Abstract
A great many plants and animals have evolved separate sexes from hermaphroditism. In species with separate sexes, the development of an individual as male or female is often controlled by a diallelic sex-determining locus (XY and ZW systems). Transitions from hermaphroditism to separate sexes must therefore have often entailed the emergence of such a locus. However, the evolutionary mechanisms governing the emergence of XY and ZW systems in ancestral hermaphroditic populations, and in particular the mechanisms leading some species to acquire an XY rather than a ZW system, remain elusive. Here, we model the co-evolution of resource allocation to male and female functions (sex allocation) with the genetic architecture of sex determination, and show that gradual evolution readily leads to the emergence of XY and ZW systems. Our model also reveals a strong influence of the shape of the relationship between resource allocation and fecundity in each sex (male and female gain curves) on whether an XY or a ZW system evolves. This is because gain curves indicate the intensity of competition for reproduction through each sex, which in turn affects selection on the genetic architecture of sex allocation. Taken together, our results advance the understanding of sexual systems by uncovering a hitherto unappreciated link between the ecology and economics of sex allocation and the genetic basis of sex determination.
Competing Interest Statement
The authors have declared no competing interest.