Polyploidy in plants
Kaarthik John
Introduction:
One of the remarkable features of living
material is their ability to perpetuate themselves. However, the ever dynamic
nature of the surrounding environment has imposed upon plants, much like other
organisms, various evolutionary and selective bottlenecks necessitating the
adoption of ways and means by organisms to keep their “race going”. A concept
which gains prominence in this regard is that of hybridization and has been an
area of much fascination since the eighteenth century. Stebbins (1958) defined
hybridization as the “crossing between individuals belonging to separate
populations which possess different adaptive norms”(27). Polyploidy, a prime
facilitator of speciation and evolution in plants and to a lesser extent in
animals, is associated with intra and inter-specific hybridization(12).The
purpose of this paper is to give a broad overview of the phenomenon of polyploidy in its entirety in plants ranging right from a brief
historical background to where it stands today. Given the importance of
polyploidy in speciation this will be looked into a little more in detail
compared to other aspects of polyploidy.
So what is polyploidy? It refers to a definite arithmetic
relationship between the chromosome numbers of related organisms (2). It has
been defined as the possession of three
or more complete sets of chromosomes and has been an important feature of
chromosome evolution in many eukaryotic taxa including plants, yeasts, insects,
amphibians, reptiles, fishes and even the mammalian genome (19). Polyploidy is
present to at least to some extent in most members of the plant kingdom being
more common in some and rather rare in others(26). The fact that it is
widespread many plants is also kind of exemplified by the wide variations in
chromosome numbers with chromosome numbers ranging from 2n = 4 to500 in
angiosperms to 2n=6 to 226 in monocots(2). Thousands of angiosperm species have
14 to 15 pairs of chromosomes (2).
The phenomenon of
polyploidy gained much of what it is today during the early part of the
twentieth century. One of the early examples of a natural polyploid was
one of De Vries’s original mutations of Oenothera lamarckiana
(mutat. gigas)(19). The first example of an artificial polyploid was by Winkler (1916) who in fact introduced the term
polyploidy (19,2). Winkler was working on vegetative grafts and chimeras of Solanum
nigrum and found that callus regenerating from cut surfaces of stem
explants were teratploid (2,19). Digby(1912) had discovered the occurrence of a
fertile type Primula kewensis from a sterile inter-specific hybrid
through chromosome doubling but failed to realize its significance in the
context of polyploidy(28). Though unaware of the ‘Primula type” fertile hybrid,
Winge (1917), from his studies on the chromosomal counts of Chenopodium
and Chrysanthemum found that chromosome numbers of related
species were multiples of some common basic number; he subsequently
proposed a hypothesis that chromosome doubling in sterile inter-specific hybrids is a means of converting them into fertile offsprings (31,3). This was subsequently verified by various
workers in artificial inter-specific
hybridizations of Nicotiana, Raphanobrassica and Gaeleopsis(2).Finally
the colchicine method of chromosome doubling was developed by Blakeslee and
Avery (1937) and became an important tool for the experimental study of
polyploidy (26).
Before going ahead what
is the main significance of polyploidy in brief? As pointed out at the very
outset it is recognized as one of the main process in the evolutionary history
of plants and to some extent other organisms(14). Though differences do exist
with regard to the nature of its role and the relative importance of different
kinds of polyploidy, an understanding of the ways in which polyploidy
operated in the past to produce new
species and races may provide useful
insights to improving our cultivated plants. In fact many of our crop species, including wheat, maize, sugar cane,
coffee, cotton and tobacco, are polyploid, either through intentional
hybridization and selective breeding (e.g. some blueberry cultivars) or as a
result of a more ancient polyploidization event(e.g. maize)( 20).
Added
to it, technological
advances in the analysis of genome structure and function has made it possible
to better analyze the genetic consequences of genome duplication. The
importance of polyploidy in such diverse fields such as cytogenetics,
physiology, breeding, cytotaxonomy and biogeography in conjunction with new
possibilities put forth by various
molecular techniques has all spurred a resurgence of interest in issues of
origin and establishment of lineages(14).
There are various
modes for the origin of polyploids. These mainly include mechanisms such as
somatic doubling during mitosis, non-reduction in meiosis leading to the
production of unreduced gametes, polyspermaty (fertilization of the egg my two
male nuclei) and endoreplication( replication of the DNA but no cytokinesis). Endoreduplication however, is more similar
to somatic doubling and is therefore not viewed as a separate mechanism by some
authors (2). These mechanisms are reviewed in greater detail below.
