Polyploidy: Evolution and Crop Improvement

Abstract

The condition of possessing more than two complete genomes in a cell has intrigued biologists almost a century. Many plant species including flowering plants are polyploids, and we know that it has a significant role in the evolution and crop improvement. It is well tolerated in many groups of eukaryotes. Polyploid ancestors have given rise to a number of flowering plants. Despite its widespread occurrence, the direct effect of polyploidy on evolutionary success of a species is still largely unknown. Many attractive hypotheses have been proposed in order to assign functionality to the increased content of a duplicated genome. Among these hypotheses are the proposals that genome doubling confers various advantages to polyploids which allow them to thrive well in environments that pose challenges to their diploid progenitors. Polyploidy is often accompanied with formation of improved varieties, developing sterile lines, restoring fertility in hybrids, enlargement and enhanced vigor, increasing allelic diversity and heterozygosity, etc. In genome-wide context for optimizing marker-assisted selection and crop plant improvement, all these factors need to be considered. This chapter attempts to give a brief overview of polyploidy, its origin, and role in evolution and crop improvement.

Full text

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© Springer India 2017
T.A. Bhat, A.A. Wani (eds.), Chromosome Structure and Aberrations,
DOI 10.1007/978-81-322-3673-3_10
J.A. Dar (*) • Z.A. Beigh
Cytogenetics and Reproductive Biology Laboratory, Department of Botany,
University of Kashmir, Srinagar 190006, J & K, India
e-mail: jahangirdar53@gmail.com
A.A. Wani
Department of Botany, University of Kashmir, Srinagar, Jammu and Kashmir, India
10
Polyploidy: Evolution and Crop
Improvement
Jahangir A. Dar, Zahoor A. Beigh, and Aijaz Ahmad Wani
Abstract
The condition of possessing more than two complete genomes in a cell has
intrigued biologists almost a century. Many plant species including flowering
plants are polyploids, and we know that it has a significant role in the evolution
and crop improvement. It is well tolerated in many groups of eukaryotes.
Polyploid ancestors have given rise to a number of flowering plants. Despite its
widespread occurrence, the direct effect of polyploidy on evolutionary success of
a species is still largely unknown. Many attractive hypotheses have been pro-
posed in order to assign functionality to the increased content of a duplicated
genome. Among these hypotheses are the proposals that genome doubling con-
fers various advantages to polyploids which allow them to thrive well in environ-
ments that pose challenges to their diploid progenitors. Polyploidy is often
accompanied with formation of improved varieties, developing sterile lines,
restoring fertility in hybrids, enlargement and enhanced vigor, increasing allelic
diversity and heterozygosity, etc. In genome-wide context for optimizing marker-
assisted selection and crop plant improvement, all these factors need to be con-
sidered. This chapter attempts to give a brief overview of polyploidy, its origin,
and role in evolution and crop improvement.
Keywords
Autopolyploidy • Allopolyploidy • Evolution • Crop improvement

202
10.1 Introduction
The organisms with more than two sets of chromosomes are called polyploids
(Acquaah 2007; Chen 2010; Comai 2005; Ramsey and Schemske 1998). Polyploidy
is widespread in nature and provides a way for adaptation and creation of new spe-
cies. According to Chen et al. (2007), many crop plants have undergone polyploidy
during their evolutionary process. According to Comai (2005), in every 100,000
plants, one plant is formed as polyploid by angiosperms at a significantly higher
frequency. For understanding the nature of polyploidy, many studies have been car-
ried out, and the present chapter seeks to throw light on the applications and impli-
cations of polyploidy in plant breeding and other commercial ventures. To
understand polyploidy a few basic points need to be defined. The complete basic set
of chromosomes is designated by “X”, while the total number of chromosomes in a
somatic cell is denoted by “2n”. A somatic cell contains twice the number of chro-
mosomes, while the gametes contain a haploid set only (Acquaah 2007; Otto and
Whitton 2000). Three types of polyploidy, namely, autopolyploidy, allopolyploidy,
and segmented allopolyploidy, have been distinguished by Stebbins (1947). In the
first one, all genomes are identical and arise via genome duplication within the same
species (Stebbins 1947; Lewis 1980). Allopolyploids contain two or more different
genomes and can arise via hybridization of two different species linked with genome
doubling (Stebbins 1947; Grant 1975). The third one, i.e., segmental allopolyploids,
carries more than two incompletely distinct genomes which can lead to the forma-
tion of both bivalents and multivalents during chromosome pairing (Stebbins 1947;
Levin 2002).
During the early part of the twentieth century, the phenomenon of polyploidy
gained much more importance. According to Ramsey and Schemske (1998), Hugo
De Vries’s original mutation of Oenothera lamarckiana was one of the earliest
examples of natural polyploidy. The occurrence of a fertile-type Primula kewensis
from a sterile interspecific hybrid through chromosome doubling was discovered by
Digby (1912), but the author failed to realize the significance of the same in the
context of polyploidy (Stebbins 1971). Many crop plants like wheat, maize, sugar-
cane, coffee, cotton, and tobacco are polyploids either through intentional hybrid-
ization and selective breeding (e.g., some blueberry cultivars) or as a result of
polyploidization event taken place in ancient times (e.g., maize) (Ramsey and
Schemske 2002). In long-lived perennials that possess various vegetative means of
propagation (Fragaria, Rubus, Artemisia, Potamogeton, etc.) and in those with fre-
quent occurrences of natural interspecific hybridizations, polyploidy seems to be
very favorable (Hilu 1993).
10.2 Changes in Chromosome Number
The changes either in one or a few chromosomes lead to aneuploidy. These changes
in chromosome numbers are determined in relation to the somatic chromosome
number (2n) of the species. Those aneuploid organisms which lack one
J.A. Dar et al.

