Polygenic barriers to sex chromosome turnover

Pavitra Muralidhar
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Abstract

In organisms with distinct sexes, sex is commonly determined by segregation of a pair of chromosomes in paternal meiosis (male heterogamety) or maternal meiosis (female heterogamety). Evolutionary transitions between sex-chromosome systems—which change the chromosome pair that determines sex, the system of heterogamety, or both—are frequent in some clades. In others, however, sex chromosome systems show long-term stability. Previous explanations of this stasis rely on evolutionary dynamics peculiar to sex linkage, such as the accumulation of deleterious mutations on the sex-specific chromosome or sexually antagonistic mutations on either sex chromosome. Here, I show that the ordinary operation of stabilizing selection on quantitative traits can, as a by-product, promote stability of sex chromosome systems. The reason is that stabilizing selection on a trait permits individual chromosomes' genetic contributions to the trait to drift upwards or downwards, subject only to the constraint that they all sum to the trait optimum. This chromosome-specific drift causes reduced fitness in individuals with unusual chromosomal constitutions, such as the novel sexual genotypes that sex chromosome turnovers always produce. The theory can explain a suite of patterns in the phylogenetic distribution of sex determining systems. Applied to interspecific hybrids, it can also explain two key observations concerning their reduced fitness: Haldane's rule and the large-X effect.
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性染色体更替的多基因障碍
在具有不同性别的生物中,性别通常是由一对染色体在父系减数分裂(雄性异配)或母系减数分裂(雌性异配)中的分离决定的。性染色体系统之间的进化转换--改变决定性别的染色体对、异配系统或两者--在某些支系中很常见。然而,在其他支系中,性染色体系统表现出长期的稳定性。以前对这种稳定性的解释依赖于性联结所特有的进化动态,如性特异性染色体上有害突变的积累或性染色体上任何一方的拮抗突变。在这里,我证明了对数量性状的稳定选择的普通操作可以作为副产品促进性染色体系统的稳定。原因在于,性状上的稳定选择允许单个染色体对性状的遗传贡献向上或向下漂移,唯一的限制条件是它们的总和必须达到性状的最佳值。这种染色体特异性漂移会导致染色体结构异常的个体的适应性降低,例如性染色体翻转总是会产生新的性基因型。该理论可以解释性别决定系统在系统发育分布中的一系列模式。将该理论应用于种间杂交种,还可以解释有关其适应性降低的两个关键观察结果:霍尔丹规则和大 X 效应。
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