Sex-linked gene expression and the reversion to hermaphroditism inCarica papayaL. (Caricaceae)

ABSTRACT One evolutionary path from hermaphroditism to dioecy is via a gynodioecious intermediate. The evolution of dioecy may also coincide with the formation of sex chromosomes that possess sex-determining loci that are physically linked in a region of suppressed recombination. Dioecious papaya ( Carica papaya ) has an XY chromosome system, where the presence of a Y chromosome determines males. However, in cultivation, papaya is gynodioecious, due to the conversion of the male Y chromosome to a hermaphroditic Y h chromosome during its domestication. We investigated gene expression linked to the X, Y, and Y h chromosomes at different floral developmental stages in order to identify differentially expressed genes (DEGs) that may be involved in the sexual reversion of males to hermaphrodites. We identified 309 sex-biased genes found on the sex chromosomes, most of which are found in the pseudoautosomal regions (PARs). Female (XX) expression in the sex determining region (SDR) was almost double that of X-linked expression in males (XY) and hermaphrodites (XY h ), which rules out dosage compensation for most sex-linked gene; although, an analysis of hemizygous X-linked loci found evidence of partial dosage compensation. Furthermore, we identified a potential candidate gene associated with both sex determination and the transition to hermaphroditism, a homolog of the MADS-box protein SHORT VEGETATIVE PHASE ( SVG ).