Institut National de Recherche Agronomique (INRA), Unité Mixte de Recherche 1099, Biology of Organisms and Populations Applied to Plant Protection, Le Rheu, France.
Mol Biol Evol. 2012 Feb;29(2):837-47. doi: 10.1093/molbev/msr252. Epub 2011 Oct 13.
Sex chromosomes play a role in many important biological processes, including sex determination, genomic conflicts, imprinting, and speciation. In particular, they exhibit several unusual properties such as inheritance pattern, hemizygosity, and reduced recombination, which influence their response to evolutionary factors (e.g., drift, selection, and demography). Here, we examine the evolutionary forces driving X chromosome evolution in aphids, an XO system where females are homozygous (XX) and males are hemizygous (X0) at sex chromosomes. We show by simulations that the unusual mode of transmission of the X chromosome in aphids, coupled with cyclical parthenogenesis, results in similar effective population sizes and predicted levels of genetic diversity for X chromosomes and autosomes under neutral evolution. These results contrast with expectations from standard XX/XY or XX/X0 systems (where the effective population size of the X is three-fourths that of autosomes) and have deep consequences for aphid X chromosome evolution. We then localized 52 microsatellite markers on the X and 351 on autosomes. We genotyped 167 individuals with 356 of these loci and found similar levels of allelic richness on the X and on the autosomes, as predicted by our simulations. In contrast, we detected higher dN and dN/dS ratio for X-linked genes compared with autosomal genes, a pattern compatible with either positive or relaxed selection. Given that both types of chromosomes have similar effective population sizes and that the single copy of the X chromosome of male aphids exposes its recessive genes to selection, some degree of positive selection seems to best explain the higher rates of evolution of X-linked genes. Overall, this study highlights the particular relevance of aphids to study the evolutionary factors driving sex chromosomes and genome evolution.
性染色体在许多重要的生物学过程中发挥作用，包括性别决定、基因组冲突、印迹和物种形成。特别是，它们表现出一些不寻常的特性，如遗传模式、半合子和重组减少，这影响了它们对进化因素（如漂变、选择和人口统计学）的反应。在这里，我们研究了驱动蚜虫性染色体进化的进化力量，蚜虫是一种 XO 系统，其中雌性是纯合的（XX），雄性是半合子（X0）。我们通过模拟表明，蚜虫中 X 染色体的不寻常传递模式，加上周期性的孤雌生殖，导致 X 染色体和常染色体在中性进化下具有相似的有效种群大小和预测的遗传多样性水平。这些结果与标准的 XX/XY 或 XX/X0 系统的预期（其中 X 的有效种群大小是常染色体的四分之三）形成对比，并对蚜虫 X 染色体的进化产生深远影响。然后，我们将 52 个微卫星标记定位在 X 染色体上，351 个标记定位在常染色体上。我们对 167 个个体的 356 个位点进行了基因分型，发现 X 染色体和常染色体上的等位基因丰富度与模拟预测的相似。相比之下，我们检测到 X 连锁基因的 dN 和 dN/dS 比值高于常染色体基因，这种模式与正选择或放松选择都兼容。鉴于两种类型的染色体具有相似的有效种群大小，并且雄性蚜虫的 X 染色体的单一拷贝使其隐性基因暴露于选择之下，某种程度的正选择似乎最能解释 X 连锁基因更高的进化率。总的来说，这项研究强调了蚜虫在研究驱动性染色体和基因组进化的进化因素方面的特殊重要性。
