Sex ratio selection and the evolution of environmental sex determination in laboratory populations of Menidia menidia

Abstract

What happens when a population with environmental sex determination (ESD) experiences a change to an environment that causes a highly unbalanced sex ratio? Theory predicts that frequency-dependent selection would increase the proportion of the minority sex and decrease the level of ESD in subsequent generations. The authors empirically modeled this process by maintaining five laboratory populations of a fish with temperature-dependent sex determination (Atlantic silverside) in extreme constant temperature environments that caused highly skewed sex ratios to occur initially. Increases in the minority sex consistently occurred from one generation to the next across all populations, first establishing and then maintaining a balanced sex ratio until termination of the experiment at 8-10 generations. Two populations that experienced high temperatures each generation displayed a loss of ESD, and in one of these ESD was virtually eliminated. This suggests that temperature-insensitive, sex-determining genes were being selected. In populations maintained in low temperature environments, however, the level of ESD did not decline. Instead, the response of sex ratio to temperature was adjusted upward or downward, perhaps by selection of sex-determining genes sensitive to higher (or lower) temperatures. -from Authors