The following points highlight the three main types of allopolyploidy. The types are: 1. Segmental Allopolyploidy 2. Genome Allopolyploidy 3. Auto-Allopolyploidy.
Type # 1. Segmental Allopolyploidy:
Some degree of homology (partial homology) may exist between some chromosome of one genome and those of the other genome. Therefore, in such polyploids both bivalents and multivalents are formed. This type of chromosome pairing is called heterogenetic pairing or allosyndetic pairing.
Segmental allopolyploids show more or less extensive segregation. The products may be sterile due to genetic imbalance. Segmental allopolyploids may show tetrasomic inheritance for genes on some chromosomes, while disomic inheritance for genes on other chromosomes. Therefore, the disomic and tetrasomic inheritance in the polyploid are evidences of segmental allopolyploidy.
It is believed that a single population has differentiated into two or more different species through a process beginning with geographical isolation. During the course of evolution, the geographically isolated populations of a species would have accumulated different genetic and chromosomal changes so that this species, say B, gave rise to two different species, say B1 and B2.
Hybridization between the species B1 and B2, followed by chromosome doubling of the resulting hybrid, will produce a segmental allopolyploid (B1B1B2B2) (Fig. 17.12).
Solarium tuberosum (4x = 48) and Delphinium gypsophilum are examples of segmental allopolyploids. Schulz-Schaeffer et al. in 1963 suggested the existence of segmental alloployploidy in the crested wheat grass complex (Agropyron spp.). They also suggested that reciprocal translocations have caused the changes in genomes in this complex.
Type # 2. Genome Allopolyploidy:
Genome allopolyploidy is the state where two or more distinct genomes are brought together through hybridization between two or more species followed by doubling of the chromosomes of the resulting hybrid. When two diploid species are crossed, the F1 hybrid contains a single copy of the genomes of the two parental species.
The F1 is often completely sterile because of irregular chromosome distribution at Al due to lack of pairing between the chromosomes of the different genomes. Chromosome doubling of the F1 hybrid produces an ailotetraploid which has two copies of each of the two parental genomes; as a result bivalents are formed in the ailotetraploid due to pairing between homologous chromosomes (autosyndesis or homo-genetic pairing) which makes it fertile.
Such a plant is generally called an amphidiploid or di-diploid or double diploid. Allopolyploidy occurs frequently in nature, and it can be experimentally produced. Most of the allopolyploid species are tetraploid, hexaploid or octaploid (Table 17.7).
One famous example of allopolyploidy occurs in the sub-tribe Trichinae. Genus Triticum has a polyploid series consisting of diploid (2*), tetraploid (4r) and hexaploid (6x) species. The common bread wheat (Triticum aestivum L.) is an allohexaploid which possesses 3 genomes A, B and D.
Genome analyses have shown that these three genomes were donated by three different diploid species, Triticum monococcum (2x = 14, AA), an unknown species (2x = 14, BB) and T. tauschii (2n = 14, DD), through hybridization and subsequent doubling of chromosome. The most widely accepted mode of origin of T. aestivum is depicted in Fig. 17.13.
During evolution divergent forms arise from a single parental population which gives rise to different genera and species. These species may again get hybridized and new polyploids originate. It has been postulated that wheat’s have arisen through the evolutionary processes as shown in Fig. 17.14.
Other examples of allopolyploidy:
Tobacco (Nicotiana tabacum) (In = 4x = 48) is an allotetraploid (amphidiploid) between two diploid wild species. Genome analysis proved that parental species of N. tabacum are N. sylvestris (2x = 24) and N. tomentosa, (Ix = 24). Crosses of both the parental species with N. tabacum, viz., N. tabacum (x N. sylvestris and N. tabacum x N. tomentosa, showed the presence of 12II + 12I during meiosis. When both the wild species were crossed together (N. sylvestris x N. tomentosa) the F1 plant showed 24I and were sterile.
Chromosome doubling of such F1 plants yielded fertile plants with 48 chromosomes, which showed 24II during meiosis and were quite similar in morphology to N. tabacum. The other cultivated tobacco, N. rustica (2x = 4x = 48), is also an amphidiploid. Genome analysis have revealed the N. rustica to be derived from the two wild species N. paniculata and N. undulata.
