Two Breeding Schemes Using 2n Gametes in Potato,Colchicine,Autotetraploids,Haploid Tuberosum,Potato Cultivars

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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 interaction in multiple allelic conditions in the autotetraploids.
To overcome this problem, following two breeding schemes were suggested by Mendiburu and Peloquin (1977): (i) 4 x x 2 x, where 4t is a cultivar and 2r is a hybrid between dihaploid and a wild diploid species.The cross gives 4 x progeny due to functioning of 2n pollen in the 2r parent. (ii) 2r x 2r, 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 schemes, 2r parent is not a dihaploid, but a hybrid (haploid x species), it is apparent that the dihaploids are first used for enriching the germplasm at- the 2x level and then for its transfer to 4 x ploidy level.
Due to disomic inheritance, there are several advantages in using 2 x hybrids derived from '2r haploid Tuberosum -x 2t: wild species' rather than crossing 2 x wild species directly with 4x potato cultivars. Since the 4x cultivar is an adapted genotype, a cross with 2 x wild species will lead to a loss of 50% of the adapted germplasm, which will be replaced by 2 n gamete from 2x species. On the other hand the 2x hybrids as above have only 50% of germplasm from 2x wild species, so that in crosses 4 x x 2 x (haploid x species), the 4t progeny (derived from 4 x cultivar x 2 x hybrid) will have only 25% unadapted germplasm from the wild species.