Development of tetraploid pure lines in potato toward heterosis breeding

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Potato is a highly heterotic tetrasomic tetraploid. Highly homozygous inbreds are useful for investigation of heterosis. In this study, applicability of S locus inhibitor gene (Sli) to develop highly homozygous diploid lines of potato and the process of inbreeding were investigated using molecular markers.The Sli gene was introduced by crossing into 32 diploid genotypes and its expression in the F_1 and S_1 progenies was investigated. F_1 hybrids with S- or A-type chloroplast DNA produced S_1 seeds (67.1% of self-pollinated F_1 plants), and by second selfing 44.2% of S_1 plants produced S_2 seeds, whereas those with W- or T-type chloroplast DNA failed to produce S_1 seeds due to cytoplasmic male sterility.A diploid clone 97H32-6 and a hybrid between 97H32-6 and a superior diploid clone produced two S_4 families and three S_5 families, respectively. The average heterozygosity levels measured using RFLP and AFLP markers decreased from the original 100% in S_0 to 10.7% in S_4 and to 8.6% … More in S_5, respectively. The heterozygosity reduction rates per generation were 38.4 and 38.5%, respectively, which were lower than the theoretically expected 50%. However, none of loci or chromosome sections was exclusively heterozygous in the advanced selfed-progenies. Thus, highly homozygous, and seed-propagated diploid potatoes could be obtained by repeated selfing using the Sli gene.Fifty-one plants of an S_3 family were evaluated for fertility-related traits and heterozygosity. At least nine loci on eight chromosome sections were associated with fertility-related quantitative trait loci (QTLs), in which both QTLs performing better in the heterozygotes and in the homozygotes were found for each fertility-related trait. By further continued selfing, QTLs performing better in heterozygotes were likely fixed to homozygotes with secondly better performance, and those performing better in homozygotes were fixed to the best genotypes, although self-fertility continued to decrease as selfed generations proceeded. Less