Evolution of Sex-Linked Regions in the Genome of Dioecious Phoenix Species

Dioecy in plants, equivalent to gonochory in animals, is characterized by male (staminate) and female (pistillate) functions on different individuals. Dioecy may have evolved several times in plants from bisexual (hermaphroditic or monoecious) ancestors. In the dioecious date palm (Phoenix dactylifera), the female individuals produce dates, and are thus the more valuable sex. Gender-linked markers have therefore been sought for decades to precociously identify the sex of date palm plants before reproductive age. We found three sex-linked microsatellite markers genetically linked that are heterozygous only in males. Male-specific alleles allowed us to identify the gender in 100% of individuals and provided genetic evidences of the existence of an XY chromosomal system with a non recombining XY-like region in the date palm genome. We investigated the evolution of suppressed recombination within the genus Phoenix as a whole by extending the analysis of P. dactylifera sex-related loci to six other species within the genus. The data indicate that, in the Phoenix species, sex chromosomes are at least partly differentiated and that the Y chromosome has started degenerating. This situation is consistent with the probable ancient origin of dioecy in the genus Phoenix, which could therefore possess one of the most ancient sex chromosomes encountered in angiosperms.