Ecological and sexual divergence in experimental populations of Chlamydomonas

Laboratory studies on speciation have revealed that selection must be disruptively applied on traits related to the mating system in order to produce deviations from random mating in experimental populations. One problem with these experiments, however, has been the complexity of the model organism used, most frequently Drosophila species. Due to the multi genic nature of the mating systems of such organisms, it has been difficult to obtain the necessary gene combinations that result in complete sexual isolation. In the present study, I have used a simple sexual organism, the unicellular green algae Chlamydomonas reinhardtii, as a model for ecological and sexual differentiation. Disruptive selection was applied on the flagella, by selecting simultaneously for photo taxis and mating, behaviours for which these organelles are of fundamental importance. An asymmetric response to selection for photo taxis and zygote production was obtained in populations selected for conditions at opposite ends of the environmental spectrum used, differentiating these two populations in both movement capacity and mating efficiency. These results are discussed in relation to previous experiments on speciation and to the implications of future experimental studies on the same subject