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The relatively young and repeated evolutionary origins of dioecy (separate sexes) in flowering plants enable investigation of molecular dynamics occurring at the earliest stages of sex chromosome evolution. With two independently young origins of dioecy in the genus,Asparagusis a model taxon for studying genetic sex-determination and sex chromosome evolution. Dioecy first evolved inAsparagus~3-4 million years ago (Ma) in the ancestor of a now widespread Eurasian clade that includes garden asparagus (Asparagus officinalis), while the second origin occurred in a smaller, geographically restricted, Mediterranean Basin clade includingAsparagus horridus. The XY sex chromosomes and sex-determination genes in garden asparagus have been well characterized, but the genetics underlying dioecy in the Mediterranean Basin clade are unknown. We generated new haplotype-resolved reference genomes for garden asparagus andA. horridus, to elucidate the sex chromosomes ofA. horridusand explore how dioecy evolved between these two closely related lineages. Analysis of theA. horridusgenome revealed an independently evolved XY system derived from different ancestral autosomes (chromosome 3) with different sex-determining genes than documented for garden asparagus (on chromosome 1). We estimate that proto-XY chromosomes evolved around 1-2 Ma in the Mediterranean Basin clade, following an ~2.1-megabase inversion between the ancestral pair. Recombination suppression and LTR retrotransposon accumulation drove the establishment and expansion of the Y-linked sex-determination region (Y-SDR) that now reaches ~9.6-megabases inA. horridus. The new garden asparagus genome revealed a Y-SDR that spans ~1.9-megabases with ten hemizygous genes. Our results evoke hemizygosity as the most probable mechanism responsible for the origin of proto-XY recombination suppression in the Eurasian clade, and that neofunctionalization of one duplicated gene (SOFF) drove the origin of dioecy. These findings support previous inference based on phylogeographic analysis revealing two recent origins of dioecy inAsparagus. Moreover, this work implicates alternative molecular mechanisms for two separate shifts to dioecy in a model taxon important for investigating young sex chromosome evolution. 

Abstract Model species continue to underpin groundbreaking plant science research. At the same time, the phylogenetic resolution of the land plant Tree of Life continues to improve. The intersection of these two research paths creates a unique opportunity to further extend the usefulness of model species across larger taxonomic groups. Here we promote the utility of the Arabidopsis thaliana model species, especially the ability to connect its genetic and functional resources, to species across the entire Brassicales order. We focus on the utility of using genomics and phylogenomics to bridge the evolution and diversification of several traits across the Brassicales to the resources in Arabidopsis, thereby extending scope from a model species by establishing a “model clade”. These Brassicales-wide traits are discussed in the context of both the model species Arabidopsis thaliana and the family Brassicaceae. We promote the utility of such a “model clade” and make suggestions for building global networks to support future studies in the model order Brassicales.