No abstract

Sex differences in ageing and lifespan are ubiquitous in nature. The "unguarded-X" hypothesis (UXh) suggests they may be partly due to the expression of recessive mutations in the hemizygous sex chromosomes of the heterogametic sex, which could help explain sex-specific ageing in a broad array of taxa. A prediction central to the UX hypothesis is that inbreeding will decrease the lifespan of the homogametic sex more than the heterogametic sex, because only in the former does inbreeding increase the expression of recessive deleterious mutations. In this study, we test this prediction by examining the effects of inbreeding on the lifespan and fitness of male and female Drosophila melanogaster across different social environments. We found that, across social environments, inbreeding resulted in a greater reduction of female than male lifespan, and that inbreeding effects on fitness did not seem to counterbalance sex-specific effects on lifespan, suggesting the former are maladaptative. Inter- and intra-sexual correlation analyses also allowed us to identify evidence of an underlying joint genetic architecture for inbreeding effects on lifespan. We discuss these results in light of the UXh and other alternative explanations, and suggest that more attention should be paid to the possibility that the "unguarded-X" may play an important role in the evolution of sex-specific lifespan.

26Sex-specific lifespans are ubiquitous across the tree of life and exhibit broad taxonomic 27 patterns that remain a puzzle, such as males living longer than females in birds and vice versa 28 in mammals. The prevailing "unguarded-X" hypothesis (UXh) explains this by differential 29 expression of recessive mutations in the X/Z chromosome of the heterogametic sex (e.g., 30females in birds and males in mammals), but has only received indirect support to date. An 31 alternative hypothesis is that the accumulation of deleterious mutations and repetitive 32 elements on the Y/W chromosome might lower the survival of the heterogametic sex ("toxic 33 Y" hypothesis). Here, we report lower survival of the heterogametic relative to the 34 homogametic sex across 138 species of birds, mammals, reptiles and amphibians, as expected 35 if sex chromosomes shape sex-specific lifespans. We then analysed bird and mammal 36 karyotypes and found that the relative sizes of the X and Z chromosomes are not associated 37 with sex-specific lifespans, contrary to UXh predictions. In contrast, we found that Y size 38 correlates negatively with male survival in mammals, where toxic Y effects are expected to be 39 particularly strong. This suggests that small Y chromosomes benefit male lifespans. Our 40 results confirm the role of sex chromosomes in explaining sex differences in lifespan, but 41indicate that, at least in mammals, this is better explained by "toxic Y" rather than UXh 42 effects. 43 44 45 46 47 48 MAIN 52Sexual dimorphism in lifespan is widespread across the tree of life, including among tetrapods 53[1]. Which sex lives longer varies considerably in both direction and magnitude. For example, 54 females live three times longer than males in the brown antechinus, a small marsupial, while 55 males are twice as likely as females to survive from one year to the next in Arabian babblers, 56 a passerine bird [2, 3]. In humans, the lifespan gap is estimated to be 4.8 years -female life 57 expectancy is 74.7 years while male life expectancy is 69.9 years [4]. Understanding the 58 dynamic nature of sex differences in lifespan across taxa remains an unsolved problem in 59 evolutionary biology. To date, empirical studies have focused on adaptive hypotheses 60 stemming from sex-specific differences in how natural selection operates on females and 61 males [1, 5-8]. Despite their prominent role in explaining sex differences in ageing, adaptive 62processes seem unable to explain the observation that the homogametic sex tends to live 63 longer than the heterogametic sex across a wide taxonomic range [1, 9]. 64 65There are at least two reasons why sex chromosomes should directly contribute to the 66 evolution of different female and male lifespans [9, 10]. First, the unguarded X hypothesis 67 (UXh) predicts increased mortality of the heterogametic sex due to the expression of 68 deleterious recessive mutations that accumulate in the non-recombining parts of the X (or Z) 69 chromosome [10]. Because these mutations are masked in the homogametic s...