Chromosome doubling can occur
either in the zygote to produce a completely polyploid individual or locally in
some apical meristems to give polyploid chimeras. Somatic polyploidy is seen in some non-meristematic plant tissues as
well.(eg: tetraploid and octoploid cells in the cortex and pith Vicia faba)(19).In
somatic doubling the main cause is mitotic non-disjunction(2).This doubling may
occur in purely vegetative tissues(as in root nodules of some leguminous
plants) or at times in a branch that may produce flowers or in early embryos
(and may therefore be carried further down)(2). Spontaneous somatic chromosome
doubling is a rare event (13) and the only well documented instance of the same
was in case of tetraploid Primula
kewensis which arose by somatic
doubling in certain flowering branches of a diploid hybrid. The phenomenon of
chromosome doubling in the zygotes was best described from heat shock
experiments in which young embryos were briefly exposed to high temperatures
(13). Zygotic chromosome doubling was first proposed by Winge and the
spontaneous appearance of tetraploids in Oenothera lamarckiana and
amplidiploid hybrids in Nicotiana were shown to be a result of zygotic
chromosome doubling (13,2).
A second major
route of polyploid formation involves gametic “non-reduction” or “meiotic
nuclear restitution” during microsporogenesis and megasporogenesis resulting in
unreduced 2n gametes. Non reduction could be due to meiotic non-disjunction
(failure of the chromosome of separate
and subsequent reduction in chromosome number), failure of cell wall
formation or formation of gametes by mitosis instead of meiosis(6).The classic
example, Raphanobrassica, originated by a one step process of fusion of
two non- reduced gametes (1,2). The production of non- reduced gametes has been
shown to be rather common in Solanum
sps.(13).
Another route may
involve non–reduction occurring in one of the germ lines (pollens or the eggs).
A tetraploid individual can then result from a two-step process (sometimes
referred to as a triploid bridge mechanism) from the fusion of an unreduced 2n
gamete with a reduced 1n gamete to give a 3n zygote followed by the subsequent
fusion of a 3n gamete with a normal 1n gamete in the next generation to give
rise to a tetraploids individual (as in artificial Galeopsis tetrahit)(2).
This is a more common route of polyploid formation from unreduced gametes
(though the frequent sterility of most triploid hybrids has lead to a
questioning of this method by some authors (13)) rather than the former one
(13,19).
The production of
non-reduced gametes is also a function of the environment and genotype. eg:
adverse growing conditions were shown to favor an increase the number of
non-reduced gametes in Gilia. An example of the influence of genotype in
modulating the production of non-reduced gametes can be seen in case of maize
wherein the gene “elongate” on chromosome 3 was found to increase the
proportion of diploid eggs produced (2,13).
Studies on
unreduced gametes in both plants and animals are getting easier with the use of
rapid screening techniques such as flow cytometry, chromosme painting and other
genomic techniques (14).
Polyspermy is
observed in many plants (19) but its contribution as a mechanism for polyploid
formation is rather rare except perhaps
in some orchids (19).
Endoreduplication is a form
of nuclear polyploidization
resulting in multiple uniform copies
of chromosomes(5). It has been known to occur in the endosperm and the
cotyledons of developing seeds, leaves and
stems of bolting plants (9). In
animals it occurs in certain tissues such as the liver cells, and megakaryocytes ( the cells which give rise
to the thrombocytes) (6).
There are a number of other factors favoring polyploidy include (but not limited to), the mode of reproduction, the mode of fertilization, the breeding system present, the growth habit of the plant, size of chromosomes etc. These are looked into in a little more detail below.
Polyploidy seems
to be favored in long lived/perennial plants possessing various vegetative
means of propagation (eg: Fragia, Rubus, Artemisia, Potamogeton etc.)
and in those with frequent occurrences of natural inter-specific
hybridizations(3).Various possible reasons have been advanced by various
workers (2,13,26) to account for the above phenomenon; one of the widely
accepted ones’ being the enhanced
chances of somatic doubling made possible in plants with enhanced lifespan
and vegetative means of reproduction.(2).
