Two Breeding Schemes Using 2n Gametes in Potato,Dihaploids,Doubled Colchicine,Inbreeding,Deleterious Genes,Interlocus Interactions

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Two Breeding Schemes Using 2n Gametes in Potato
In the above analytical method, where, improved dihaploids are doubled by colchicine, there is reduced vigour of dihaploids due to inbreeding and expression of deleterious genes. This leads to failure in recovery of sufficient vigour in the tetraploids on chromosome doubling, because vigour depended on intra-and interlocus interactions in multiple allelic condition in the autotetraploids. To overcome this problem, following two breeding schemes were suggested by Memdiburu and Peloquin(1977) : (i) 4x × 2x, where 4x is cultivar and 2x is a hybrid between dihaploid and a wild diploid species. The cross gives 4x progeny due to functioning of 2n pollen in the 2x parent. (ii) 2x × 2x, where both parents are hybrids derived from crosses between dihaploids and wild species.

The 4x progeny in this case is derived due to functioning of 2n eggs in one parent and 2n pollen in the other parent. Since, in both these scheme, 2x parent is not a dihaploids but a hybrid (haploid × species), it is apparent that the dihaploids are first used for enriching the germplasm at the 2x level and then the enriched 2x is used for its transfer to 4x ploidy level. Due to disomic inheritance, there are several advantages in using 2x hybrids derived from ‘2x haploid Tuberosum × 2x wild species’.

In the first of the two schemes discussed above, i.e., in 4x i.e., in 4x × 2x crosses, 2x wild species is used directly with 4x potato cultivars. Since the 4x cultivar is an adapted genotype, a cross with 2x wild species will lead to a loss of 50% of the adapted germplasm, which will be replaced by unreduced 2n gamete from 2x species. On the other hand the 2x hybrids as above have only 50% of germplasm from 2x wild species, so that the 4x progeny (derived from 4x cultivar × 2x hybrid) will have only 25% unadapted germplasm from the wild species.

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