IFM Biology, AVIAN Behavioural Genomics and Physiology Group, Linköping University, Linköping, Sweden.
Evolution. 2018 Mar;72(3):568-577. doi: 10.1111/evo.13434. Epub 2018 Feb 12.
Life span 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 life span 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 life span 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 life span.
许多物种的寿命存在性别差异。为了解释这一有趣的现象，提出了三种假说，涉及不同的驱动因素：性选择、细胞质基因组的不对称遗传和 X(Z)染色体的半合性（无保护的 X 假说）。其中，无保护的 X 假说受到的实验关注最少。该假说认为，异配性别寿命缩短是因为其单个 X(Z)染色体上的隐性有害等位基因无条件表达。在黑腹果蝇中，X 染色体异常大（约占基因组的 20%），为评估涉及 X 染色体的理论提供了强大的模型。在这里，我们通过仅使 X 染色体纯合的雌性果蝇来表达隐性 X 连锁等位基因，从而迫使来自实验室种群的雌性果蝇以与雄性相同的程度表达，以此来检验无保护的 X 假说。我们没有发现 X 染色体纯合导致寿命或卵至成虫活力降低的证据。相比之下，常染色体纯合的雄性和雌性都表现出相似的、显著的这些性状的减少。无保护的 X 假说的逻辑无可争议，但我们的结果表明，隐性有害 X 连锁等位基因对异配性别表现的抑制程度太小，无法解释寿命方面的一般性别差异。
