The sex chromosomes often follow unusual evolutionary trajectories. In particular, the sex-limited chromosomes frequently exhibit a small but unusual gene content in numerous species, where many genes have undergone massive gene amplification. The reasons for this remain elusive with a number of recent studies implicating meiotic drive, sperm competition, genetic drift, and gene conversion in the expansion of gene families. However, our understanding is primarily based on Y chromosome studies as few studies have systematically tested for copy number variation on W chromosomes. Here, we conduct a comprehensive investigation into the abundance, variability, and evolution of ampliconic genes on the avian W. First, we quantified gene copy number and variability across the duck W chromosome. We find a limited number of gene families as well as conservation in W-linked gene copy number across duck breeds, indicating that gene amplification may not be such a general feature of sex chromosome evolution as Y studies would initially suggest. Next, we investigated the evolution of HINTW, a prominent ampliconic gene family hypothesized to play a role in female reproduction and oogenesis. In particular, we investigated the factors driving the expansion of HINTW using contrasts between modern chicken and duck breeds selected for different female-specific selection regimes and their wild ancestors. Although we find the potential for selection related to fecundity in explaining small-scale gene amplification of HINTW in the chicken, purifying selection seems to be the dominant mode of evolution in the duck. Together, this challenges the assumption that HINTW is key for female fecundity across the avian phylogeny.

中文翻译：

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禽W染色体上的多拷贝基因家族进化

性染色体通常遵循不寻常的进化轨迹。特别是，在许多物种中，性别限制染色体经常表现出少量但不寻常的基因含量，其中许多基因都经历了大规模的基因扩增。其原因仍然难以捉摸，最近的一些研究涉及减数分裂驱动、精子竞争、遗传漂变和基因家族扩展中的基因转换。然而，我们的理解主要基于 Y 染色体研究，因为很少有研究系统地测试 W 染色体上的拷贝数变异。在这里，我们对禽 W 上扩增基因的丰度、变异性和进化进行了全面调查。首先，我们量化了鸭 W 染色体上的基因拷贝数和变异性。我们发现有限数量的基因家族以及在鸭品种中 W 连锁基因拷贝数的保守性，表明基因扩增可能不像 Y 研究最初表明的那样是性染色体进化的一般特征。接下来，我们研究了 HINTW 的进化，这是一个突出的扩增基因家族，被假设在女性生殖和卵子发生中发挥作用。特别是，我们使用为不同女性特异性选择制度选择的现代鸡和鸭品种与其野生祖先之间的对比，研究了推动 HINTW 扩张的因素。虽然我们在解释鸡中 HINTW 的小规模基因扩增时发现了与繁殖力相关的选择潜力，但纯化选择似乎是鸭子进化的主要模式。一起，