POSTMEIOTIC RESTITUTION IN 2N-EGG FORMATION OF DIPLOID POTATO

Four diploid (2n = 2x = 24) interspecific F-1 hybrids of tuberous Sola num species were tested for the modes of origin of 2n-eggs. The four h ybrids were heterozygous for the genetic marker amylose-free starch (A mf/amf) on chromosome 8. By crossing these hybrids with tetraploid S. tuberosum parents (2n = 4x = 48) that were nulliplex for this marker, i.e. Amf/amf x amf/amf/amf/amf crosses, tetraploid progenies were gene rated and classified for starch phenotypes of microspores. Based on th e segregation of the amf marker gene, the tetraploid progenies were cl assified into nulliplex, simplex and duplex genotypes. In the progenie s of three F-1 hybrids, the simplex genotypes predominated and the ori gin of 2n-eggs could be explained as the result of second division res titution (SDR). But in the progeny of one F-1 hybrid (a S. microdontum x S. tuberosum hybrid), there were only nulliplex and duplex genotype s, indicating complete homozygosity of the 2n-eggs (Amf/Amf or amf/amf ) for this locus. In order to genotype the 2n-eggs also for other loci on the same chromosome, a tetraploid progeny was generated from a cro ss between this hybrid and a tetraploid S. tuberosum parent (Tetra 4) and analysed for four RFLP loci on chromosome 8. This analysis showed that all 2n-eggs of the S. microdontum x S. tuberosum hybrid were homo zygous for all four loci, as was observed for the amf locus. From the same analysis it was evident that crossovers had occurred between the two genomes of this F-1 hybrid. These homozygous and recombinant genot ypes indicated that the 2n-eggs had originated from the doubling of th e chromosome number in the normal haploid products of meiosis. Followi ng the terminology of first and second division restitution (FDR and S DR), this new mode of origin of 2n-eggs in diploid potato is called po st-meiotic restitution (PMR).