Heterologous transition and the evolution of a new sex chromosome in swordtail fish

In a more than 30 year evolutionary experiment we tested the genomic consequences of hybridisation and selection between fish with different sex chromosome systems. This experiment mimicked an evolutionary scenario, hybridisation with repeated backcrossing, that has likely contributed to the evolution of two freshwater fish species, Xiphophorus clemenciae and X. monticolus. We find that introgression and imposing selection for pigmentation phenotypes on backcrossed individuals results in the retention of an unexpectedly large maternally derived genomic region. Our results show that during the hybridisation process the sex-determining region of the X chromosome from one parental species (the platyfish, X. maculatus) was translocated to an autosome in the hybrids (with X. hellerii) and has led to the evolution of a new sex chromosome, which acts as a W. Our results highlight the complexity of factors contributing to patterns observed in the hybrid genomes, and for the first time experimentally demonstrate that hybridisation can catalyze rapid evolution of genome architecture.

本研究开展了一项历时三十余年的演化实验，探究了携带不同性染色体系统的鱼类之间的杂交与选择所引发的基因组效应。本实验模拟了一种可能推动了两种淡水鱼类——剑尾鱼（Xiphophorus clemenciae）与山栖剑尾鱼（X. monticolus）——演化的场景：即杂交后进行多次回交。我们的研究发现，在回交个体中施加基因渐渗（introgression）与色素沉着表型选择后，会意外保留一块规模超出预期的母源基因组区域。研究结果显示，在杂交过程中，其中一个亲本物种（花斑剑尾鱼，X. maculatus）的X染色体性别决定区域易位至与霍氏剑尾鱼（X. hellerii）杂交产生的杂种的常染色体上，并由此演化出一条新的性染色体，其功能等效于W染色体。本研究结果凸显了杂种基因组观测模式背后驱动因素的复杂性，并首次通过实验证实，杂交能够推动基因组架构发生快速演化。