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Botanica Pacica. A journal of plant science and conservaon. 2023. 12(S)
Floral development in Chamaelirium japonicum
In the bud, the anthers are organized in two levels: the outer
above the inner ones (Fig. 3A–E). At the lower side of the
ower, the inner stamens become pushed outside the perianth.
By the time of gynoecium initiation, the ower is more
or less symmetrical with all the tepals of equal size (Fig. 5).
Differences in tepal size and the curvature of the ower
become pronounced during gynoecium growth (Fig. 3).
DISCUSSION
Developmental data allow better understanding the
inorescence and oral morphology in Chamaelirium japo-
ni cum. First of all, the inorescence is bracteate and each
ower is subtended by a bract which is almost indiscernible
at maturity. The owers are initiated simultaneously with
Figure 5 Perianth and androecium development in Chamaelirium japonicum. A–D – owers with initiated tepals, note position of transversal
tepals – left forms with outer transversal tepal to the left (A and D), right forms with outer transversal tepal to the right (B and C). E –
initiation of outer stamens. F – ower shortly before initiation of inner stamens. G – part of inorescence at stage of outer tepal initiation
showing left and right ower forms. H – initiation of inner stamens. I – stage later than in H, beginning of enlargement of transversal and
adaxial tepals, ower becomes zygomorphic. br – ower-subtending bract, it – inner tepal, it^ – transversal inner tepal, it* – small inner tepal,
ist – inner stamen, ot –outer tepal, ot^ – transversal outer tepal, ot* – small outer tepal, ost – outer stamen. Scale bars – 30 µm in A–F, H–I
and 100 µm in G

8Botanica Pacica. A journal of plant science and conservaon. 2023. 12(S)
Remizowa et al.
their subtending bracts. This phenomenon is rare but not
unique in angiosperms. Such developmental pattern, some-
times even in form of common primordia, has been repor-
ted for some Alismatales (summarized in Remizowa et al.
2013a,b), magnoliids (Tucker 1975, 1981) and eudicots
(Sokoloff et al. 2007, Claßen-Bockhoff & Bull-Hereñu
2013, Claßen-Bockhoff 2016). Simultaneous initiation of
ower and ower-suntending bract is more common for
dense inorescences and for lineages with tendency to
ower-subtending bract reduction; the ower-subtending
bracts are either small or absent in different representatives
of the group.
In general, oral orientation in monocots with tri me-
rous owers is highly dependent on the presence of a oral
prophyll (bracteole) (Eichler 1875, Engler 1888, Remizowa
et al. 2006, 2013a). In trimerous owers subtended by a
bract, the median tepal of the outer whorl usually occu-
pies an abaxial position if the bracteole is absent (Fig. 1G)
and it occurs in either transverse position or an inter me-
diate (between median and transverse) if a bracteole is
pre sent (Fig. 1H). Although owers of Ch. japonicum do
possess a ower-subtending bract and do not possess a
brac teole, their oral orientation is more similar to that
of brac teolate monocot owers (Fig. 1F). The bracteole,
if present, is always situated in the same radius as one of
the inner tepals, and if the bracteole position is unstable,
oral orientation is also unstable: the ower ‘follows’ the
brac teole (Remizowa et al. 2013a). Apparently, this is not the
case for Ch. japonicum. The bracteoles are entirely absent at
any developmental stage in the species studied here. One
can suppose that the bracteole is morphologically sup pres-
sed and thus cannot be traced even during early ower
development (a ‘cryptic’ bracteole, see Choob 2022, for a
discussion on cryptic prophylls in monocots). This hypo-
thesis is problematic having in mind that bracteoles are
absent in all closest relatives of Ch. japonicum as well as in all
members of the family Melanthiaceae (Tamura 1998, Tanaka
2017a). Chamaelirium is so far the only known mem ber of his
family with such a oral diagram, though details of ower
orientation should be investigated in detail in other species
of the genus. Other members of the family de ve lop owers
with outer median tepal in abaxial position (Fig. 1G), which
is typical for monocot owers lacking a brac te ole (see gures
in Eichler 1875, Engler 1888, Endress 1995).