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chromosome pair (2n−2) are termed as nullisomic. While those aneuploids that lack
a single chromosome (2n−1) are known as monosomic. A double monosomic indi-
vidual lacks two chromosomes which belong to two different chromosome pairs
(2n−1−1). An aneuploid individual that contains one extra chromosome (2n + 1) is
known as trisomic and that having two extra chromosomes from two different chro-
mosome pairs is called double trisomic (2n + 1 + 1). A tetrasomic individual has one
pair of extra chromosomes (2n + 2). On the other hand, euploidy involves change in
complete set of genome which is an exact multiple of the basic chromosome num-
ber of the concerned species. It is generally called polyploidy. A polyploid indi-
vidual having all the genomes identical is called an autopolyploid. On the other
hand, allopolyploids have two or more different genomes present. Euploids may
have 3, 4, 5, 6, 7, 8, or more genomes making up their somatic chromosome number.
The terminology of heteroploidy in common use is summarized in Table 10.1.
Table 10.1 Type of variations in chromosome number
Term Type of change Symbol
Aneuploid One or few chromosomes extra or missing from
2n
2n ± few
Nullisomic One chromosome pair missing 2n−2
Monosomic One chromosome missing 2n−1
Double monosomic One chromosome from each of the two different
chromosome pairs missing
2n−1−1
Trisomic One chromosome extra 2n + 1
Double trisomic One chromosome for each of two different
chromosome pairs extra
2n + 1 + 1
Tetrasomic One chromosome pair extra 2n + 2
Euploid Number of genomes or copies of a single genome
more or less than two
Monoploid One copy of a single genome X
Haploid Gametic chromosome complement of the species N
Polyploid More than two copies of one genome
Autopolyploid Genomes identical with each other
Autotriploid Three copies of one genome 3x
Autotetraploid Four copies of one genome 4x
Autopentaploid Five copies of one genome 5x
Autohexaploid Six copies of one genome 6x
Allopolyploid Two or more distinct genomes (generally each
genome has two copies)
Allotetraploid Two distinct genomes 2x1 + 2x2
Allohexaploid Three distinct genomes 2x1 + 2x2 + 2x3
Allooctoploid Four distinct genomes 2x1 + 2x2 + 2x3 +
2x4
Source: Singh BD (2012), Plant Breeding Principles and Methods
10 Polyploidy: Evolution and Crop Improvement

