Development of reproductive barriers in sympatry.

Freshwater zooplankter Brachionus plicatilis is able to inhabit different habitats and locally adapt to their environmental conditions. It also shows a high degree of population structuring in small geographical regions. Here we try to shed light on the evolution of reproductive isolation in populations of B. plicatilis with presumptive gene flow among locally adapted populations. We have conducted laboratory experiments on admixed pairwise populations that differ in predictability of the water regime. We have assessed the potential for within-population reproductive preferences as a deviation of genotypes from Hardy–Weinberg equilibrium in diapausing eggs, a product of sexual reproduction. We expected heterozygote deficit to increase with environmental distance. We have found signs for incipient reproductive isolation in one third of our admixed populations, however no correlation with environmental distance was found, nor with genetic or geographic predictor variables. The overall inbreeding coefficient showed a tendency for within-population crosses preferences to decrease over time. [ABSTRACT FROM AUTHOR]