Surrounding phyllomes (ower-subtending bracts and
brac teoles) are crucial not only in establishing oral orien-
tation but also for sequence of tepal initiation (Remi-
zo wa et al. 2013a). In species with only the ower-sub-
ten ding bract developed, the oral development is often
uni directional along the median plane, and the organs are
initiated later or delayed on the abaxial side of the ower.
This unidirectionality is a source for monosymmetry in
early development and sometimes a precondition for the
evolutionary origin of zygomorphic owers (Endress
Figure 6 Early gynoecium development in Chamaelirium japonicum. A–B – gynoecium initiation as three separate carpel primordia. C – slightly
later than in B. D – beginning of zonal growth below free carpel parts, one carpel removed. E–F – elongation of free plicate carpel parts
(asymplicate zone, future styles) and symplicate zone (future ovary). Scale bars – 30 µm

9
Botanica Pacica. A journal of plant science and conservaon. 2023. 12(S)
Floral development in Chamaelirium japonicum
1995, 1999, 2012, Rudall & Bateman 2004, Remizowa et al.
2013a). Floral organ initiation is usually sequential at least in
the outer tepals in species with a transversal or nearly trans-
versal oral prophyll. The rst outer tepal is initiated strict ly
opposite the oral prophyll with divergence angle 180º, the
second and the third outer tepals are initiated between the
inorescence axis and the oral prophyll and between the
ower-subtending bract and oral prophyll (Remizowa et
al. 2013a). Flowers of Ch. japonicum do not t any of these
patterns. There are neither unidirectional development nor
spiral organ initiation. All organs within their whorls are
initiated simultaneously.
Changes of oral symmetry occur during development
in many taxa including monocots (summarized in Endress
1999, 2008, 2012, Remizowa et al. 2013b). Symmetry of
o wers in early development is inuenced by surrounding
parts of the inorescence. The ultimate ower shape deve-
lops later and, in some owers, (predominantly with posi-
tio nal zygomorphy) becomes prominent shortly before
anthesis. The ower of Ch. japonicum demonstrates several
changes of symmetry with regard to the ower-subtending
bract and inorescence axis during the development (Fig. 7).
Even before organ initiation, the oral meristem changes
its shape. At earliest stages, it is monosymmetric with the
median symmetry plane. Just before inception of the outer
tepals, the ower meristem is asymmetric (Fig. 7A–C).
The asymmetry is followed by polysymmetry. The period
of poly symmetry lasts until the differential growth of
tepals. Fi nally, the ower becomes asymmetric, having no
symmetry plane.
Developmental data allowed a better understanding of
the nature of zygomorphy in Chamaelirium. The term ‘zygo-
mor phy’ implies a bilateral symmetry (monosymmetry)
along plane of zygomorphy (usually median) accompanied
by different appearance of dorsal and ventral sides of the
ower (Endress 1999, 2001, 2012, Jabbour et al. 2009,
Citerne et al. 2010, Bukhari et al. 2017). Flowers of Ch. japo-
nicum demonstrate only the latter – lower (ventral) tepals
are smaller than the dorsal (upper) ones. Another unusual
feature is the even number of organs responsible for zygo-
mor phy. Ch. japonicum is (at least to our knowledge) the only
monocot having two small tepals belonging to two different
whorls. Large tepals are located on the upper part of the
ower, they are of the same colour as other parts of the
ower and do not serve as landing area for pollinators being
widely spaced. In terms of oral symmetry, the owers
of Ch. japonicum are not zygomorphic and can be inter-
pre ted as asymmetric by reduction (Endress 1999, 2012),
though no complete loss of any organs can be found here.
Despite structural asymmetry, owers visually appear as
zygo morphic due to differences between the lower and the
upper ower halves.