204
10.3 Origin of Polyploidy
Polyploidy is originated by different means. Somatic doubling during mitosis, non-
reduction in meiosis leading to the formation of unreduced gametes, polyspermy
(fertilization of the egg by two male nuclei), and endoreduplication (replication of
the DNA but no cytokinesis) are some of the basic processes which give rise to
polyploidy. According to Grant (1981), some authors have reported endoreduplica-
tion and somatic doubling more similar and have not viewed these as separate
mechanisms. Chromosome doubling can occur either in the zygote or in some api-
cal meristems to produce complete polyploids and polyploidy chimeras, respec-
tively. Ramsey and Schemske (1998) have reported somatic polyploidy in some
nonmeristematic plant tissues (e.g., tetraploid and octoploid cells in the cortex and
pith of Vicia faba). According to Grant (1981), mitotic nondisjunction is the main
cause of somatic doubling. Somatic doubling can occur in purely vegetative tissues,
in branches that may produce flowers, or in early embryonic divisions (Grant 1981).
Chromosome doubling in the zygotes was best described from heat shock experi-
ments in which young embryos were briefly exposed to high temperatures (Lewis
1980).
10.3.1 Origin of Autopolyploidy
Autoployploidy can be defined as the individuals with multiple sets of chromo-
somes derived from a single species. Autopolyploids can occur spontaneously in
nature in low frequency and can be artificially induced by various means such as
heat and chemical treatments, decapitation, and selection from twin seedlings. In
autopolyploidy, the chromosomes fail to separate due to meiotic failure resulting in
gametes with twice as many chromosomes as normal (2n). Autopolyploids can be
formed by unreduced 2n gametes which are produced due to gametic nonreduction
or meiotic nuclear restitution during micro- and megasporogenesis. Figure 10.1
shows the origin of autopolyploidy from two nonreduced gametes.
10.3.2 Origin of Allopolyploidy
The polyploids with chromosomes derived from different species are called allo-
polyploids. The fusion of reduced 1n gamete with unreduced 2n gamete gives rise
to 3n zygote followed by the subsequent fusion of 1n reduced gamete with 3n gam-
ete in the next generation giving rise to a tetraploid individual. This two-step pro-
cess of allopolyploid production is sometimes referred to as a triploid bridge. The
diagrammatic representation of allopolyploid formation is given in Fig. 10.2.
Environment and genotype have the influence on the formation of nonreduced
gametes. For example, an increase in the number of nonreduced gametes in Gilia
J.A. Dar et al.

205
has been shown to be favored by adverse growing conditions. In case of maize, the
gene elongate on chromosome 3 was found to increase the proportion of diploid
eggs which is an example of genotype in modulating the nonreduced gamete pro-
duction (Grant 1981; Lewis 1980). The rapid screening techniques like flow cytom-
etry, chromosome pairing, and other genomic techniques are helpful in studies on
unreduced gametes in both plants and animals (Mable 2003). According to Ramsey
and Schemske (1998), the contribution of polyspermy as a mechanism of poly-
ploidy formation is rare except in some orchids. Endoreduplication has been known
to occur in endosperm and the cotyledons of developing seeds, leaves, and stems of
bolting plants (Larkins et al. 2001).
Fig. 10.1 Showing the origin of autopolyploid from unreduced gametes (Source: Campbell’s
Biology, page 454, 5th Edition)
Fig. 10.2 Showing the origin of allopolyploid from unreduced and reduced gametes (Source:
Campbell’s Biology, page 454, 5th Edition)
10 Polyploidy: Evolution and Crop Improvement

206
10.4 Methods for Inducing Polyploidy
In the1930s it was discovered that colchicine inhibits the formation of spindle fibers
and temporarily arrests chromosomes at the anaphase stage (Blakeslee and Avery
1937). At this point, the chromosomes have replicated, but cell division has not yet
taken place resulting in the formation of polyploidy cells. Other mitotic inhibitors,
namely, oryzalin, trifluralin, amiprophos-methyl, and N2O gas, have also been iden-
tified and used as doubling agents (Bouvier et al. 1994; Van Tuyl et al. 1992; Taylor
et al. 1976). There are various methods for applying these doubling agents. One of
the easiest and most effective methods is to work with a large number of seedlings
having small, actively growing meristems. Seedlings can be soaked or the apical
meristems can be submerged with different concentrations, durations, or frequen-
cies of a given doubling agent. Shoots of older plants can be treated, but it is often
less successful and results in a greater percentage of cytochimeras. Treatment of
smaller axillary or subaxillary meristems is sometimes more effective. Chemical
solutions can be applied to buds using cotton, agar, or lanolin or by dipping branch
tips into a solution for a few hours or days. Surfactants, wetting agents, and other
carriers (dimethyl sulfoxide) are sometimes used to enhance efficacy. Heat or cold
treatment, X-ray, or gamma ray irradiation may also induce polyploidy in low fre-
quencies. Triploid branches have been produced in Datura by cold treatment.
Exposure of maize plants or ears to high temperature (38–45 °C) at the time of first
zygotic division produces 2–5 % tetraploid progeny (Randolph 1941). Similarly
heat treatments in barley, wheat, rye, and some other crop species have been suc-
cessfully used for inducing polyploidy.
10.5 Role of Polyploidy in Plant Evolution
In comparison to the gradual evolutionary process whereby species evolve by small
spontaneous mutations accumulated over time in the population, new species of
plants can also arise rapidly. The most common mechanism for abrupt speciation is
through the formation of natural polyploids. Once a tetraploid arises in a popula-
tion, it can generally hybridize with other tetraploids. However, these tetraploids are
reproductively isolated from their parental species. Tetraploids that cross with dip-
loids of the parental species will result in triploids that are typically sterile. This
phenomenon provides reproductive barrier between the polyploids and the parental
species which is a driving force for speciation. Various estimates suggest that about
47–70 % flowering plants are of polyploidy origin (Grant 1971; Goldblatt 1980;
Ramsey and Schemske 1998). For example, the plants in the rosaceous subfamily
Maloideae (Malus, Pyrus, Photinia, etc.) are believed to have originated from
ancient allopolyploids since they have n = 17 basic chromosome number, whereas
plants in other subfamilies of Rosaceae have n = 8 or 9 (Rowley 1993). In many
genera, different species will have different ploidy levels representing a series of
polyploids. In Chrysanthemum different species have chromosome numbers of 2n =
18, 36, 54, 72, 90, and 198 – all multiples of a basic chromosome number of 9.
J.A. Dar et al.

