Frequent sexual reproduction limits adaptation in outcrossed populations of the alga Chlamydomonas reinhardtii

Sexual reproduction can facilitate adaptation by reshuffling genetic variation. However, sexual reproduction can also bear costs. Such costs come in two forms: direct costs and evolutionary costs. Direct costs are associated with the cost of producing males (twofold cost of sex), the cost of meiosis, and the typically slower cell division during sexual reproduction of single-celled organisms. Evolutionary costs occur when too frequent sexual reproduction would hinder adaptation, by breaking apart adaptive allele combinations. Whereas the direct costs of sexual reproduction have been studied repeatedly in theoretical studies, the evolutionary costs of sex remain less well understood. We investigate here how the frequency of sexual reproduction affects adaptation to a non-stressful and a stressful environment in populations of the green alga Chlamydomonas reinhardtii, while minimizing the direct costs of sexual reproduction. Contrary to several previous studies, we found that an increasing frequency of sexual reproduction hindered adaptation of populations. In populations experiencing the highest frequency of sexual reproduction, adaptation was entirely prevented. These findings suggest that there were strong evolutionary costs associated with too frequent sexual reproduction in our populations. This observation may help to explain why in many facultative sexual species, there is a low frequency of sexual reproduction.