Cotton belongs to the genus Gossypium which contains 33 diploid (2x = 26) and 6 ailotetraploid (2n = Ax = 52) possess. Of these, two diploids, namely, G. herbaceum and G. arboreum, and two allotetraploids, viz., G. hirsutum and G. barbadense, are cultivated for their fibre and seed, whereas the other species are wild. Among the diploids, 21 are Old World species, and 12 and New World species.
All the 6 ailotetraploid species belong to the New World.
The Old World diploid species have large chromosomes, while the New World diploid species have large chromosomes. The genomes of Old World species have been designated as A, B, C, E, F and G, whereas the genomes of New World species are designated as D. The 6 ailotetraploid species (2n = 4x = 52) possess 13 pairs of large and 13 pairs of small chromosomes.
Hybrids between Asiatic species (AA) and the American allotetraploid species (AADD) showed 13II of large chromosomes plus 13I of small chromosomes, indicating that the large chromosomes of Asiatic species are homologous to the large chromosomes of the allotetraploids.
Similarly, small chromosomes (D) of the New World species were found to be homologous to the small chromosomes (D) of the allotetraploids. Thus the tetraploid cotton species are amphidiploids of Asiatic diploid cotton and New World diploid cotton (Table 17.7). The most probable origin of the allotetraploid species G. hirsutum is shown in Fig. 17.15.
There are two theories regarding the origin of tetraploid cotton in the New World. One hypothesis assumes that amphidiploids originated at the time when areas of distribution of the two parental forms overlapped, prior to the drifting of the supercontinent Pangea, or they originated in the late Cretaceous or early.
Tertiary period in Polynesian Island by migration of G. arboreum type across a trans-Pacific land bridge. The other hypothesis assumes that G. arboreum was introduced into the American continent by an early civilization cross Pacific and hybridization occurred between the cultivated Asiatic diploid and wild American diploid species.
It is considered that A-genome species originated from a progenitor D-genome species through macroevolution. Hybridization and polyploidy occurred within a short geological time period, prior to or at the time of separation of continents during the Mesozoic era. Later, the diploidization of the gene regulatory system of the hybrids formed different amphidiploid species.
The genus Brassica belongs to the family Cruciferae and consists of both diploid and polyploid species. There are three elementary species, namely, B. nigra, (2n = 16; BB), B. oleracea (2n = 18; CC) and B. campestris (2n = 20; AA).
Chromosome pairing studies have revealed that these species of Brassica evolved from some ancestral species with “x = 6”, though the process of dysploidy. Dysploidy may be defined as the condition where basic chromosome number differs within a population or species. The three elementary species gave rise to amphidiploids (allotetraploids) through natural hybridization and subsequent chromosome doubling.
The ailotetraploid species are Brassica carinata (2n = 34, BBCC), B. juncea (2n = 36, AABB), and B. napus (2n; = 38, AACC). Cytogenetic relationships among different Brassica species were established by Nagaharu U, and is represented as the famous “U-triangle” after his name (Fig. 17.16).
Experimental allopolyploids have been synthesized in the (Jrucifers through interspecific and inter-generic crosses. Synthetic B. carinata, B. juncea and B. napus have been produced by crossing the concerned parental species and chromosome doubling of the resulting hybrids. Other allopolyploids synthesized are: naporapa (In = 58, AAAACC), napoleracea (In = 56, AACCCC), napocampestris (2n = 58, AAAACC) etc.
Brassica napocampestris (2n = 58, AAAACC):
It is an auto-allohexaploid obtained from the cross between B. napus ssp. raifpera (2n = 38, AACC) and B. campestris (2n = 20, AA). It was initially synthesized by Frandsen and Winge in 1932. Meiosis shows various types of associations of chromosomes, such as, hexavalents, quadrivalents, trivalents and bivalents. Lagging chromosomes are also observed at Al. B. napocampestris is intermediate between the parents regarding leaf and inflorescence characteristics. Fertility is low but the plant has promise as fodder crop.
Inter-generic allopolyploids using Raphanus and Brassica have been successfully synthesized as described below.