Cross
fertilization and allogamy were argued to be factors favoring polyploidy.
Autogamy however was thought to
restrict it (26). There have been opposing views as well on the same. For example in the tribe Midinae
(Compositae) polyploidy was found to be highly developed in the autogamous genus Madia but
almost absent in the allogamous species of Layia and Hemizonia
(26).
As regards
latitude and altitude, the proportion of polyploids has been found to increase
with latitude and altitude; but this has not always been found to be true with
respect to altitude(3). Various reasons which have been put forth to explain
the above trend in the distribution of polyploids some of them being the better adaptability of polyploids to colder
climates and changes that might have taken place in the Pleistocene period etc.
Various ecological
factors also have a bearing on the distribution of polyploids eg: polyploids
were found to be more frequently distributed in wet soils and meadows as
opposed to more stable habitats with drier soils or forest communities
respectively (2).
With regard to the
breeding system since the main mode of origin of allopolyploids in annuals is
by the fusion of unreduced gametes, the presence of an outcrossing breeding
system tends to reduce the chances of union of unreduced gametes (2).
A perennial growth habit tends to favor polyploidy
as opposed to an annual growth habit, probably due to the fact that having a
long life span increases the chances that rare events will occur (e.g.
polyploidization following hybridization), and allows for mating between
polyploids and their offspring (2,18).
A reciprocal
relationship has been observed between cell size, chromosome size and
chromosome numbers in polyploids (2).Added to these a few other factors related
to the genotype and the environment which had been looked into during the
discussion on “the production of unreduced gametes” also come into play in.
Classification
of polyploids:
Kihara & Ono(1926) first described two distinct types of polyploids: “autopolyploids’ and “allopolyploids”(8).Autopolyploidy is the doubling of the same chromosome set while allopolyploidy is the product of inter-specific hybridization; it is the product of doubling in a species hybrid and is therefore a polyploid containing separate sets of non-homologous chromosomes. The frequency of multivalent formation at synapsis was initially emphasized as a criterion for distinguishing auto- and allopolyploidy (30). Later other basis for classification were also put forth. But nevertheless the classification of a plant into an auto or allotetraploid simply based on criteria such as resemblance in external morphology to some diploids, the behavior of chromosomes at meiosis etc. does not give any information as to the behavior or phylogenetic origin of the species and may therefore be of little practical value. More extensive classifications were subsequently given by other workers (Simonet, Lilienfield and Clausen, Keck and Hiesey) and the classification has expanded ever since (30). Below is generally accepted classification of the different classes of polyploids:
Autopolyploids: Here the genomes coming together in the polyploid are identical. Here again they may be a strict /true autoployploid (AAAA) or an interracial autopolyploid (AAAA). Autopolyploids are also called polysomicpolyploids and can occur at the level of triploidy or anywhere upwards (7,2).
Amphiploid: This was a term coined by Clausen, Keck and
Hiesey (1945) and includes (6,7,2)
true
allopolyploids, segmental allopolyploids and autoallopolyploids and aneuploids.
A true
allopolyploid or a disomic polyploid is
a polyploid species derived from hybridization of parents that had structural
dissimilarity between their basic genomes. These can occur at any level from
tetraploidy upwards.eg: AA X BB ® AABB
In a segmental
allopolyploid the genomes in the
species are partially homologous to
each other and therefore exhibit partial multivalent formation. eg: A1A1
X A2A2 ® A1A1A2A2
Autoallopolyploids are polyploids which combine the characteristics from both autopolyploids and allopolyploids, such as. eg:AA X BBBB ® AABBBB
Aneuploids involve the gain or loss of a single chromosomes
Paleopolyploid: This refers to an ancient polyploid that later became a diploid again due to sequence divergence between duplicated chromosomes as in the human genome. They generally have large basic chromosome numbers (4,32).
Neoployploids: newly formed auto and allopolyploids(13,20).
A polyploid could have individuals with a series of ploidy levels within the species thereby giving rise to a ‘ploidy series’. The ploidy series may consist of individuals with even or odd multiples of the basic chromosome number(eg: Chrysanhemum (x=9); series 2x, 4x, 6x, 8x,10x) or odd multiples of the basic chromosome number ( eg: Crepis occidentalis (x=11); series 2x, 3x, 4x, 5x, 7x and 8x forms)(2).
