Department of Biology, Texas A&M University, College Station, TX, United States.
Interdisciplinary Program in Genetics and Genomics, Texas A&M University, College Station, TX, United States.
J Hered. 2024 Aug 20;115(5):524-531. doi: 10.1093/jhered/esae025.
Chromosome number is a fundamental genomic trait that is often the first recorded characteristic of a genome. Across large clades, a common pattern emerges: many or even most lineages exhibit relative stasis, while a handful of lineages or species exhibit striking variation. Despite recent developments in comparative methods, most of this heterogeneity is still poorly understood. It is essential to understand why some lineages have rapid rates of chromosome number evolution, as it can impact a variety of other traits. Previous research suggests that biased female meiotic drive may shape rates of karyotype evolution in some mammals. However, Carnivora exhibits variation that this female meiotic drive model cannot explain. We hypothesize that variation in effective population size may underlie rate variation in Carnivora. To test this hypothesis, we estimated rates of fusions and fissions while accounting for range size, which we use as a proxy for effective population size. We reason fusions and fissions are deleterious or underdominant and that only in lineages with small range sizes will these changes be able to fix due to genetic drift. In this study, we find that the rates of fusions and fissions are elevated in taxa with small range sizes relative to those with large range sizes. Based on these findings, we conclude that 1) naturally occurring structural mutations that change chromosome number are underdominant or mildly deleterious, and 2) when population sizes are small, structural rearrangements may play an important role in speciation and reduction in gene flow among populations.
染色体数目是一个基本的基因组特征，通常是记录基因组的第一个特征。在大的进化枝中，出现了一个常见的模式：许多甚至大多数谱系表现出相对的稳定性，而少数谱系或物种表现出惊人的变化。尽管最近在比较方法方面取得了进展，但大多数这种异质性仍然理解得很差。了解为什么一些谱系的染色体数目进化速度很快是至关重要的，因为这可能会影响到许多其他特征。先前的研究表明，雌性减数分裂驱动的偏倚可能会影响某些哺乳动物的核型进化速度。然而，食肉目表现出的变异是这种雌性减数分裂驱动模型无法解释的。我们假设有效种群大小的变化可能是导致食肉目进化速度变化的原因。为了检验这一假设，我们在考虑范围大小（作为有效种群大小的替代指标）的情况下，估计了融合和裂变的速率。我们认为融合和裂变是有害的或亚显性的，只有在范围较小的谱系中，由于遗传漂变，这些变化才能固定下来。在这项研究中，我们发现融合和裂变的速率在范围较小的类群中比在范围较大的类群中更高。基于这些发现，我们得出结论：1）自然发生的改变染色体数目的结构突变是亚显性的或轻度有害的，2）当种群规模较小时，结构重排可能在物种形成和种群间基因流动减少中发挥重要作用。
