Breeding of Vegetable, Spice and Tuber crops (2+1)

Application of Autopolyploidy in Crop Improvement

• Triploids. Triploids are produced by hybridization between tetraploid and diploid strains. They are generally highly sterile, except in a few cases. This feature is useful in the production of seedless watermelons. In certain species, they may be more vigorous than the normal diploids, e.g.in sugarbeets.


• Seedless watermelons are grown commercially in Japan. They are produced by crossing tetraploid (4x, used as female) and diploid (2x, used as male) lines, since the reciprocal cross (2x x 4x) is not successful. The triploid plants do not produce true seeds; almost all the seeds are small, white rudimentary structures like cucumber (Cucumis sativus) seeds. But a few normal sized seeds may occur, which are generally empty. For good fruit setting, pollination is essential. For this purpose, diploid lines are planted in the ratio 1 diploid; 5 triploid plants. There are several problems, viz, genetic instability of 4x lines, irregular fruit shape, a tendency towards hollowness of fruits, production of empty seeds and the labour involved in triploid seed production (by hand-pollination).


• Triploid sugarbeets (B.vulgaris) produce larger roots and more sugar per unit area than do diploids, while tetraploids produce smaller roots and lower yields than diploids. Apparently, 3x is the optimum level of ploidy in sugarbeets. Triploid sugarbeet varieties have been grown commercially in Europe and Japan, but their popularity is declining rapidly. The triploid varieties are mixtures of triploid, diploid and other ploidy level plants. Seed production of triploid sugarbeet is difficult because the beet flower is small. Triploid seed may be produced in one of the following two ways: (1) using 4x plants as female and 2x as male or (2) using 4x as male and 2x as female. The first combination gives lower seed yield but a higher proportion of triploids, while the second gives higher seed yield but a lower proportion of triploids. Commercial triploid sugarbeet seed is produced by interplanting 4x and 2x lines in the ratio 3:1, and seed from both 4x and 2x plants is harvested. This seed consists of about 75% triploid (3x) seeds. Triploids sugarbeet may give 10-15 per cent higher yields than diploids.


• Pusa Giant Berseem is the first autopolyploid variety released for general cultivation in India. It yields 20-30 per cent more green fodder than the diploid berseem varieties.
1. Autoployploidy is more likely to succeed in species with lower chromosome numbers
than in those with higher chromosome numbers.
2. Cross-pollinating species are generally more responsive than self pollinating species.
3. Crops grown for vegetative parts are more likely to succeed as polyploids than those
grown for seeds.


Application in Crop Improvement

1. Aneuploids are useful in the studies on effects of loss or gain of an entire chromosome or a chromosome arm on the phenotype of an individual.
2. Aneuploids are useful in locating a linkage group and a gene to a particular chromosome. By using a secondary or tertiary trisomic, the gene may be located to one of the two arms of a chromosome, or even to a part of the chromosome arm. The most important application of aneuploids is in locating genes on particular chromosomes; this will be considered in some detail.
3. Aneuploids are useful in identifying the chromosome involved in translocations.
4. They are useful in the production of substitution lines. Chromosome substitution may be desirable for studying the effects of individual chromosomes of a variety or for the transfer of the genes carried by specific chromosomes or a variety into another one