Aneuploid series represent succession of allopolyploids based on different basic chromosome numbers. (eg: Stipa 2n=22,24,28,32,36….82) (32). Dibasic polyploids are the sum of two different diploid numbers (eg: Brassica oleracea( 2n=18) and Brassica campestris (2n=20) and their tetraploid derivative B. napus (2n =4x=38) (2).
Polyploids often tolerate the loss of one or more chromosome pairs which at times may give rise to modified polyploid series, what Darlington called a ‘polyploid drop’(eg: a modified series found in Hesperis where different species have gametic numbers of n =7,14,13 and 12) (2).
Consequences/Implications of polyploidy
Polyploidy has
been regarded as a major force in evolution and speciation (22,23). It is
estimated that between 47% and 70% of angiosperm species are polyploid (19) and
this shoots up to as high as 95% in Pteridophytes (reaches the highest known
levels in the plant kingdom) (11,19). Polyploidy is rather common in some families
like Rubiaceae, Compositae, Iridaceae, Gramineae etc. It is uncommon but
present in other families such as Caesalpinaceae, Passifloraceae and Fagaceae
(2). Homosporous pteridophytes are found to have higher base numbers as
compared to heterosporous types(13). Again the highest percentage is found in
perennial herbs and a smaller proportion is found in annuals and woody plants
(26,28). It has been studied to a lesser extent in thallophytes (algae) but
polyploids have been also been observed in Cladophora and Chara (26). A few
cases of polyploidy have been observed Rhodophycophyta and Phaeophycophyta. In
bryophytes mosses seems to be a classical example for the occurrence of natural
and artificial polyploids(26) but is rather uncommon in liverworts (2). Stebbins
(26) even pointed out to inter-generic differences in the frequency of
polyploids and cited the example of Saliaceae where polyploidy was common in Salix but
rare in Populus. In Gymnosperms it is rare. It is unkown in cycads and Ginko
but is known in Ephedra (2), Pseudolarix amabilis(26) and Seqoia sempervirens(26).
Inspite of the prevalence of high rates of polyploidy in many
of the plant species there have been opposing views on the as well on
relative contribution of polyploidy towards the process of speciation(18).
These are exemplified in the statements below:
...polyploidy
has contributed little to progressive evolution.
(Stebbins GL.
1971,pp.132, 28)
....polyploidy,
far from playing a secondary role in evolution, has provided
the additional,
uncommitted gene loci necessary for major steps in the
evolution of
animals.
(Schultz RJ.
1980,22)
The field of
polyploidy therefore formed an area of interest and controversy for more than
50 years. The application of molecular techniques is now contributing to
reshape many of our traditional tenets of polyploidy which is looked into in
greater detail in some of the sections below.
The evolutionary
forces involved in speciation can broadly be grouped into those producing
variation and those tending to fix it. The forces mainly contributing to bring
about variation include hybridization and novel mutations while those tending
to fix it include selection with wide outcrossing, inbreeding by self
fertilization and selection, inbreeding by assortative mating etc.(2). The
variation generating and variation fixing forces can be combined in various
permutations and combinations (2). The process of polyploidization can
essentially be seen as a variation generating force involving extensive intra
and inter-specific hybridizations as mentioned earlier(13). Naturally occurring
inter-specific hybrids, presumably F1s occur in all major groups of plants (2).
Examples for artificially induced polyploids range from the classical Raphanobrassica
example of Karpechenko to many of the modern crop plants (31). The
contribution of polyploidy in the evolution of new species can be better
illustrated by looking into some specific examples below.
The classic
intergeneric cross between Raphanus (radish, 2n=18) and Brassica (cabbage,2n=18)
caused the formation of a sterile F1 hybrid with only univalents. However
subsequent chromosome doubling resulted in a fertile allotetraploid, Raphanobrassica
(2n=36), which behaved similar to normal diploids and therefore provided a clear example of how hybridization between
two species followed by chromosome
doubling could cataclysmally (cataclysmic evolution) bring about the creation
of a new species(31).Though an artificially created allopolyploid species, it
definitely epitomized all criteria required of a legitimate species encountered
in nature(31). Primula kewensis also represented another example of an
artificial allotetraploid created from a previously sterile hybrid (got from a
cross between two diploid species).
