Rapid evolution of sex role specialization in a hermaphrodite under sex-limited selection

The evolution of separate sexes from hermaphroditism is thought to have occurred independently many times, and is linked to the evolution of sex chromosomes. Even though we have a good understanding of the theoretical steps in the evolution of sex chromosomes from a hermaphrodite ancestor, the initial stages are still hard to study because many sex chromosome systems are old. We addressed this problem by experimentally selecting a hermaphrodite via sex-limited experimental evolution for several generations, simulating the early stages in the evolution of a sex chromosome. More specifically, we used a GFP (green fluorescent protein) marker as a proxy for a sex-determining locus, and selected replicate populations of the simultaneously hermaphroditic flatworm Macrostomum lignano for fitness via the male sex role, female sex role, or both (i.e. a control). After 14 generations, a fitness assay revealed clear evidence for incipient sex role specialization, presumably reflecting the release from constraints usually imposed by selection on the other sex role. Importantly, however, this was not simply explained by differential sex allocation in the different selection regimes - insofar as morphological traits reflect the underlying trade-off over resource allocation to the male and female sex functions - because testis and ovary sizes did not diverge among treatments. Our study shows that sex role specialization can occur rapidly as a result of sex-limited selection, which is consistent with genetic constraints between sex-roles, and in line with the first predicted steps towards the evolution of a new sex chromosome system.