X-Y divergence of house fly (Musca domestica) proto-sex chromosomes follows distinct evolutionary trajectories despite residing in the same genome
Sex determination systems and sex chromosomes frequently differ between species. One cause of these differences is new sex determining genes that drive evolutionary turnover of sex chromosomes. At the earliest stages of this turnover, X and Y (or Z and W) chromosomes start out as nearly identical homologs, and they can diverge via chromosomal rearrangements (e.g., inversions) that suppress X-Y…
The house fly, Musca domestica, possesses six chromosomes that can all function as Y chromosomes, offering a unique opportunity to study sex chromosome evolution within a single genomic context. In this study, researchers generated haplotype-resolved chromosome assemblies from five strains of the house fly, each carrying a distinct Y chromosome: IM, IIM, IIIM, VM, and YM.
Their findings reveal instances of both canonical and non-canonical evolutionary trajectories in sex chromosome divergence. On one chromosome (IIM), an inversion was found to be linked to higher levels of X-Y divergence, yet this inversion did not encompass the male-determining region. Conversely, the other three sex chromosomes (IM, IIIM, and VM) exhibited substantial X-Y divergence across near their entire lengths, without any evidence of inversions.
The YM chromosome stood apart from the rest, characterized by significant Y-specific sequences and a male-determining gene located at the chromosome's end. Despite sharing a strong cytological resemblance to the Muller element F found in other fly species, YM and its X counterpart displayed a high degree of divergence. This study underscores the coexistence of diverse modes of sex chromosome evolution within a single genome, challenging the notion of a uniform evolutionary pattern.
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