The contribution of
natural polyploidy to the creation of new species can best be seen from the
stories of various crop plants such as wheat (Triticum aestivum),
tobacco (Nicotiana tabaccum),
and upland cotton (Gossypium hirsutum) to name a few (31).
The origin of allohexaploid bread wheat (Triticum
aestivum)occurred from three diploid grass species coupled with chromosome doubling as shown in
the figure below (Fig1)(31).
The New World
cottons (2n=52) are also supposed to have risen from an
allopolyploidization event between Asiatic cottons (2n=26) and
American cottons (2n=26)(1).
Similarly many of
the mustard species such as Brassica
juncea(2n=36), B.napus (2n=38) etc
are supposed to have risen from
inter-specific crosses between B.nigra (2n=16) and B.campestris(2n=20)
and B.oleraceae(2n=18) and B.campestris(2n=20) respectively(1).
Triticum monococcum X Triticum searsi
2n =14 Aegilops speltoides ?
(AA) 2n =14
(BB)
(gemetes) A B
¯ ¯
---------------------------
AB (sterile hybrid)
¯
doubled
¯
Triticum tdiscocoides X Triticum tauschii (DD)
2n =28 Aegilops squarrosa ? |
AABB 2n =14
DD
¯ ¯
(gametes) AB D
ABD ( sterile hybrid)
¯ (doubled)
Triticum aestivum
2n=42
(AABBDD)
Fig 1: The role of polyploidy in the evolution
of new species; the origin of bread wheat.
Having looked into
some specific examples as to how polyploid was involved in some of the early
speciation events in may be useful to review in brief some of the traditional
views on polyploidy and restructuring of some of these which have been made
possible by modern molecular techniques.
For long polyploid
events were considered rare and most of the polyploid species were thought to
have single origin (23). Polyploids in general were regarded to have the
ability to colonize a wider range of habitats and survive better in harsh and
unstable climates as compared to their diploid progenitors probably due to
increased heterozygosity and genic and biochemical flexibility provided by the
presence of additional alleles (16,25,29).Due to the ‘buffering effect’ of
multiple genomes, mutations and recombination etc. they were thought to contribute lesser to the development of new adaptive complexes than
diploids.(considering equal mutation rates in diploids and polyploids)
(25,24).There again, allopolyploids were considered a major force in evolution
while autopolyploids on the other hand were considered rare and rather
maladaptive (24).
But as mentioned
earlier many of these views on seem to be undergoing transformation largely due
to data available from molecular approaches such as RFLPs, GISH, chromosome
painting etc. Polyploids in fact
are thought to represent a relatively frequent class of mutation, one that
occasionally establishes within populations when its phenotypic effects are
relatively mild (18,20). Or conversely polyploidy may be common because
polyploid species may evolve faster or in more novel directions ( 18, 20).
Most of the
polyploid species are now seen to be of multiple origin with a few exceptions
such as Arachis hypogea and salt
water grass (Spartina anglica). The exact extent of multiple origins is
unknown but in most cases is thought to be underestimated (24).Studies on
recent allopolyploidy in Tragopogon indicate that multiple origins can
occur frequently over a short time and area.(just past about 50 years in a
small region of eastern Washington)(23).The concept of recurrent
polyploidization and subsequent further interbreeding of genotypes is best seen
in the Artic flora. Multiple polyploid events from genetically and
morphological differentiated diploid populations yield a complex of different
genotypes at the ploidy level and these genotypes in turn ultimately come into
contact and hybridize (24).
Another discovery
is the occurrence of extensive genomic reorganization in polyploids. Chromosome
painting, genetic mapping and comparative genetics have shown the occurrence of
extensive intra and intergenomic reorganizations. Nine intergenomic translocations
were detected in allotetraploid tobacco
(Nicotiana). Soybean, an ancient tetraploid also revealed significant
intra-chromosomal rearrangement (24). In another experiment by Song et al.
using synthetic polyploids of Brassica to study the evolution of early
generations after polyploidization, it was found that extensive genomic changes
were detected in F5 as compared to the F2 plants as indicated by the loss/gain
of parental restriction fragments and
also by the appearance of new fragments(25). Transposable elements (TE) might
also facilitate genome restructuring
since duplicates of all genes in polyploids
might buffer them from the
deleterious consequences of transposition and TE will tend to multiply and be maintained (15).(eg: in cotton the spread of A genome repeats to D genome
is thought to be mainly by transposition)(16).
