Department of Biology, Lund University, Lund, Sweden.
Science for Life Laboratory, Department of Cell and Molecular Biology, Uppsala University, Uppsala, Sweden.
PLoS Genet. 2023 Jun 30;19(6):e1010801. doi: 10.1371/journal.pgen.1010801. eCollection 2023 Jun.
Sex chromosomes have evolved repeatedly across the tree of life and often exhibit extreme size dimorphism due to genetic degeneration of the sex-limited chromosome (e.g. the W chromosome of some birds and Y chromosome of mammals). However, in some lineages, ancient sex-limited chromosomes have escaped degeneration. Here, we study the evolutionary maintenance of sex chromosomes in the ostrich (Struthio camelus), where the W remains 65% the size of the Z chromosome, despite being more than 100 million years old. Using genome-wide resequencing data, we show that the population scaled recombination rate of the pseudoautosomal region (PAR) is higher than similar sized autosomes and is correlated with pedigree-based recombination rate in the heterogametic females, but not homogametic males. Genetic variation within the sex-linked region (SLR) (π = 0.001) was significantly lower than in the PAR, consistent with recombination cessation. Conversely, genetic variation across the PAR (π = 0.0016) was similar to that of autosomes and dependent on local recombination rates, GC content and to a lesser extent, gene density. In particular, the region close to the SLR was as genetically diverse as autosomes, likely due to high recombination rates around the PAR boundary restricting genetic linkage with the SLR to only ~50Kb. The potential for alleles with antagonistic fitness effects in males and females to drive chromosome degeneration is therefore limited. While some regions of the PAR had divergent male-female allele frequencies, suggestive of sexually antagonistic alleles, coalescent simulations showed this was broadly consistent with neutral genetic processes. Our results indicate that the degeneration of the large and ancient sex chromosomes of the ostrich may have been slowed by high recombination in the female PAR, reducing the scope for the accumulation of sexually antagonistic variation to generate selection for recombination cessation.
性染色体在生命之树上反复进化，由于遗传退化，性限制染色体（例如某些鸟类的 W 染色体和哺乳动物的 Y 染色体）通常表现出极端的大小二态性。然而，在某些谱系中，古老的性限制染色体逃脱了退化。在这里，我们研究了鸵鸟（Struthio camelus）中性染色体的进化维持，其中 W 染色体的大小仍然是 Z 染色体的 65%，尽管它已经有超过 1 亿年的历史了。使用全基因组重测序数据，我们表明，假常染色体区域（PAR）的群体规模重组率高于相似大小的常染色体，并且与异配子雌性中的系谱重组率相关，但与同配子雄性无关。性连锁区域（SLR）内的遗传变异（π = 0.001）明显低于 PAR，这与重组停止一致。相反，PAR 内的遗传变异（π = 0.0016）与常染色体相似，并且取决于局部重组率、GC 含量以及在较小程度上取决于基因密度。特别是，靠近 SLR 的区域与常染色体一样具有遗传多样性，这可能是由于 PAR 边界周围的高重组率将遗传连锁限制在 SLR 只有约 50Kb 以内。因此，雄性和雌性中具有拮抗适应度效应的等位基因驱动染色体退化的潜力是有限的。虽然 PAR 的一些区域具有不同的雌雄等位基因频率，提示存在性拮抗等位基因，但合并模拟表明，这广泛上与中性遗传过程一致。我们的研究结果表明，鸵鸟大而古老的性染色体的退化可能由于雌性 PAR 中的高重组而减缓，减少了积累性拮抗变异的范围，从而产生了对重组停止的选择。
