Data from: Rewinding the ratchet: Rare recombination locally rescues neo-W degeneration and generates plateaus of sex-chromosome divergence

Natural selection is less efficient in the absence of recombination. As a
result, non-recombining sequences, such as sex chromosomes, tend to
degenerate over time. Although the outcomes of recombination arrest are
typically observed after many millions of generations, recent neo-sex
chromosomes can give insight into the early stages of this process. Here
we investigate the evolution of neo-sex chromosomes in the Spanish marbled
white butterfly, Melanargia ines, where a Z-autosome fusion has turned the
homologous autosome into a non-recombining neo-W chromosome. We show that
these neo-sex chromosomes are likely limited to the Iberian population of
M. ines, and that they arose around the time when this population split
from North-African populations, around 1.5 million years ago.
Recombination arrest of the neo-W chromosome has led to an excess of
premature stop codons and frameshift mutations, and reduced gene
expression compared to the neo-Z chromosome. Surprisingly, we identified
two regions of 1 Mb at one end of the neo-W that are both less diverged
from the neo-Z and less degraded than the rest of the chromosome,
suggesting a history of rare but repeated genetic exchange between the two
neo-sex chromosomes. These plateaus of neo-sex chromosome divergence
suggest that neo-W degradation can be locally reversed by rare
recombination between neo-W and neo-Z chromosomes.