Two sexes, one genome: the evolutionary dynamics of intralocus sexual conflict

Abstract

As the evolutionary interests of males and females are frequently divergent, a trait value that is optimal for the fitness of one sex is often not optimal for the other. A shared genome also means that the same genes may underlie the same trait in both sexes. This can give rise to a form of sexual antagonism, known as intralocus sexual conflict (IASC). Here, a tug-of-war over allelic expression can occur, preventing the sexes from reaching optimal trait values, thereby causing sex-specific reductions in fitness. For some traits, it appears that IASC can be resolved via sex-specific regulation of genes that subsequently permits sexual dimorphism; however, it seems that whole-genome resolution may be impossible, due to the genetic architecture of certain traits, and possibly due to the changing dynamics of selection. In this review, we explore the evolutionary mechanisms of, and barriers to, IASC resolution. We also address the broader consequences of this evolutionary feud, the possible interactions between intra- and interlocus sexual conflict (IRSC: a form of sexual antagonism involving different loci in each sex), and draw attention to issues that arise from using proxies as measurements of conflict. In particular, it is clear that the sex-specific fitness consequences of sexual dimorphism require characterization before making assumptions concerning how this relates to IASC. Although empirical data have shown consistent evidence of the fitness effects of IASC, it is essential that we identify the alleles mediating these effects in order to show IASC in its true sense, which is a “conflict over shared genes.”

Intralocus Sexual Conflict and the Tragedy of the Commons in Seed Beetles

“…The lack of relationship between SD and productivity is, however, also consistent with a scenario where any potential population-level benefits of increased SD in terms of alleviated IaSC are balanced by increased IeSC (Arnqvist and Rowe 2005;Pennell and Morrow 2013;see fig. 1).…”

“…However, a resolution to IaSC would allow both sexes to reach their independent phenotypic optima (e.g., increased courtship intensity and high mating rates in males versus increased mating resistance and low remating rates in females). If such optima involve male mating traits that inflict harm on females, such as male aggression, resolved IaSC could result in elevated IeSC and increased detriment to females, leading to depressed population viability (Kokko and Brooks 2003;Rankin et al 2011;Pennell and Morrow 2013;Chenoweth et al 2015). An illustrative example is given in figure 1.…”

Impact of gene polymorphisms of gonadotropins and their receptors on human reproductive success

“…Intralocus sexual conflict occurs when traits encoded by the same genetic locus result in opposite effects in males and females, in terms of reproductive success (Pennell & Morrow 2013). This was experimentally demonstrated in animal models, revealing that high levels of the sex hormone testosterone result in different, sex-related reproductive success in the bank vole Myodes glareolus (Mills et al 2012).…”

Maternal inheritance of mitochondria: implications for male fertility?

“…The genetic correlation between the sexes is expected to be strongly positive for most traits (Bonduriansky & Chenoweth 2009). This is because the sexes share the same genomes, both nuclear and mitochondrial (Pennell & Morrow 2013), and the genes encoding trait function are likely to have much the same role in each of the sexes. Thus, if a mutation was to arise in a mitochondrial gene that encodes a critical element of OXPHOS function and has a negative effect on males, then it is reasonable to assume that under most scenarios, this same mutation would exert a similarly negative effect on females.…”