Polyploidy and Evolution of Species

The below mentioned article provides a close view on the Polyploidy and Evolution of Species.

The polyploidy has played an important role in evolution of new varieties and species in nature. Angiosperms and Pteridophytes have very high numbers of polyploid species in nature. More than 35 per cent angiosperms are polyploids.

According to Manton (1950), among all the plant groups, ferns show highest degree of polyploidy. It is generally noted that with the increase in chromosome number the adaptability and variabilities of species increase progressively.

Many mutations in diploids are deleterious or fatal, but in polyploids, where a gene is represented more than twice the chances of appearance of deleterious effects due to mutation are lesser.

Autopolyploidy has limited contribution in the evolution of plant species. Some of our present day crops, many forage grasses and several ornamental plants are polyploids.

Some of the autopolyploid crops are listed in following Table 23.6:

Allopolyploidy has played vital role in the evolution of species. A good number of crop species seem to have evolved through allopolyploidy. This is evident from the widespread occurrence of allopolyploids in various genera of plants and their high adaptability in nature.

It is estimated that about one-third of the flowering plants are polyploids and a great majority of them are allopolyploids. The importance of allopolyploidy in evolution of species was first pointed out by Winge in 1917. Subsequent studies have supported this contention.

Cytological studies in many species have made it possible to trace the evolutionary history of many allopolyploid species and their diploid parental species have been identified with some degree of certainty. The parental diploid species of allopolyploid is based primarily on the pairing between the chromosomes of the diploid species and allopolyploid species.

When the chromosomes of a diploid species pair with some of the chromosomes of the allopolyploid species, the homology between the chromosomes of two species is established this suggests that the diploid species may be one of the parent species of allopolyploid species.

But in some cases, owing to considerable chromosome differentiation in diploid and allopolyploid species in course of time, pairing between the chromosomes of the two species might be greatly reduced.

The identification of parental diploid species can also be made by involving the suspected parent species in hybridization and synthesizing an allopolyploid from the interspecific hybrid and then comparing the synthetic allopolyploid with naturally occurring allopolyploid. The synthetic allopolyploid does often resemble the natural allopolyploid species.

This has been demonstrated in Brassica in which a synthetic allopolyploid developed from the interspecific hybrid of cross B. oleracea (cabbage) x B campestris (rape) was found to resemble very closely with B. napus (turnip). However, the resemblance between natural and synthetic allopolyploids may not be close in all cases.

The other methods for determining the resemblance between the diploid parents and allopolyploid species are biochemical techniques, such as by electrophoretic analysis of their protein and enzyme patterns and from chromosome banding patterns.

It is now possible to reconstruct the probable method of origin of certain species that have arisen through natural hybridization and polyploidization. There are many cultivated allopolyploid species, such as wheat, tobacco, cotton and Brassica, whose possible evolutionary history can be inferred with high certainty.

Evolution of Bread Wheat:

The origin of bread wheat (Triticum aestivum) has been most extensively investigated. The common bread wheat (T. aestivum) is a hexaploid species (2 n =6x = 42) having three different sets of genomes (AABBDD) which seem to have been contributed by three different diploid species (Sears, Kihara, Percival etc.).

The species of wheat (Triticum) fall into the following three categories: diploids, tetraploids and hexaploids (Table 23.7).

It is generally believed that the genome A present in the diploid wheats is more or less the same as those present in tetraploid and hexaploid wheats and the genome B of tetraploid Emmer wheats is also more or less similar to genome B found in hexaploid wheats.

The pairing behaviour of chromosomes during meiosis in diploid, tetraploid and hexaploid wheats supports this. The chromosome pairing in triploid hybrids between diploid and tetraploid wheats results in 7 II (bivalents) and 7 II (univalents) associations.

The pentaploid hybrids (5n) resulted from a cross between tetraploid and hexaploid wheats show 14 IIs and 7 Is. Though these facts are generally accepted by the majority of cytogeneticists, yet there is no agreement on the point regarding the diploid parent species which contributed genomes A, B and D to hexaploid wheats.

It has been proposed that the source of A genome of common bread wheat is T. monococcum, that of B genome is Aegilops speltoides and that of D genome is Aegilops squarrosa.

The possible evolutionary history of hexaploid wheat is as follows:

The genome D present in bread wheat is supposed to have been derived from a grass Aegilops squarrosa (2n = 14) which grows in the region extending from Armenia to Afghanistan. The different varieties of wheat with higher chromosome numbers have apparently arisen from this type and other related grasses through hybridization followed by chromosome doubling.

J. Percival is of the opinion that bread wheat (T. aestivum) originated through hybridization between Emmer wheat (2n = 28 chromosomes) and goat grass (In = 14). Experimental results have been obtained by E.S. Mc. Fadden, Sears (1948) and Kihara that definitely established the origin of one type of bread wheat T. spelta.

These investigators doubled the chromosome numbers of Emmer wheat and goat grass by colchicine treatment in order to get diploid gametes from these two plant species. By crossing these two colchicine treated plants they found a fertile hybrid with 42 chromosomes (28 from Emmer wheat and 14 from goat grass).

The hybrid resembled bread wheat and was given name T. spelta. These researches showed how a moderately useful wheat and a useless grass were combined in nature to produce most valuable variety of wheat. The origin of wheat is still a matter of great speculation and is very confusing even today.

Evolution of Allopolyploid Species of Brassica:

The species of Brassica, show a good degree of variation in their somatic chromosome numbers (Table 23.8).

The possible origin of different polyploid species of Brassica has been illustrated in the famous species triangle proposed by N.U. (1935) (Fig. 23.11). According to this NU’s triangle, three species B. nigra, B. campestris and B. oleracea have contributed genomes for different amphidiploid species and represent the three tips of the triangle.

The amphidiploid species are presented midway between their parental species. B.juncea is an amphidiploid from a cross B.nigra (n = 8) x B. campestris (n = 10) (fig. 23.12), B.napus (n = 19) is an amphidiploid from the cross B.oleracea (n=9) and B. campestris (n=n10) and B. carinata (n= 17) is amphidiploid from a cross B.nigra (n= 8) x B.oleracea (n=9).

It has been experimentally possible to get synthetic fertile amphidiploids according to the above scheme which resembled natural amphidiploids.

Evolution of Cotton Species:

The genus Gossypium is represented by some wild and some cultivated species.

The chromosome numbers of some species have been presented in (Table 23.9):

G. hirsutum, G. barbadense, and G.tomentosum (n = 26) appear to have arisen through natural hybridization between the diploid species (n = 13) from old world or Asiatic cotton and New world or American cotton and subsequent chromosome doubling of interspecific hybrid.

A possible origin of G. hirsutum is from the cross G. arborium n = 13 (Asiatic cotton), x G. thurberi (n = 13) followed by chromosome doubling of the hybrid.

Evolution of Tobacco:

Tobacco (Nicotiana tabacum) (2n = 48) is probably an amphidiploid from a natural cross N. sylvestris (2n = 24) x N. tomentosa (2n = 24). This is evident from the chromosome behaviour during meiosis in the triploid hybrids of the crosses N. tabacum x N. sylvestris and N. tabacum x N. tomentosa which formed 12 IIs and 12 univalents during prophase I.

This indicates similarity between chromosomes of N. tabacum, N. sylvestris and N. tomentosa. Morphologically, the amphidiploids of the hybrids from cross between N. sylvestris and N. tomentosa are similar to N. tabacum in many respects.

The species N. tabacum has undergone considerable differentiation due to gene mutations and modification (possibly due to loss of some duplicated segments) in genomes of the two parents.