In addition to
gene restructuring, in a polyploid organism there could be several fates for
redundant genes including regulatory or functional divergence which will
preserve the increased gene number but may also lead to ‘copy number dependent
gene silencing’; an epigenetic change. (16,24). Extensive silencing may finally
cause a polyploid to no longer appear to be structured as an allopolyploid as
in Zea mays. Gene silencing from epigenetic modifications is thought to
be reversible (16).
Also it has long
been held that polyploid species have the ability to colonize a wider range of
habitats and survive better in harsh and unstable climates (better heat, cold
drought resistance etc.(29). But an opposing view has been presented by
Stebbins who postulated that the better
ability is due to secondary contacts between
previously isolated populations which generate more aggressive and
adapted gene combinations that have been buffered and maintained(29). In a 39
year experiment with diploid and tetraploid Ehrharta erecta he found
that the polyploid was inferior in behavior to their diploid ancestors under
field conditions(29).
The high rates of polyploid occurrence has been looked into and explained on different grounds. Incompatible gene or chromosome combinations that may be brought together in allopolyploid genomes cannot be easily flushed out through Mendelian segregation and the elimination of such DNA sequences and the alteration of DNA methylation patterns permit fertility restoration in some allopolyploids (21).
Another contradicting view in the context of polyploids with respect to evolution is the fact that autoploids are not maladaptive to evolution. Important genetic attributes such as increased enzyme multiplicity and increased allelic diversity in fact make them a strong success in nature (24).
The effects of polyploidy on evolution have been one of the one key areas of focus of many workers and authors. Nevertheless polyploidy does have implications on various other aspects some of which are looked into below.
Cytology: Polyploids in general tend to have larger cells compared to their diploid progenitors (12). As regards the effect of polyploidy on the volume of the cell there seem to be conflicting reports with some stating the lack of any effect cell volume while others reporting an increase in cell volume (upto eight times that found in haploids)(12). Overall there seems to be a reduction if the surface area to volume ratio (12,13) which in turn has a bearing on enzymes such as ornithine transcarbymylase, tryptophan synthase, invertase and acid phosphatse related to cell volume and cell surface area respectively. Metabolism and growth seems to be rather retarded in polyploids (12,13) due to altered geometric relations between the nucleus and the rest of the cell. An inverse relationship between DNA content and development rate has also been found (13). Polyploids also seem to have reduced number of cell divisions during development (l3).
Gene activity: There have been reports of variable results with regard to the effect of polyploidy on gene activity, the total RNA and protein contents (17). Differential amplification of RNA cistrons were reported with increasing ploidy levels in Datura innoxia and autotetraploid tomato (13). With regard to effect of chromosome doubling on enzyme behavior “ambiguous” results have been reported.The levels of various enzymes involved in the electron transport chain, photosynthesis and photorespiration were found to increase, decrease or remain unchanged in autotetraploid tomatoes (13). The CO2 exchange rate has generally been reported to decline with ploidy level but has been found to increase in Festuca arundinacea.
The effects of chromosome doubling may exhibit differences between gametophytes and sporophytes of the same species and also between different strains within the same species (12).
Growth substances: As regards the effect of polyploidy on various growth hormones lower contents of abscisic acid and an auxin like substances were found in autotetraploid Lycopersicon pimpinellifolium as compared to diploids(13).
Water balance: A higher water content and a lower osmotic pressure was found in autotetraploid tomatoes and also in Petunia (26). Not much was know about differences in the rates of water uptake but it was found that transpiration rates were lower in polyploids.
Responses to different stress:
Parameters such as whole plant weight, relative water
content etc. decrease to smaller extent in polyploids as compared to
diploids(13). Generally polyploids seem
to be more tolerant to drought than their diploid progenitors (12). But
contradictory views have been presented by Stebbins who observed that
tetraploids of Ehrharta erecta persisted only on shady well drained soils
whereas the diploid s survived in more
harsh environments (29). As regards cold tolerance it was found that
autotetraploids of Brassica campestris, Raphanus sativus and a
few others were more cold resistant while tetraploids of Trifolium repens were less resistant.(12) Polyploids plants
were also found to be more resistant to mutagenesis and irriadiation(13).