207
Polyploids have adaptive and evolutionary advantages due to certain factors.
They are significantly more heterozygous than their diploid counterparts. The het-
erozygosity can be a key factor in growth, performance, and adaptability of a poly-
ploidy plant. Allopolyploids can contribute to heterosis or hybrid vigor due to
dissimilarity in genes. All polyploid individuals have a certain amount of genetic
redundancy; extra copies of genes can mutate and diverge resulting in new traits
without compromising essential functions. Polyploid populations often reveal
extensive genomic rearrangement including the origin of novel DNA regions
(Arnold 1997; Song et al. 1995; Wendel 2000). Ancient polyploids can eventually
undergo such changes to the extent that they effectively become diploidized where
diploid gene ratios are restored.
Polyploid plants also tend to be more self fertile and apomictic. Since polyploids
usually arise at a low frequency, greater self fertility and apomixes would help to
compensate for their minority disadvantage (Briggs and Walters 1977) and would
provide further benefits in areas where breeding systems are compromised in stress-
ful environments. Furthermore, inbreeding is less harmful for allopolyploid plants
due to their greater heterozygous nature. One question that frequently arises is
whether polyploids inherently have greater stress tolerance or not. For example, it
has often been observed that unequal number of polyploids is found in stressful
conditions like cold and dry regions. Some argue that this is a spurious correlation
(Sanford 1983) or possibly the result of intermixing of species and formation of
allopolyploids during glacial periods (Stebbins 1984). However, polyploids may
also have positive characteristics that provide some benefits helpful in adaptation.
Molecular studies have confirmed that allopolyploids exhibit enzyme multiplicity
(Soltis and Soltis 1993). Since allopolyploids represent a fusion of two different
genomes, these polyploids can potentially produce all of the enzymes produced by
each parent as well as new hybrid enzymes. This enzyme multiplicity may give
polyploids with greater biochemical flexibility, possibly extending the range of
environments in which the plant can grow (Roose and Gottlieb 1976). Other changes
in expression of genes, altered regulatory interactions, and rapid genetic and epigen-
etic changes could further contribute to increased variation and new phenotypes
(Osborn et al. 2003).
10.5.1 Autopolyploidy and Evolution
Autopolyploidy has played a limited role in evolution of plant species. Some of our
present-day crop species are considered to be autopolyploids. Autotetraploids
appear to have been more successful as crops than other forms of autopolyploidy. In
addition many forage grasses and several ornamentals are most likely autopoly-
ploids. Recent studies using genomic in situ hybridization (GISH), however, have
revealed peanut and coffee to be allopolyploids. GISH is a powerful tool for inves-
tigation of genome organization and evolutionary relationships. The diploid pro-
genitors (parental species) of Arachis hypogaea are A. villosa and A. ipaensis (Raina
and Mukai 1999). Similarly the most likely diploid progenitors of Coffea arabica
10 Polyploidy: Evolution and Crop Improvement

