Genetic strategies to determine the mode of 2n egg formation in diploid potatoes

Abstract

In this study, nineteen diploid potato clones (Solanum spp. 2n=2x=24) were identified as 2n egg producers on the basis of fruit set in 2x–4x crosses. The segregation of three genes mapped close to the centromere, Got-1 (1.1 cM), Pgm-2 (2.0 cM), and Sdh-1 (8.3 cM), were analyzed in the tetraploid offspring in these 2x–4x crosses to discriminate between First Division Restitution (FDR) and Second Division Restitution (SDR) modes of 2n egg formation. The co-dominant nature of these markers lead to more precise estimates of the recombinational frequencies as a result of completely classifying the segregating progenies. 2x–4x data revealed a predominance of SDR mechanisms occurring in 20 of the 21 families analyzed. With a SDR mode established, half-tetrad analysis (HTA) of four distal loci, 6-Pgdh-3, Mdh-1, Pgi-1, and Aps-1, revealed two SDR segregation patterns in some of the families. One pattern fit the expectations for the distal arm position. The gene-centromere map distances based upon SDR modes in the families following this pattern, were generally close to 4x−2x (FDR) estimates suggesting similar recombination rates between micro- and mega-sporogenesis. Heterozygosity transmission, on average, was 39.1%. In the other segregation pattern, in which the diploid parents were derived from S. chacoense PI 230580, higher than expected homozygosity levels were found in the female 2n gametophyte populations. A post-meiotic doubling of the reduced megaspore, which generates homozygous 2n eggs, is suggested to operate in three families. The common genetic background of the diploid clones suggested a heritable nature of this mechanism. Pooled data from these three deviant families calculated that 1.8% of the heterozygosity was transmitted to the tetraploid progeny.