The mechanisms underlying sex determination are astonishingly plastic. Particularly the triggers for the molecular machinery, which recalls either the male or female developmental program, are highly variable and have evolved independently and repeatedly. Fish show a huge variety of sex determination systems, including both genetic and environmental triggers. The advent of sex chromosomes is assumed to stabilize genetic sex determination. However, because sex chromosomes are notoriously cluttered with repetitive DNA and pseudogenes, the study of their evolution is hampered. Here we reconstruct the birth of a Y chromosome present in the Atlantic herring. The region is tiny (230 kb) and contains only three intact genes. The candidate male-determining gene BMPR1BBY encodes a truncated form of a BMP1B receptor, which originated by gene duplication and translocation and underwent rapid protein evolution. BMPR1BBY phosphorylates SMADs in the absence of ligand and thus has the potential to induce testis formation. The Y region also contains two genes encoding subunits of the sperm-specific Ca²⁺ channel CatSper required for male fertility. The herring Y chromosome conforms with a characteristic feature of many sex chromosomes, namely, suppressed recombination between a sex-determining factor and genes that are beneficial for the given sex. However, the herring Y differs from other sex chromosomes in that suppression of recombination is restricted to an ∼500-kb region harboring the male-specific and sex-associated regions. As a consequence, any degeneration on the herring Y chromosome is restricted to those genes located in the small region affected by suppressed recombination.

性别确定的基础机制令人惊讶。特别是，引起男性或女性发展计划的分子机制的触发因素是高度可变的，并且独立且反复地发展。鱼显示出各种各样的性别决定系统，包括遗传和环境触发因素。假定性染色体的出现可以稳定遗传性别的确定。然而，由于性染色体以重复的DNA和假基因而臭名昭著，因此对其进化的研究受到了阻碍。在这里，我们重建了大西洋鲱鱼中存在的Y染色体的诞生。该区域很小（230 kb），仅包含三个完整基因。候选男性决定基因BMPR1BBY编码BMP1B受体的截短形式，该形式由基因复制和易位引起，并经历了快速的蛋白质进化。BMPR1BBY在不存在配体的情况下使SMAD磷酸化，因此具有诱导睾丸形成的潜力。Y区还包含两个编码精子特异性Ca ²⁺亚基的基因男性生育所需的CatSper通道。鲱鱼Y染色体符合许多性染色体的特征，即，性别决定因素与对特定性别有益的基因之间的重组受到抑制。但是，鲱鱼Y与其他性染色体的不同之处在于，对重组的抑制仅限于一个包含男性特异性和性别相关区域的〜500kb区域。结果，鲱鱼Y染色体上的任何变性都限于位于受抑制重组影响的小区域中的那些基因。