Work

The tortured past and present of young polymorphic neo-sex chromosomes of the mountain pine beetle

Year: 2026

Type: article

Abstract: Neo-sex chromosomes have proven powerful for studying the early stages of sex chromosome evolution and differentiation, yet if these dynamic processes could contribute to the emergence of new species remains unclear. The mountain pine beetle (Dendroctonus ponderosae), a major forest pest, is suspected of harboring neo-Y diversity that contributes to cryptic species boundaries. Here, we generated six chromosome-level assemblies (male and female from the three neo-Y populations) to examine the evolutionary processes contributing to differences between, and within, the neo-X and neo-Y chromosomes. Using comparative genomics approaches, we find the large sex chromosomes formed through sequential fusions with three large autosomes, with recombination cessation occurring between the neo-X and neo-Y at multiple distinct time points (∼4.3 to ∼8.6 MYA). Comparative analyses reveal that while the neo-X chromosomes remain largely collinear across populations, only ∼65% of the highly rearranged neo-Y can be aligned to the neo-X. We find dramatic structural divergence between the three neo-Ys which likely contributes to the extensive degeneration despite only moderate accumulation of repeats. All populations retain a single pseudoautosomal region (PAR), though PAR size and gene content also vary due to neo-Y specific rearrangements. We document a pattern of rapid, ongoing, and variable gene loss across neo-Y chromosomes in the mountain pine beetle. Some genes undergoing degeneration are associated with early-stage reproductive isolation phenotypes previously characterized in crossing experiments. Together, these results highlight how rapid genome restructuring through chromosomal fusions and Y chromosome degeneration can occur at evolutionary time scales relevant to the evolution of new species.

Source: Evolution Letters

DOI: 10.1093/evlett/qrag040