The evolution of a sex chromosome in asparagus
Harkess, Alex Edward
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The overwhelming majority of flowering plants reproduce through the production of hermaphroditic flowers. A small percentage of angiosperm species instead are dioecious, producing either male or female flowers on individual plants. Dioecy can be mediated at the molecular level by a sex chromosome that genetically differentiates males and females. Sex chromosomes evolve from autosomes, and this conversion is hypothesized to involve mutations in one or more linked genes that determine sex. Given the complexities of anther and ovule development, the full suite of sex determination genes has not been described for any dioecious plant. Here we explore the conversion from autosome to an XY sex chromosome using garden asparagus (Asparagus officinalis), an ideal model system for studying the earliest events in sex chromosome evolution given that it recently evolved a sex chromosome pair. Focusing first on broad trends, genomic characterization of several hermaphroditic and dioecious species across the Asparagus genus revealed an increase in retrotransposon content coincident with the evolution of dioecy. To identify putative sex determination genes on this Y chromosome, we then explore the timing of male and female sterility events in garden asparagus, hypothesizing that anther sterility in females likely occurs before pollen microsporogenesis. Finally, we assemble and annotate a high quality reference genome for garden asparagus, and perform a suite of mutant analyses to identify two genes in a non-recombining region of the Y that are ultimately responsible for sex determination.