Data from: Sex differences in lifespan: females homozygous for the X chromosome do not suffer the shorter lifespan predicted by the unguarded X hypothesis

Lifespan differs between the sexes in many species. Three hypotheses to explain this interesting pattern have been proposed, involving different drivers: sexual selection, asymmetrical inheritance of cytoplasmic genomes, and hemizygosity of the X(Z) chromosome (the unguarded X hypothesis). Of these, the unguarded X has received the least experimental attention. This hypothesis suggests that the heterogametic sex suffers a shortened lifespan because recessive deleterious alleles on its single X(Z) chromosome are expressed unconditionally. In Drosophila melanogaster, the X chromosome is unusually large (~20% of the genome), providing a powerful model for evaluating theories involving the X. Here, we test the unguarded X hypothesis by forcing D. melanogaster females from a laboratory population to express recessive X-linked alleles to the same degree as males, using females exclusively made homozygous for the X chromosome. We find no evidence for reduced lifespan or egg-to-adult viability due to X homozygozity. In contrast, males and females homozygous for an autosome both suffer similar, significant reductions in those traits. The logic of the unguarded X hypothesis is indisputable, but our results suggest that the degree to which recessive deleterious X-linked alleles depress performance in the heterogametic sex appears too small to explain general sex differences in lifespan.

诸多物种中，两性的寿命均存在显著差异。针对这一有趣现象，学界已提出三种基于不同驱动机制的假说：性选择、细胞质基因组的不对称遗传，以及X(Z)染色体的半合子状态（unguarded X hypothesis，未受保护X染色体假说）。其中，未受保护X染色体假说得到的实验关注最少。该假说提出，异配性别寿命更短的原因在于：其仅携带一条X(Z)染色体，其上的隐性有害等位基因会无条件表达。在黑腹果蝇（Drosophila melanogaster）中，X染色体体型异常庞大（约占基因组的20%），为验证涉及X染色体的相关理论提供了极具说服力的实验模型。本研究通过将实验室种群的黑腹果蝇雌性个体仅针对X染色体构建为纯合状态，使其与雄性个体同等程度地表达隐性X连锁等位基因，以此检验未受保护X染色体假说。我们未发现因X染色体纯合导致寿命缩短或卵至成虫存活率下降的相关证据。与之相反，常染色体纯合的雌雄个体均出现了相似且显著的上述性状衰退。未受保护X染色体假说的逻辑无可辩驳，但本研究结果显示，隐性有害X连锁等位基因对异配性别生存表现的抑制程度，似乎不足以解释两性寿命的普遍差异。