Evolution of multiple sex-chromosomes associated with dynamic genome reshuffling in Leptidea wood-white butterflies

Sex chromosome systems tend to be highly conserved and knowledge about their evolution typically comes from macroevolutionary inferences. Rapidly evolving complex sex chromosome systems represent a rare opportunity to study the mechanisms of sex chromosome evolution at unprecedented resolution. Three cryptic species of wood white butterflies – Leptidea juvernica, L. sinapis, and L. reali – have each a unique set of multiple sex chromosomes with 3–4 W and 3–4 Z chromosomes. Using a transcriptome-based microarray for comparative genomic hybridization (array-CGH) and a library of bacterial artificial chromosome (BAC) clones, both developed in L. juvernica, we identified Z-linked Leptidea orthologs of Bombyx mori genes and mapped them by fluorescence in situ hybridization (FISH) with BAC probes on multiple Z chromosomes. In all three species, we determined synteny blocks of autosomal origin and reconstructed the evolution of multiple sex chromosomes. In addition, we identified W-homologs of Z-linked orthologs and characterized their molecular differentiation. Our results suggest that the multiple sex chromosome system evolved in a common ancestor of these three Leptidea species as a result of dynamic genome reshuffling through repeated rearrangements between the sex chromosomes and autosomes, including translocations and fissions. Thus, the sex chromosome turnover could not play a role in reproductive isolation between the Leptidea species studied. However, we suggest that subsequent species-specific rearrangements of multiple sex chromosomes, along with different rates of neo-W chromosome degeneration and significantly increased number of Z-linked genes could accelerate the accumulation of genetic incompatibilities between populations and promote their divergence resulting in speciation.

性染色体系统（sex chromosome systems）通常具有高度保守性，目前学界对其演化的认知大多源自宏观演化推断。快速演化的复杂型性染色体系统，为我们以空前分辨率探究性染色体演化机制提供了难得的研究契机。三种隐存的溪粉蝶属（Leptidea）物种——L. juvernica、L. sinapis与L. reali——各自拥有独特的多套性染色体组，包含3~4条W染色体与3~4条Z染色体。研究团队利用基于转录组开发的阵列比较基因组杂交（array-CGH）技术，以及一套源自L. juvernica的细菌人工染色体（bacterial artificial chromosome, BAC）克隆文库，鉴定出了家蚕（Bombyx mori）基因的溪粉蝶属Z连锁同源基因，并通过荧光原位杂交（fluorescence in situ hybridization, FISH）技术，以BAC探针将这些基因定位至多条Z染色体上。在这三个物种中，我们均确定了常染色体来源的共线性区块，并重构了多套性染色体的演化历程。此外，我们还鉴定出了Z连锁同源基因的W同源序列，并对其分子分化特征进行了系统表征。本研究结果表明，这三种溪粉蝶的共同祖先中，多套性染色体系统经由性染色体与常染色体间的反复重排（包括易位与染色体断裂）引发的动态基因组改组演化而来。因此，性染色体周转并未在本次研究的溪粉蝶物种间起到生殖隔离的作用。不过我们推测，后续发生的物种特异性多套性染色体重排，连同新形成的W染色体的差异化退化速率，以及Z连锁基因数量的显著增加，可能会加速种群间遗传不相容性的积累，并推动种群分化，最终促进物种形成。