The effects of sex on extinction dynamics and evolutionary rescue ofChlamydomonas reinhardtiidepend on the rate of environmental change

The continued existence of sex despite many costs it entails still lacks an adequate explanation. Previous experimental studies demonstrated that sex enhances the rate of adaptation in changing environments. To the best of our knowledge, no experimental study that investigated the effects of sex on adaptation has manipulated the rate of environmental change, which is negatively correlated with the probability of population survival. Since the patterns of adaptation (adaptive walk) depend on the rate of environmental change, the overall effects of sex may also be altered by this factor. To investigate the interplay of sex and the rate of environmental deterioration, we carried out a long-term selection experiment with a unicellular alga ( Chlamydomonas reinhardtii) , by manipulating mode of reproduction (asexual or sexual populations) and the rate of environmental deterioration (an increase of salt concentration). We monitored both the population size and extinction dynamics and estimated the probability of evolutionary rescue. We detected a faster decline of population size in the asexual group relative to both the obligate sexual and facultative sexual group, irrespective of the rate of environmental deterioration. The results revealed significant interaction between mode of reproduction and the rate of environmental deterioration on extinction rate of experimental populations. Obligate sexual reproduction was advantageous under the intermediate rate of environmental deterioration, while facultative sexuality was favoured under the high rate of environmental deterioration. The populations within the obligate sexual group were most likely to adapt to grow in conditions lethal for the ancestral populations, irrespective of the rate of environmental deterioration. To the best of our knowledge, this is the first study which indicates that different modes of sexual reproduction will be beneficial (slower extinction rate) at different rates of environmental deterioration and that obligate sexuality maximizes the probability of adaptation irrespective of the rate of environmental deterioration.