MOE Laboratory of Biosystems Homeostasis & Protection, Life Sciences Institute, Zhejiang University, Hangzhou, China.
Department of Molecular Evolution and Development, University of Vienna, Vienna, Austria.
Nat Ecol Evol. 2019 May;3(5):834-844. doi: 10.1038/s41559-019-0850-1. Epub 2019 Apr 1.
Songbirds have a species number close to that of mammals and are classic models for studying speciation and sexual selection. Sex chromosomes are hotspots of both processes, yet their evolutionary history in songbirds remains unclear. We characterized genomes of 11 songbird species, with 5 genomes of bird-of-paradise species. We conclude that songbird sex chromosomes have undergone four periods of recombination suppression before species radiation, producing a gradient of pairwise sequence divergence termed 'evolutionary strata'. The latest stratum was probably due to a songbird-specific burst of retrotransposon CR1-E1 elements at its boundary, instead of the chromosome inversion generally assumed for suppressing sex-linked recombination. The formation of evolutionary strata has reshaped the genomic architecture of both sex chromosomes. We find stepwise variations of Z-linked inversions, repeat and guanine-cytosine (GC) contents, as well as W-linked gene loss rate associated with the age of strata. A few W-linked genes have been preserved for their essential functions, indicated by higher and broader expression of lizard orthologues compared with those of other sex-linked genes. We also find a different degree of accelerated evolution of Z-linked genes versus autosomal genes among species, potentially reflecting diversified intensity of sexual selection. Our results uncover the dynamic evolutionary history of songbird sex chromosomes and provide insights into the mechanisms of recombination suppression.
鸣禽的物种数量接近哺乳动物，是研究物种形成和性选择的经典模式生物。性染色体是这两个过程的热点，但它们在鸣禽中的进化历史仍不清楚。我们对 11 种鸣禽物种的基因组进行了测序，其中包括 5 种天堂鸟物种。我们的研究结果表明，在物种辐射之前，鸣禽的性染色体经历了四次重组抑制期，产生了一个被称为“进化层”的序列差异梯度。最近的一个层可能是由于边界处的一个鸣禽特异性的 CR1-E1 逆转座子爆发，而不是通常认为的抑制性连锁重组的染色体倒位。进化层的形成重塑了性染色体的基因组结构。我们发现 Z 连锁倒位、重复和鸟嘌呤-胞嘧啶（GC）含量以及 W 连锁基因丢失率与层的年龄呈逐步变化。少数 W 连锁基因因其重要功能而得以保留，与其他性连锁基因相比，蜥蜴直系同源基因的表达更高且更广泛。我们还发现，在不同物种中，Z 连锁基因相对于常染色体基因的进化速度存在差异，这可能反映了性选择强度的多样化。我们的研究结果揭示了鸣禽性染色体的动态进化历史，并为重组抑制机制提供了新的见解。