Early development shows that all tepals are initiated
and the ower itself is polysymmetrical at certain stages
of deve lop ment. Asymmetry by reduction is established
relatively early in oral development. It is expressed mainly
in the perianth. Although the orientation and development
of the ower in Ch. japonicum are unusual for monocots,
in which only the ower-subtending bract is present, the
inuence of the ower subtending bract on the abaxial side
of the ower is still tracible. The two tepals adjacent to
the ower-subtending bract stop their growth at a certain
moment. The growth of the larger, non-reduced tepals
proceeds simultaneously with the curvature of the ower.
Another manifestation of reduction is in gynoecium.
The lower carpel is somehow reduced, but this feature
is apparently variable. The size of the lower carpel is the
same as that of the other two carpels, but either the style is
absent, or the carpel contains one (or more than one) ovule,
or a combination of both.
The type of oral orientation and development of o-
wers of Ch. japonicum that differs from those in most other
mono cots cannot be directly explained by either theory of
oral patterning. Endress (1995, 1999) speculates that the
se quence of initiation of oral organs depends on the shape
of the space available. This space in turn depends on the in-
o res cence type – symmetrical in racemose inorescences
and asymmetrical in thyrses. Others hypothesize that sur-
roun ding structures govern oral development, and the
space available for developing ower depends on relative
po si tions of the inorescence axis, ower-subtending
bract and bracteole(s) (Ronse De Craene 2010, 2018, 2022,
Remizowa et al. 2013a, Bull-Hereñu et al. 2022, Choob
2022, Walch & Blaise 2022). Ch. japonicum contradicts
both these hypo the ses. The ower orientation and the
shape of the ower me ri stem prior organ initiation are as
Figure 7 Changes of ower symmetry during oral development in Chamaelirium japonicum. A – ower initiation, bilateral along median
plane. B – bean-shaped oral meristem, bilateral along median plane. C – oral meristem just before perianth initiation, asymmetrical. D-E –
perianth and androecium initiation, the upper and lower halves are equally developed, the ower is actinomorphic. Oblique symmetry planes
not shown. F – mature zygomorphic ower, asymmetrical. Symmetry planes are shown by grey lines

10 Botanica Pacica. A journal of plant science and conservaon. 2023. 12(S)
Remizowa et al.
in bracteolate owers despite bracteoles are lacking. The
young owers are free from any pressure apart that from
ower-subtending bract. Never theless, the oral meristem
autonomously becomes triangular to produce outer tepal
primordia which are of different size.
Developmental behavior of owers of Ch. japonicum
re sembles constitutional zygomorphy as there are strong
dif fe rences in tepals that are expressed relatively early in
the development. Visual zygomorphy is neither dorsal nor
vent ral as none of the organs occupy a median position, but
organs are larger on adaxial side of the ower. In owers
with median plane of constitutional zygomorphy (the most
com mon type), the differences between abaxial and adaxial
sides of the ower often affect both perianth and stamens
(End ress 1999, 2012). This is not the case in Ch. japonicum.
Similar to owers with constitutional zygomorphy, the o-
wers of Ch. japonicum exhibit change of oral symmetry
during their development. The main difference is in ower
orien ta tion that allows even numbers of unequal organs –
two small + four large tepals. This is only possible having
two adaxial, two transversal and two abaxial tepals. The dif-
fe rence in tepal size in Ch. japonicum is developmentally es-
tab lished relative to transversal plane (Fig. 7D–E). Typical
zygo morphic owers demonstrate both bilateral symmetry
along zygomorphy plane and differences between abaxial
and adaxial sides (Bukhari et al. 2017). Flowers of Ch. japo-
ni cum have only differences between abaxial and adaxial
sides, and strictly speaking they are asymmetric. On the
other hand, ‘zygomorphy’ in Ch. japonicum is not so deep as
in typical owers with elaborate zygomorphy. Apparently,
the symmetry may be easily changed here as in groups with
po sitional zygomorphy, but a sigmoidal stamen curvature
is absent here. Indeed, subspecies of Ch. japonicum differ
in the degree of reduction of lower tepals (Tanaka 2013,
2017a, Averyanov & Tanaka 2014, Wu et al. 2016, Liu et al.
2018, Tong et al. 2020, Qin et al. 2023).