Polyploids seem to be more tolerant to poor soils, since most of them are slow
growers and therefore have more modest nutrient requirements (12). Most
polyploids in general are more
resistant to pathogens and pests due to
enhanced production of various
secondary plant metabolites. Polyploids are also more resistant to herbicides,
which may be due to increased heterozygosity and increased genetic redundancy
(13).
Other aspects:
There seems to be
reduced pollen and seed viability in autopolyploids as compared to diploid
ancestors. There is an alteration in the primary and secondary metabolism.
Autopolyploids of many drug plants have increased quantities of alkaloids per
dry weight. Chromosome doubling may also alter the secondary metabolism in a
qualitative manner(12) (eg: differences in glycoflavone profiles). Another effect
appears to be reduction in the amount of branching, which occurs frequently not
universally(30). Seeds of most tetraploids are larger than those of diploids;
however the percent germination may be variable being lower in some (eg: Lycopersicum
esculentum) or higher in others (Oryza punctata). The leaves and
other appendages of polyploids in general seem to be more thicker, shorter and
broader(26). Polyploidy also seems to affect incompatibility relationships of
some self–incompatible species. Any incompatibility in the diploid parents such
as in Raphanus and Brassica seems to be carried down equally to
the autotetraploid also(29).
A more thorough treatment of these effects can be found in12,13 and 26.
Some of the crop plants cited by Stebbins (1950), which
are polyploids, include potato, coffee, banana, peanut, tobacco, wheat, oats,
sugarcane, plum, loganberry and strawberry (26). After observing the polyploid
nature of different crop plants and also the bigger size and vigour of the same
in many cases, plant breeders started getting interested in the artificial
induction of polyploids. Though this was initially marred by the lack of a
suitable method for the induction of polyploidy, the discovery of the “colchcine
method” served to remove this hurdle to a large extent (1). Polyploids are now
found in alsike clover , rey, turnip, dill, spinach, apple, radish, grapes,
zinnias, sugarbeets, petunia, Datura, Polulus, tea, watermelons, various
forage grasses etc. to name just a few (1). It must be pointed out that in many
of the seed crops polyploids produced had lower fertility rates than their diploid prototypes also in general
there is an optimum range of polyploidy beyond which growth may be depressed
with increasing chromosome numbers
(1,33). But in plants and flowers where seed cost are minor compared to the
beauty of the flowers or other valuable features of other plant parts reduced
fertility may not be a critical factor(1).
Conclusion:
Polyploidy is an
exciting phenomenon with intriguingly, a lot of practical potential. As
mentioned earlier it has been an area of interest right from the eighteenth
century and continues to be an area of interest as seen from the increasing
number of publications being generated continually on various aspects of
polyploidy over the last decade(14).Technological advancement and various
molecular tools(such as GISH, RFLP, flow cytometry etc.) seem to be helping us
gain better insights into various aspects of polyploidy in plants and animals
and also help reshape many of the traditionally held tenets. While it is
suggested by some that that certain factors such as the high rates of polyploid
occurrence in plants, the absence of ethical issues etc. may make plants very
good models to elucidate the molecular mechanisms of polyploid formation (11)
yet others have suggested a ‘cross-disciplinary’ approach involving both plants
and animals in looking into the origin and establishment of polyploids lineages
as a more efficient strategy(14).
Many discoveries
of the past in conjunction with the current information being generated seem to
be setting the stage for a new series of questions such as the extent of
geneflow in polyploid lineages of separate origin, the rapidity of genome
restructuring, its prevalence in neoployploids and the like.
Many of our crop
plants are polyploids and a number of them have and are being synthetically
produced. Yet, it has been impossible to predict with certainity, which diploid
genomes when merged will coexist stably. Therefore transforming the present
‘hit and miss’ process into a more exact science will form an area of focus in
future. Also a more detailed analysis of gene expression changes in many of the
newly formed polyploids could help uncover the impact of new variation on
polyploid evolution. An understanding the timing and frequency of changes in
many of the newly formed polyploids along with the mechanisms involved in
diploidization(10)may also facilitate gain a more thorough insight into the
evolutionary impact of rapid genome changes in neoployploids along with
strategies possible for manipulation.
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