Thread by @JacobPhD

New paper! A sex-determining DNA sequence in wild strawberries has repeatedly jumped to new genomic locations, leading to increasingly differentiated sex chromosomes. Here’s the summary (thread).
journals.plos.org/plosbiology/ar…

Some wild strawberry species have distinct sexes (male left, female right), but their recent ancestors and close relatives (including the🍓we eat) do not. This setup lets us study how and why a perfectly good hermaphrodite evolves to stop making both gametes. Photos: Gerry Carr.

Using classical genetic mapping, @dr_tialynn has led years of research showing that different genomic locations can control sex. But, since these polyploid strawberry plants have eight similar copies of the baseline genome, it’s tough to characterize any single chromosome.

So we asked: is any DNA unique to females across these various populations with different sex chromsomes? Yes! All females all share a short sequence, never seen in males, containing two genes. Males have no analogously unique DNA. Thus, this gene cassette must determine sex.

This evolutionary tree shows how these sex-determining cassettes are related. Each clade (α, β, and γ) is associated with a different genomic location (1Mb, 13Mb, and 37Mb). The sequence matches position 1Mb, presumably the ancestral location. But then it moved. At least twice.

The number of female-specific genes increases with each jump, as “souvenirs” from past locations are brought along. Thus, these jumps lead directly to greater divergence between sex chromosomes, which in other species (like human X and Y) can eventually become extreme.

Is this process favored by natural selection? Or just random rearrangement of the genome? That remains to be seen, but we speculate that it could be adaptive by locking genes into tight association with sex, allowing for more sex-specific traits.

This study is a cumulation of a huge amount of work, mostly by people who are not me: my coauthors, but also numerous colleagues and members of @AaronListon's and @dr_tialynn’s labs who laid the groundwork. I’m honored to have been part of such a fantastic team.