Raphanobrassica is the first artificial fertile allopolyploid obtained from the cross between radish, Raplianus sativus (2n = 18, RR) and cabbage, B. oleracea (2n = 18, CC). It was produced by Karpechenko in 1928. This plant (2n = 4x = 36, RRCC) showed 18II with occasional 17II+2I during meiosis. In some cases, occasional quadrivalents, trivalents and univalents were also observed.
However, the fertility of this ailotetraploid is low due to genetic and chromosomal imbalances. The main aim of synthesis of Raphanobrassica is to combine the vigour, high quality and disease resistance of radish with high productivity of cabbage; it is likely to succeed as a fodder crop.
Brassicoraphanus (2n = 38, AARR) is the amphidiploid produced through hybridization of B. campestris ssp. chinensis (2n = 20, AA) with Raphanus sativus (2n = 18, RR); it was originally synthesized by Terasawa in 1932. Fertility is low in these polyploids, but they can be utilized as fodder.
Triticale (X Triticosecale Wittmack):
Triticale (X Triticosecole Wittmack) is the artificially synthesized allopolyploid between wheat and rye. It is regarded as the first man-made cereal of commercial value. Since 1930, it has progressed from the cereal of botanical importance to the cereal of agronomical importance during 1980s. An excellent review on the various aspects of triticales has been presented by Gupta and Priyadarshan in 1982.
Triticale has been synthesized at various ploidy levels, such as, tetraploid, hexaploid, octaploid and decaploid (Fig. 17.17).
Allohexaploid triticales (2n = 6x = 42) are synthesized by crossing tetraploid wheat’s with Secale cereale followed by doubling of the chromosomes of the hybrid (Fig. 17-17). The octaploid triticales (2n = 8x = 56) are produced through hybridization between hexaploid wheat and rye (Fig. 17.17).
The decaploid triticales (2n = 10x = 70) were originally synthesized by Muntzing in 1955 by crossing octaploid triticales with rye, followed by chromosome doubling of the resulting hybrids (Fig. 17.17). Tetraploid triticales are produced by crossing hexaploid triticales (AABBRR) with diploid rye followed by selfing of the F1 hybrid (ABRR). The selling can give rise to tetraploid triticales having mixed genomes A and B (with variable number of chromosomes of each genome) and a genome of 7 pairs of rye chromosomes; it is represented as “(AB) (AB) RR”.
Tetraploid triticales have chromosomal stability and fertility but they are poor in agronomic performance. However, they can be used for the improvement of hexaploid and octaploid triticales. Hexaploid triticales possess some desirable characters such as, winter hardiness, early flowering and early maturity. Seeds have higher protein and lysine content as compared to hexaploid wheat.
However, baking quality is not as good as that of wheat. Octaploid triticales also have winter hardiness, early flowering and early maturity. They produce larger kernels with high protein content. Baking quality of octaploid triticales is good and they are cultivated in China.
Both hexaploid and octaploid triticales show superiority over wheat regarding their performance at high altitudes (2000 m and above). However, they show meiotic instability, partial sterility, shriveled kernels and pre-harvest sprouting in rainy weather.
Decaploid triticale strains show irregular meiosis leading to reversion to plants of lower chromosome numbers. They are poor in vigour and fertility and thus, they have no agronomic value.
Type # 3. Auto-Allopolyploidy:
This term was used by Kostoff in 1939 to denote a condition where an allopolyploid individual also shows the characteristics of auto-polyploidy for one or more genomes. If there are two genomes, e.g. A and B, the auto-allopolyploid may be AAAABB, AABBBB or AAAABBBB. Thus auto-allopolyploidy is possible from the level of hexaploidy (6x) and above.
Decaploid triticale (AABBDDRRRR) is an example of auto-allopolyploidy; this plant has two copies each of the three genome of bread wheat (ABD) and four copies of that of rye genome (R). Thus the rye genome is in autotetraploid condition. Some of the artificially synthesized allopolyploids in Brassica are also auto-allopolyploids, such as, B. napocampestris, (2x = 58, AAAACC) and B. napoleracea (2n = 56, AACCCC).