Highly conserved Z and molecularly diverged W chromosomes in the fish genus Triportheus (Characiformes, Triportheidae)

The main objectives of this study were to test: (1) whether the W-chromosome differentiation matches to species’ evolutionary divergence (phylogenetic concordance) and (2) whether sex chromosomes share a common ancestor within a congeneric group. The monophyletic genus Triportheus (Characiformes, Triportheidae) was the model group for this study. All species in this genus so far analyzed have ZW sex chromosome system, where the Z is always the largest chromosome of the karyotype, whereas the W chromosome is highly variable ranging from almost homomorphic to highly heteromorphic. We applied conventional and molecular cytogenetic approaches including C-banding, ribosomal DNA mapping, comparative genomic hybridization (CGH) and cross-species whole chromosome painting (WCP) to test our questions. We developed Z- and W-chromosome paints from T. auritus for cross-species WCP and performed CGH in a representative species (T. signatus) to decipher level of homologies and rates of differentiation of W chromosomes. Our study revealed that the ZW sex chromosome system had a common origin, showing highly conserved Z chromosomes and remarkably divergent W chromosomes. Notably, the W chromosomes have evolved to different shapes and sequence contents within ~15–25 Myr of divergence time. Such differentiation highlights a dynamic process of W-chromosome evolution within congeneric species of Triportheus.

本研究的核心研究目标为验证两项假说：其一，W染色体的分化模式是否与物种的进化分化（系统发育一致性，phylogenetic concordance）相匹配；其二，同属类群内的性染色体是否拥有共同的祖先。本研究选取单系类群三鳍脂鲤属（Triportheus，脂鲤目，三鳍脂鲤科）作为研究模型。截至目前，该属所有经分析的物种均具有ZW型性染色体系统：Z染色体始终为核型（karyotype）中最大的染色体，而W染色体则呈现高度多态性，形态从近乎同形到高度异形不等。本研究采用常规与分子细胞遗传学技术手段，包括C带染色、核糖体DNA定位、比较基因组杂交（comparative genomic hybridization, CGH）以及跨物种全染色体涂染（cross-species whole chromosome painting, WCP），以验证前述研究问题。我们从耳状三鳍脂鲤（T. auritus）中制备了Z、W染色体涂染探针，用于跨物种WCP实验，并在代表性物种西格纳斯三鳍脂鲤（T. signatus）中开展了CGH实验，以此解析W染色体的同源性水平与分化速率。本研究结果显示，ZW型性染色体系统具有共同起源：Z染色体高度保守，而W染色体则呈现显著的分化差异。值得注意的是，在约1500万至2500万年的分化时间内，W染色体已演化出各异的形态与序列组成。这类分化现象凸显了三鳍脂鲤属同属物种W染色体演化的动态过程。