208
are the wild species C. congensis and C. eugenioides (Raina et al. 1998). A similar
analysis of other putative autotetraploids may reveal them to be allopolyploid in
nature. Molecular analysis like genome sequencing and comparative genomic stud-
ies reveal that most species of angiosperms and vertebrates have experienced whole
genome duplications, followed by loss of most of the duplicated regions of the
genome. In this process, their genomes have retained considerable amounts of
duplications, which have expanded the range of genetic diversity in these species.
For example, Oryza sativa and Arabidopsis thaliana have experienced whole
genome duplications three times at approximately 70 million years ago, 65 million
years ago, and 40 million years ago. A large part of the duplicated genome has been
lost since then, but a large number of duplicated genes exist even today. These
duplicated genes have diverged to various degrees and become subfunctional, i.e.,
they show reduced levels of expression as confirmed by transcriptome analyses.
10.5.2 Allopolyploidy and Evolution
Allopolyploidy has played an important role in evolution. Allopolyploidy occurs in
various genera of plants and has enjoyed considerable success in natural popula-
tions. It is expected that one third of the angiosperms are polyploids, and a huge
number of them are allopolyploids. Allopolyploids have been more successful as
crop species than autopolyploids. Some of the present-day allopolyploid crop spe-
cies are given in Table 10.2.
Some naturally occurring allopolyploid crops include wheat, cotton, tobacco,
mustard, oat, etc. Interspecific crossing followed by chromosome doubling in nature
has resulted in the origin of allopolyploid crop species. The evolutionary origins of
some natural allopolyploid crops are described below:
(i) Bread wheat (Triticum aestivum)
Evolutionary origin of bread wheat has been the most extensively investigated.
Identity of the diploid species contributing the three different genomes (A, B, and
D) of Triticum aestivum has been investigated by many workers more notably by
Sears, Kihara, and others. It is generally accepted that the genome A present in
diploid wheat is similar to those present in tetraploid and hexaploid wheat. Further,
the genome B of tetraploid emmer wheat is found similar to that in hexaploid wheat.
This is evident from chromosome pairing in crosses among diploid, tetraploid, and
hexaploid wheat. Hybrid between diploid and tetraploid wheat shows 7II and 7I,
while those between tetraploid and hexaploid wheat shows about 14II and 7I. It is
believed that A genome of wheat has come from Triticum monococcum (2n = 14),
D genome from Triticum tauschii (2n = 14), and B genome from unknown source
probably from an extinct species (2n = 14) (Fig. 10.3).
J.A. Dar et al.

209
Table 10.2 Some allopolyploid crop species and their gametic chromosome numbers
Scientific name Common name
Gametic ch.
no. Cultivated/wild
Avena strigosa Sand oats 7 Wild
A. barbata Slender wild oats 14 Wild
A. sativa Cultivated oats 21 Cultivated
A. byzantine Cultivated red oats 21 Cultivated
Brassica nigra Black sarson 8 (B)aCultivated
B. oleracea Cabbage, cauliflower,
etc.
9 (C) Cultivated
B. campestris Turnip rape 10 (A) Cultivated
B. carinata Abyssinian cabbage 17 Wild
B. juncea Rai, Indian mustard 18 Cultivated
B. napus Rape 19 Cultivated
Gossypium arboreum Asiatic (desi) cotton 13 (A2) Cultivated
G. herbaceum Asiatic cotton 13 (A1) Cultivated
G. thurberi Wild American cotton 13 (D1) Wild
G. barbadense Sea island (Egyptian)
cotton
26 (A2D2) Cultivated
G. hirsutum American upland
cotton
26 (A1D1) Cultivated
Hordeum vulgare Cultivated barley 7 Cultivated
H. jubatum Squirrel tail barley 14 Wild
H. nodosum Foxtail barley 21 Wild
Medicago hispida California burclover 7 Cultivated
M. lupulina Black medic 8, 16 Cultivated
M. falcate Yellow alfalfa 8, 16 Cultivated
Nicotiana sylvestris Wild tobacco 12 Wild
N. tomentosa Wild tobacco 12 Wild
N. tabacum Cultivated tobacco 24 Cultivated
N. rustica Cultivated tobacco 24 Cultivated
N. bigelovii Wild tobacco 24 Wild
N. debneyi Wild tobacco 24 Wild
Prunus americana American plum 8 Cultivated
P. avium Sweet cherry 8 Cultivated
P. persica Peach 8 Cultivated
P. cerasus Sour cherry 16 Cultivated
P. domestica European plum 16 Cultivated
Saccharum officinarum Noble canes 40 Cultivated
S. barberi Indian canes 41, 45, 46, 58,
62
Cultivated
S. sinensis Indian canes 58, 59 Cultivated
S. spontaneum Wild canes 20–64 Wild
(continued)
10 Polyploidy: Evolution and Crop Improvement

