Data from: The evolution of XY-recombination: sexually antagonistic selection versus deleterious mutation load

Recombination arrest between X and Y chromosomes, driven by sexually antagonistic genes, is expected to induce their progressive differentiation. However, in contrast to birds and mammals (which display the predicted pattern), most cold-blooded vertebrates have homomorphic sex chromosomes. Two main hypotheses have been proposed to account for this, namely high turnover rates of sex determining systems and occasional XY recombination. Using individual-based simulations, we formalize the evolution of XY recombination (here mediated by sex reversal; the “fountain-of-youth” model) under the contrasting forces of sexually antagonistic selection and deleterious mutations. The shift between the domains of elimination and accumulation occurs at much lower selection coefficients for the Y than for the X. In the absence of dosage compensation, mildly deleterious mutations accumulating on the Y depress male fitness, thereby providing incentives for XY recombination. Under our settings, this occurs via “demasculinization” of the Y, allowing recombination in XY (sex-reversed) females. As we also show, this generates a conflict with the X, which co-evolves to oppose sex reversal. The resulting rare events of XY sex reversal are enough to purge the Y from its load of deleterious mutations. Our results support the “fountain of youth” as a plausible mechanism to account for the maintenance of sex-chromosome homomorphy.

由性拮抗基因（sexually antagonistic genes）驱动的X与Y染色体间的重组抑制，本应引发二者逐步分化。然而，与呈现该预期模式的鸟类和哺乳类不同，绝大多数冷血脊椎动物拥有同形性染色体（homomorphic sex chromosomes）。目前已有两种主要假说被提出以解释这一现象：一是性别决定系统的高周转率，二是偶尔发生的XY重组。本研究采用基于个体的模拟（individual-based simulations）方法，在性拮抗选择与有害突变（deleterious mutations）的对立选择压力下，形式化阐释了XY重组的演化过程——此处XY重组由性反转介导，即“青春之泉”模型（fountain-of-youth model）。相较于X染色体，Y染色体在清除与积累的阈值区间发生转换时，所需的选择系数（selection coefficients）要低得多。在不存在剂量补偿（dosage compensation）的情况下，Y染色体上积累的轻度有害突变会降低雄性适合度（male fitness），从而为XY重组提供了演化动力。在本研究设定的条件下，这一过程通过Y染色体的去雄性化（demasculinization）实现，使得XY个体（性反转后的雌性）能够发生重组。正如本研究所示，这会与X染色体产生演化冲突——X染色体会协同演化以对抗性反转。由此产生的罕见XY性反转事件，足以清除Y染色体携带的有害突变负荷。本研究结果支持“青春之泉”假说可作为维持性染色体同形性的合理机制。