Department of Evolutionary Studies of Biosystems, SOKENDAI, The Graduate University for Advanced Studies, Hayama, Kanagawa 240-0193, Japan.
Genetics. 2022 Jan 4;220(1). doi: 10.1093/genetics/iyab204.
Muller's ratchet is a process in which deleterious mutations are fixed irreversibly in the absence of recombination. The degeneration of the Y chromosome, and the gradual loss of its genes, can be explained by Muller's ratchet. However, most theories consider single-copy genes, and may not be applicable to Y chromosomes, which have a number of duplicated genes in many species, which are probably undergoing concerted evolution by gene conversion. We developed a model of Muller's ratchet to explore the evolution of the Y chromosome. The model assumes a nonrecombining chromosome with both single-copy and duplicated genes. We used analytical and simulation approaches to obtain the rate of gene loss in this model, with special attention to the role of gene conversion. Homogenization by gene conversion makes both duplicated copies either mutated or intact. The former promotes the ratchet, and the latter retards, and we ask which of these counteracting forces dominates under which conditions. We found that the effect of gene conversion is complex, and depends upon the fitness effect of gene duplication. When duplication has no effect on fitness, gene conversion accelerates the ratchet of both single-copy and duplicated genes. If duplication has an additive fitness effect, the ratchet of single-copy genes is accelerated by gene duplication, regardless of the gene conversion rate, whereas gene conversion slows the degeneration of duplicated genes. Our results suggest that the evolution of the Y chromosome involves several parameters, including the fitness effect of gene duplication by increasing dosage and gene conversion rate.
穆勒棘轮是一种在没有重组的情况下，有害突变不可逆固定的过程。Y 染色体的退化和其基因的逐渐丢失，可以用穆勒棘轮来解释。然而，大多数理论考虑的是单拷贝基因，可能不适用于 Y 染色体，许多物种的 Y 染色体都有许多重复的基因，这些基因可能通过基因转换进行协同进化。我们开发了一个穆勒棘轮模型来探索 Y 染色体的进化。该模型假设一个不重组的染色体，既有单拷贝基因又有重复基因。我们使用分析和模拟方法来获得该模型中基因丢失的速率，特别关注基因转换的作用。基因转换的同质化使重复拷贝要么突变，要么完整。前者促进棘轮，后者则阻碍棘轮，我们想知道在什么条件下哪种对抗力量占主导地位。我们发现，基因转换的影响是复杂的，取决于基因重复的适合度效应。当重复对适合度没有影响时，基因转换会加速单拷贝和重复基因的棘轮效应。如果重复有附加的适合度效应，那么无论基因转换率如何，单拷贝基因的棘轮效应都会被基因重复所加速，而基因转换则会减缓重复基因的退化。我们的研究结果表明，Y 染色体的进化涉及几个参数，包括基因重复通过增加剂量和基因转换率对适合度的影响。