210
(ii) Tobacco (Nicotiana tabacum)
The genus Nicotiana comprises about 76 currently recognized naturally occur-
ring species that are subdivided into 13 sections (Knapp et al. 2004). Nicotiana
tabacum (n = 14) is a classic amphidiploid species originated from a hybridization
event between Nicotiana sylvestris and Nicotiana tomentosa, both the species are
diploid with n = 12 (Fig. 10.4). It has been reported that the maternal parent and the
other donor of the S genome is Nicotiana sylvestris (Bland et al. 1985; Olmstead
and Palmer 1991; Aoki and Ito 2000; Yukawa et al. 2006), whereas the section
Tomentosae (Nicotiana tomentosiformis, Nicotiana otophora, or an introgressive
hybrid between the two) has contributed the T genome (Kenton et al. 1993; Riechers
and Timko 1999; Lim et al. 2000; Kitamura et al. 2001; Ren and Timko 2001).
Table 10.2 (continued)
Scientific name Common name
Gametic ch.
no. Cultivated/wild
S. robustum Wild canes 30–74 Wild
Sorghum versicolor Wild sorghum 5 Wild
S. bicolor Jowar 10 Cultivated
S. halepense Johnson grass 20 Cultivated
Trifolium pratense Red clover 7 Cultivated
T. alexandrinum Berseem clover 8 Cultivated
T. repens White clover 16 Cultivated
T. medium Zigzag clover 40, 48, 42, 49 Cultivated
Triticcum monococcum Wild einkorn 7 (A) Wild
T. turgidum var. dicoccoides Wild emmer 14 (AB) Wild
T. turgidum var. dicoccum Emmer wheat 14 (AB) Cultivated
T. turgidum var. turgidum Solid stem wheat 14 (AB) Cultivated
T. turgidum var. carthlicum Persian wheat 14 (AB) Cultivated
T. turgidum var. polonicum Polish wheat 14 (AB) Cultivated
T. turgidum var. durum Durum wheat 14 (AB) Cultivated
T. timopheevii – 14 (AG) Cultivated
T. timopheevii var.
americanum
– 14 (AG) Wild
T. aestivum var. spelta Spelt wheat 21 (ABD) Cultivated
T. aestivum var. aestivum Common bread wheat 21 (ABD) Cultivated
T. aestivum var. macha – 21 (ABD) Wild
T. aestivum var. compactum Club wheat 21 (ABD) Cultivated
T. aestivum var.
sphaerococcum
Indian dwarf wheat 21 (ABD) Wild
T. zhukovskyi – 21 (AAG) Cultivated
aLetters within parentheses denote the genomes present in the species
Source: Singh BD (2012), Plant Breeding, Principles and Methods
J.A. Dar et al.

211
(iii) Cotton (Gossypium hirsutum)
All the diploid species of genus Gossypium have haploid chromosome number of
13 and fall into seven different genome types, designed A–G based on chromosome
pairing relationships (Beasley 1942; Endrizzi et al. 1984). A total of five tetraploid (n
= 2x = 26) species are recognized in Gossypium. According to Kimber (1961), all
tetraploid species show disomic chromosome pairing. Chromosome pairing in inter-
specific crosses between diploid and tetraploid cotton suggests that tetraploids contain
two different genomes, which resemble the A genome of G. hirsutum (n = 13) and D
genome of G. raimondii (n = 13), respectively. Both the A and D genome species
diverged from a common ancestor about 6–11 million years ago (Wendeil 1989). The
putative A x D polyploidization event occurred in the New World, about 1.1–1.9 mil-
lion years ago, in which A genome donor which is native to the old world served as
the female parent (Wendeil 1989; Wendeil and Albert 1992). The five allotetraploid
species (G. hirsutum, G. barbadense, G. darwini, G. mustelinum, and G. tomentosum)
are thought to have originated by diversification at the polyploidy level (Fig. 10.5).
(iv) Amphidiploid Brassica species
An interesting example of the role of allopolyploidy in the evolution of different
Brassica species is presented in the Brassica triangle (Morinaga 1934) (Fig. 10.6).
As per the scheme, Brassica juncea (n = 18) is an amphidiploid from an interspe-
cific cross between Brassica nigra (n = 8) and Brassica campestris (n = 10), whereas
an interspecific cross between Brassica oleracea (n = 9) and Brassica campestris
(n = 10) has given rise to amphidiploid Brassica napus (n = 19). On the other hand
Brassica carinata (n = 17) is a result of an interspecific cross between Brassica
nigra (n = 8) and Brassica oleracea (n = 9).
Fig. 10.3 Evolution of bread wheat (Triticum aestivum) (Source: http://www.ibri.org/Books/
Pun_Evolution/Figures/Fig03-12.gif)
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212
10.6 Polyploidy and Crop Improvement
Polyploidy has found some practical applications in improvement of crops, includ-
ing the development of commercial varieties in India as well as in some other coun-
tries, utilization as a bridging species, creation of new crop species, widening the
genetic base of existing polyploids, etc. Some important examples are discussed
below.
(a) Autotetraploids (triploid sugar beet and watermelon): Triploid sugar beets have
larger roots as compared to diploids, but they maintain the sugar content of
diploids and thus yield more sugar per unit area. Such triploids are produced by
inter planting diploid and tetraploid plants. Autotetraploid sugar beets have
Fig. 10.4 Evolution of
Nicotiana tabacum
Fig. 10.5 Evolution of cotton (Gossypium hirsutum)
J.A. Dar et al.

