Recurrent neo-sex chromosome evolution in kiwifruit.

Sex chromosome evolution is thought to be tightly associated with the acquisition and maintenance of sexual dimorphisms. Plant sex chromosomes have evolved independently in many lineages1,2 and can provide a powerful comparative framework to study this. We assembled and annotated genome sequences of three kiwifruit species (genus Actinidia) and uncovered recurrent sex chromosome turnovers in multiple lineages. Specifically, we observed structural evolution of the neo-Y chromosomes, which was driven via rapid bursts of transposable element insertions. Surprisingly, sexual dimorphisms were conserved in the different species studied, despite the fact that the partially sex-linked genes differ between them. Using gene editing in kiwifruit, we demonstrated that one of the two Y-chromosome-encoded sex-determining genes, Shy Girl, shows pleiotropic effects that can explain the conserved sexual dimorphisms. These plant sex chromosomes therefore maintain sexual dimorphisms through the conservation of a single gene, without a process involving interactions between separate sex-determining genes and genes for sexually dimorphic traits.

Two Y-chromosome-encoded genes determine sex in kiwifruit.

Dioecy, the presence of male and female individuals, has evolved independently in multiple flowering plant lineages1-3. Although theoretical models for the evolution of dioecy, such as the 'two-mutations' model, are well established4,5, little is known about the specific genes determining sex and their evolutionary history3. Kiwifruit, a major tree crop consumed worldwide, is a dioecious species. In kiwifruit we previously identified a Y-encoded sex-determinant candidate gene acting as the suppressor of feminization (SuF), named Shy Girl (SyGI)6. Here, we identify a second Y-encoded sex-determinant that we named Friendly Boy (FrBy), which exhibits strong expression in tapetal cells. Gene-editing and complementation analyses in Arabidopsis thaliana and Nicotiana tabacum indicated that FrBy acts for the maintenance of male (M) functions, independently of SyGI, and that these functions are conserved across angiosperm species. We further characterized the genomic architecture of the small (<1 megabase pairs (Mb)) male-specific region of the Y chromosome (MSY), which harbours only two genes expressed extensively in developing gynoecia and androecia, respectively: SyGI and FrBy. Re-sequencing of the genome of a natural hermaphrodite kiwifruit revealed that this individual is genetically male but carries deletion(s) of parts of the Y chromosome, including SyGI. Additionally, expression of FrBy in female kiwifruit resulted in hermaphrodite plants. These results clearly indicate that Y-encoded SyGI and FrBy act independently as the SuF and M factors in kiwifruit, respectively, and provide insight into not only the evolutionary path leading to a two-factor sex-determination system, but also a new breeding approach for dioecious species.

A Y-Encoded Suppressor of Feminization Arose via Lineage-Specific Duplication of a Cytokinin Response Regulator in Kiwifruit.

Dioecy, the presence of male and female flowers on distinct individuals, has evolved independently in multiple plant lineages, and the genes involved in this differential development are just starting to be uncovered in a few species. Here, we used genomic approaches to investigate this pathway in kiwifruits (genus Actinidia). Genome-wide cataloging of male-specific subsequences, combined with transcriptome analysis, led to the identification of a type-C cytokinin response regulator as a potential sex determinant gene in this genus. Functional transgenic analyses in two model systems, Arabidopsis thaliana and Nicotiana tabacum, indicated that this gene acts as a dominant suppressor of carpel development, prompting us to name it Shy Girl (SyGI). Evolutionary analyses in a panel of Actinidia species revealed that SyGI is located in the Y-specific region of the genome and probably arose from a lineage-specific gene duplication. Comparisons with the duplicated autosomal counterpart, and with orthologs from other angiosperms, suggest that the SyGI-specific duplication and subsequent evolution of cis-elements may have played a key role in the acquisition of separate sexes in this species.