Unraveling the Gender Specification Pathway in Garden Asparagus (Asparagus officinalis L.)

Garden asparagus (Asparagus officinalis) is a dioecious vegetable crop species with a recently evolved, homomorphic sex chromosome pair. A non-recombining region on the proto-Y chromosome actively differentiates males (XY) from females (XX). All floral buds initiate as hermaphrodites, and anthers degenerate in female flowers during tapetum development, whereas pistil development is arrested in a later stage in male flowers. This temporal separation of sterility events suggests that at least two sex determination genes are linked in a non-recombining region, which has been genetically mapped to a small (<2Mb) region on the sex chromosomes. Given that the highly repetitive nature of sex chromosomes complicates genome assembly, we take an alternative approach using transcriptome sequencing to identify sex-specific gene expression. Comparing RNA-Seq from female (XX), male (XY), and “supermale” (YY) spear tips, we found a large number of genes exhibiting significant upregulation in males. Among these were homologs of genes with known function in the anther development pathway. These results have allowed us to identify a developmental point in the anther development pathway that is interrupted in female flower development. Genes in the anther pathway have been mapped to genome scaffolds and found to be autosomal, suggesting that an upstream regulator many be responsible for the male-biased expression patterns. We are currently utilizing a Genotype-by-Sequencing approach to order and orient sex-linked and autosomal scaffolds derived from a 120X coverage draft genome assembly, which will greatly aid in the identification of genes mapping to the sex determination region.