Reduced recombination in potato, causing sterility or uniform 2n gametes, is associated with mutant StMSH4 alleles

During meiosis homologous chromosomes should be separated in a balanced way to ensure that each gamete will receive a complete chromosomal set. Desynapsis is a failure to maintain chromosome pairing until metaphase I because of a limited number of crossing-over, which leads to unbalanced chromosome segregation and the formation of sterile gametes. However, when desynapsis is combined with another meiotic mutation that restitutes the first meriotic division, then highly heterozygous fertile 2n gametes are produced. Such a modified meiosis, which approximates mitosis in its ability to maintain parental heterozygosity, is of great interest to breeders. Desynapsis inheritance was described as recessive, and the trait was tentatively mapped to chromosome 8 in a historical diploid S. tuberosum bi-parent population. We generated from the same cross, a large population segregating for desynapsis and 2n pollen production and, genotyped it using a skim-sequencing approach. During two growing seasons, desynapsis was assessed by combining microscopic observations on stainability and diameter of pollen grains indicative of their viability and ploidy. The trait was then mapped to a region of chromosome 8 colocalizing with the ortholog of AtMSH4, involved in the type I crossing over pathway. A seven base-pair insertion in the second exon of StMSH4 was identified in the allele associated with desynapsis in this population. An identical allele, apart from a 3,819 bp insertion in place of the seven bp one, was identified in clone RH89-039-16. Complementation test performed between StMSH4 alleles with the long and the short insertion resulted in desynapsis indicting that both alleles are non-functional.