213
smaller roots as compared to diploid. Triploids can be produced when tetraploid
and diploid are crossed or when something goes erroneous in meiosis and unre-
duced gametes are produced (with 2n chromosomes) which unite with gametes
carrying haploid (n) chromosomes. Triploidy can also be beneficial in water-
melons. Diploid watermelons (Citrullus lanatus) have 22 chromosomes per
somatic cell and are fully fertile and produce a huge number of seeds per fruit.
However, natural parthenogenesis is not known for watermelons. Hence
Kihara’s (1951) technique of producing seedless triploid watermelons can be
utilized. The autotetraploid lines are planted alternately with diploids in isola-
tion. Tetraploids are used as seed parent. The seeds produced in triploids from
tetraploids are viable. However, when diploid is used as female or seed parent,
then the program of triploid seed production is unproductive. Because of mei-
otic abnormalities, triploids cannot produce true seeds, but rudimentary struc-
tures similar to seeds of cucumber (small and white). For raising a commercial
successful crop of triploid seedless watermelon, it is necessary to interplant
diploid variety because fruit setting on triploids depends upon stimulus pro-
vided by the pollen.
(b) Autotetraploids: The only successfully developed autotetraploid among the
grain crops is the rye (Secale cereale). Tetraploid rye has better qualities than
diploids like larger kernels, superior ability to emerge under adverse conditions,
and higher protein content. It is grown in Sweden and Germany. Similarly tet-
raploid grapes with larger fruits and fewer seeds per berry than diploids have
been developed in California, USA. Tetraploid strains of red clover grown in
Sweden have given higher hay yields than corresponding diploids among the
forage crops. Pusa giant berseem (a variety of Egyptian clover) has been
released in India for higher fodder yield. In cases of ornamental plants like
phlox, dahlia, snapdragon, etc., induced autopolyploidy has been most
successful as they have bigger flower size, longer blooming period, and rela-
Fig. 10.6 Evolution of
Brassica species and the
relationships between
diploid and naturally
occurring amphidiploid
species of Brassica
10 Polyploidy: Evolution and Crop Improvement

214
tively longer lasting flowers (Kehr 1996). Autotetraploidy has also been suc-
cessful in crops like banana, maize, potato, and turnip.
(c) Overcoming barriers to hybridization: Due to differences in ploidy levels
between prospective parents, the desirable crosses are difficult to obtain. Such
interploid barriers appear to arise from abnormal endosperm formation. In spe-
cies where there is a block at interploid level, seeds will often only develop
normally if there is a 1 paternal: 2 maternal ratio in the genomic makeup of the
endosperm, which would be the normal case for two diploid parents (Ramsey
and Schemske 1998). Seeds that do not meet this criterion are often immature
or aborted. In some cases this ratio is not accurate, but the larger the variation,
the lower the viability of the seeds (Sanford 1983). In cases where the blocks
due to the difference in ploidy level exist, barriers to hybridization may be over-
come by manipulating the ploidy levels to match prior to hybridization.
(d) Developing sterile cultivars: A significant threat to certain ecosystems is the
introduction of invasive species. An ideal approach for addressing this problem
is the development of sterile forms of important nursery crops. There are vari-
ous methods for developing sterile plants. Among them one of the rapid and
efficient approaches is to create polyploidy. In most cases, these sterile plants
function normally except reproduction particularly meiosis. In spite of these
complexities, autotetraploids of some species can produce seeds that are fertile.
In such a situation, triploids can be created by hybridizing tetraploids with dip-
loids. In some species triploid development can be complicated due to the inter-
ploid block that prevents the normal development of triploid embryo. However,
embryo culture is one of the techniques that can be used to produce sterile
triploid plants. Another approach for triploid development is the regeneration of
plants from endosperm. In most angiospermic seeds, the embryo is diploid, and
the adjoining endosperm originates from the fusion of three haploid nuclei (one
from male gametophyte and two from female) resulting in triploid tissue. This
tissue can be excised from developing seeds and cultured in vitro to eventually
give rise to regenerated embryos and plantlets. This method has been successful
in various plants like citrus, kiwifruit, loquat, etc.
(e) Restoring fertility in wide hybrids: It is not necessary for hybrids between dif-
ferent taxa to be sterile. This often occurs due to failure of the chromosomes to
pair correctly during meiosis – referred to as chromosomal sterility. The fertility
can be restored by doubling the chromosomes of a wide hybrid. This approach
has been used successfully in Rhododendron and Chitalpa tashkentensis
(Contreras 2006; Olsen 2006). However, in some cases this technique has been
successful in restoring fertility, as was the case with tetraploid hybrids of
Alstroemeria aurea × A. caryophyllaceae (Lu and Bridgen 1997).
(f) Enhancing pest resistance and stress tolerance: Polyploids have played an
important role in adaptability and resistance to biotic and abiotic stresses (Levin
1983). In some cases polyploids have demonstrated greater resistance to pests
and pathogens, greater nutrient uptake efficiency, better drought resistance, and
superior cold tolerance. There are a number of strategies for inducing polyploids
as a means of enhancing adaptability. The expression and concentration of cer-
J.A. Dar et al.

