BREEDING FOR 2N-EGG PRODUCTION IN HAPLOID-X SPECIES 2X-POTATO HYBRIDS

The identification of diploid (2x), Tuberosum haploid x wild species hybrids (HS) which produce high frequencies of 2n eggs and have good tuber appearance is important for obtaining 4x progeny from 2x x 2x crosses in potato. More than 1700 HS clones were screened for the occurrence and frequency of 2n eggs through 2x x 4x crosses during two pollination periods. The resulting seed/fruit provides an estimate of 2n egg frequency. About 650 HS clones produced 2n eggs; 63 had 10-114 seeds/fruit; and 27 were selected for good tuber type. There was a large variation in seed set after 2x x 4x crosses, both within and between clones. Comparison of fruit and seed set between the two pollination periods using the same 595 clones indicated that environmental factor significantly affected 2n egg frequency. The mode of 2n egg formation was mainly due to omission of the second meiotic division controlled by a recessive gene (os). The gene frequency for os varied from 0.28 to 0.76 among six, 2x taxa. Both additive and dominance variance were significant for seed set after 2x x 4x crosses. Narrow sense heritability was estimated as 0.24. The average degree of dominance was equal to 1.66 which could indicate overdominance. Alternatively, this could be due to linkage disequilibrium, i.e. pseudo-overdominance. However, dominance variance was greater than the additive variance indicating recurrent selection can be useful for improving the frequency of 2n egg production in the 2x population. The selected individuals, based on their phenotypic performance, should be progeny tested to identify the best parents for use in the next cycle of recombination.