Mapping of the genes for dioecism and monoecism in Spinacia oleracea L.: evidence that both genes are closely linked.

Spinach is basically a dioecious species, with occasional monoecious plants in some populations. Sexual dimorphism in dioecious spinach plants is controlled by an allelic pair termed X and Y located on the short arm of the longest chromosome (x = 6). Ten AFLP markers, closely linked to the X/Y locus, were identified using bulked segregant analysis, four of which were revealed to co-segregate with Y in the present mapping population. We mapped the AFLP markers and two known male-specific DNAs to a 13.4 cM region encompassing the locus. These markers will be the basis for positional cloning of the sex-determination gene. We also showed that a single, incompletely dominant gene is responsible for the highly staminate monoecious character. The gene was found to be located at a distance of 4.3 cM from microsatellite marker SO4, which mapped 1.6 cM from the X/Y locus. This indicates that the monoecious gene seems not to be allelic to but closely linked to the X/Y gene pair. SO4 will enable breeders to efficiently select highly male monoecious plants for preferential use as the pollen parent for hybrid seed production.