215
tain secondary metabolites and defense chemicals can be enhanced by increas-
ing the chromosome number and related gene dose. However, this is not always
true, and little is generally known about the relationship between gene dose,
gene silencing, and expression secondary metabolites. An important method
would be to develop allopolyploids between plants with diverse endogenous
secondary metabolites. A unique and valuable characteristic of allopolyploids
is that they often produce all the enzymes and metabolites (including defense
chemicals) of both parents. This could be particularly effective for combining
the characteristics of pest resistance of two species and contributing to a much
broader and more horizontal form of pest resistance. The same approach may
be useful for enhancing tolerance to certain environmental stresses.
(g) Increased allelic diversity and heterozygosity: Polyploidy has played an impor-
tant role in increasing the allelic copy number and heterozygosity leading to
novel phenotypes. Allelic diversity also increases during allopolyploidy, when
two (or more) different genomes are present in a common nucleus. According
to Osborn et al. (2003), the oil seed production in B. napus is positively affected
by intergenomic heterozygosity. The QTL for seed yield and other traits in
other populations of B. napus is also affected by intergenomic heterozygosity
(Udall et al. 2006; Quijada et al. 2006). The tetraploid cotton also dominates the
global market in terms of fiber production because they produce longer, finer,
and stronger fiber than do their diploid relatives. According to Jiang et al.
(1998), several QTL located on the D genome suggested that D genome loci
had been used for the synthesis of fiber subsequent to polyploidy formation.
(h) Creation of new crop species: New crop species can be developed by poly-
ploidy as triticale as the best example which is an allopolyploid between
Triticum aestivum and Secale cereal. Poland, Germany, and France mainly cul-
tivate the triticale varieties. An induced polyploid variety Raphanobrassica was
of no use as the desired traits were not obtained from the cross. Another new
autotetraploid variety was produced in kiwi (Actinidia chinensis) with the help
of colchicine treatment, highlighting the considerable potential of this method
to produce new cultivars with satisfactory fruit size (Wu et al. 2012).
10.7 Conclusions
Despite the prevalent occurrence of polyploidy in nature and the occurrence of its
footprints in all angiospermic genomes, the question of effects of polyploidy on the
evolutionary route of a species is still unclear. Earlier questions about the role of
polyploidy in response to environmental stress or whether genome doubling is
advantageous or disadvantageous to evolutionary success are being revisited using
current genomic tools. Studies based on molecular levels are evident for genomic
change on numerous levels of regulation related to polyploidization. However, the
effects of polyploidy on fitness under different environmental conditions are not
known still in many cases, and there is little evidence that observed transcriptional
and genomic changes actually lead to faster evolution or greater adaptation in
10 Polyploidy: Evolution and Crop Improvement

216
natural populations. Polyploids are looking generally different from their progeni-
tors in morphological, ecological, physiological, and cytological characteristics that
can contribute both to exploitation of a new niche and to reproductive isolation.
Therefore, polyploidy is a major mechanism for adaptation and speciation in plants.
The development of new crops and the interspecific gene transfer and also the origin
of new crops can be traced with the help of polyploidy breeding. Thus, polyploidy
is an interesting field of study to demonstrate the evolution of crop plants and utilize
their variability in the field of crop breeding.
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