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 may be 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 in animals because many well-studied animal 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. After 14 generations, a fitness assay revealed evidence of incipient sex role specialization in the female-selected lines, presumably reflecting the release from constraints usually imposed by selection on the other sex role. Importantly, however, this was not simply explained by morphology because testis and ovary sizes did not diverge among treatments. There was no evidence of a cha..., Experimental evolution protocol
The experimental evolution lines consist of four replicate populations (denoted 1-4) within each selection regime (female-limited, male-limited, and control), resulting in 12 populations in total (denoted F1-4, M1-4, and C1-4). Populations with the same replicate number are not more closely genetically related to each other than to other replicates, but they are related in terms of handling (i.e. culturing was during the same time of the day, and worms with the same replicate number were placed in the same area of the incubator).
A new generation starts with 48 mature, GFP(+) individuals from each replicate population being crossed with two worms each from the matched “source” population. Source populations are maintained in the same way as the other laboratory lines described above, and have the same genetic origin as the GFP(+) individuals from the same combination of treatment and replicate. They are used to provide mating partners for the GFP(+) indi...,