Identifying the Sex Determination Genes in Asparagus

Garden asparagus (Asparagus officinalis) is a dioecious species with a recently evolved, homomorphic sex chromosome pair which makes it ideal for studying the earliest events in sex chromosome evolution. A proposed evolutionary path from hermaphroditism to dioecy and a sex chromosome pair would involve the origin of a Y chromsome through cessation of recombination between a suppressor of female function and a promoter of male function.  We have explored this hypothesis in garden asparagus by genetically mapping sex determination to a small (<2Mb) non-recombining region on the proto-Y chromosome that actively differentiates males (XY) from females (XX). We have identified two independent male-to-hermaphrodite mutants that implicate a single gene in this non-recombining region on the Y as responsible for dominantly interrupting pistil development. Anther development is not affected in these mutants. To identify the gene(s) responsible for anther sterility in XX females, we assessed gene expression profiles in male and female spears and identified known anther development genes that exhibit male-biased expression.  A small subset of these genes do indeed map to sex linked scaffolds in the asparagus genome assembly, but their role in gender determination is still under investigation. Nonetheless, by identifying a female suppressor gene that does not influence anther development, these results support the hypothesis that at least two genes are necessary in the conversion of an autosome to a sex chromosome pair.