Postmeiotic restitution in 2n-egg formation of diploid potato

Abstract

Four diploid (2n=2x=24) interspecific F1 hybrids of tuberous Solanum species were tested for the modes of origin of 2n-eggs. The four hybrids were heterozygous for the genetic marker amylose-free starch (Amf/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×amf/amf/amf/amf crosses, tetraploid progenies were generated and classified for starch phenotypes of microspores. Based on the segregation of the amf marker gene, the tetraploid progenies were classified into nulliplex, simplex and duplex genotypes. In the progenies of three F1 hybrids, the simplex genotypes predominated and the origin of 2n-eggs could be explained as the result of second division restitution (SDR). But in the progeny of one F1 hybrid (a S. microdontum×S. tuberosum hybrid), there were only nulliplex and duplex genotypes, 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 cross 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×S. tuberosum hybrid were homozygous 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 F1 hybrid. These homozygous and recombinant genotypes indicated that the 2n-eggs had originated from the doubling of the chromosome number in the normal haploid products of meiosis. Following the terminology of first and second division restitution (FDR and SDR), this new mode of origin of 2n-eggs in diploid potato is called post-meiotic restitution (PMR).