Ancient U/V sex chromosomes reveal slow expansion of recombination suppression in Polytrichaceae mosses.

Ancient U/V sex chromosomes in Polytrichaceae mosses originated about 124 million years ago, yet their non-recombining SDRs show limited expansion and retain hundreds of genes.

• Why it matters: Haploid sex chromosomes are less studied than diploid systems, and Polytrichaceae U/V chromosomes let researchers test how long-term recombination suppression shapes genomes without diploid heterogamety confounds.
• What they did: Using chromosome-scale genomes from ten Polytrichaceae species, the study identified homologous U and V chromosomes and compared SDR boundaries, gene content, and signatures of recombination suppression.
• The result: SDRs show strong recombination-suppression signatures (e.g., reduced gene density and diversity, transposable-element accumulation, high synonymous divergence) but limited functional degeneration, implying recombination arrest alone did not expand SDRs extensively.

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