The oral shape of Ch. japonicum is so peculiar that it
must have a functional signicance. However, inferring
such a sig nicance is surprisingly difcult. Flowers with
pe rianth parts longer on the upper part are relatively rare
in angiosperms. There are two main hypotheses explaining
adap tive implications of this trait (Neal et al. 1998, Leins &
Erbar 2010, Reyes et al. 2016). The dangerous lower mar-
gin hypothesis implies that a species pollinated by ying
vertebrates could benet from having its lower mar gin
smaller than the upper one as it avoids visits from in sects
(some of which may be nectar thieves) and makes polli na tion
easier for larger pollinators that cannot land on the ower
(Reyes et al. 2016). The nectar protection hypothesis im plies
that the longer upper perianth parts protect oral nec tar in
rainy weather. Both hypotheses clearly have no thing to do
with Ch. japonicum, because its owers produce no nectar and
the only recorded visitors are insects (Maki 1993b.).
What is the reason for unusual ower groundplan in
Ch. japo nicum? It is hardly evolved as an adaptation for insect
pollination. There is no landing platform, no special colour
marks, no nectar, the anthers are not gathered together as
in owers with buzz-pollination. Direct observations of
pollination by insects are lacking but cross-pollination is
suggested via indirect evidences (Maki 1993a, b, 1996, Maki
& Masuda 1994). Clearly, cross-pollination must occur in
dioecious and self-incompatible members of the ge nus. A
plausible explanation here is pendent owers. Me dian aba-
xial organs are mechanically problematic as they interfere
oral curvature. The ower-subtending bract though being
small prevents organ formation not only in the median
aba xial position but along median plane in general. Thus,
pen dent ower is a pre-adaptation for unusual ower orien-
tation in this case. However, the organ positions are set
before the actual curvature on the ower. The crucial stage
is the one when the oral meristem is wider than the ower-
subtending bract and bean-shaped.
In monosymmetric owers of eudicots and at least
some monocots with median plane of zygomorphy, the
dif fe rence between adaxial and abaxial sides is controlled
by CYCLOIDEA-like genes (Cubas 2004, Jabbour et al.
2009, Preston & Hileman 2009, Hoshino et al. 2014). In
Ch. japo nicum, zygomorphy is not so deeply expressed as in
mo del plants. The genetic background of zygomorphy of
Ch. japonicum is unclear and it worth investigating whether
the same genes are involved.
Last decades brought several new species of Chamae-
li rium (Huang et al. 2011, Tanaka 2013, 2017a, Averyanov
& Tanaka 2014, Wu et al. 2016, Liu et al. 2018, Tong et al.
2020, Qin et al. 2023). Although the exact oral orientation
is unclear from the protologues, the images available show
the same ower orientation for all species irrespective of
ower symmetry. A recently described dioecious species,
Ch. jiuwanshanense (Qin et al. 2023), is intriguing with respect
to patterns of oral symmetry and certainly deserves de-
tailed developmental investigations. The number of long
tepals varies between 3 and 5 (6). Remarkably, the long te-
pals are not always restricted to one side of the ower. At
least some owers possess three equally spaced tepals that
apparently belong to the same whorl. Some owers illust-
ra ted by Qin et al. (2023) could be true monosymmetric.
The pedicels of Ch. jiuwanshanense are conspicuous and up-
right. Therefore, not spatial constraints related to ower
cur vature similar to those proposed here for Ch. japonicum
should take place in Ch. jiuwanshanense. Species of Chamae-
li rium demonstrate a range of ower morphologies which
includes different patterns of tepal reduction and acti no-
mor phy/zygomorphy. These two characters seem to vary
in de pen dently, which makes Chamaelirium a perfect model to
study how owers play with symmetry and organ numbers.
ACKNOWLEDGEMENTS
We thank staff of Electron Microscopy Laboratory
of Moscow State University for their support in SEM
studies. This paper is dedicated to the memory of Alexey
Shipunov, who collected developmental material used in
this study. This work was supported by the Russian Science
Foundation (project 19-14-00055-P).
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