Nathan W Burke, Russell Bonduriansky
In most animals, females need to mate to reproduce. But some populations have lost sex altogether and reproduce only asexually. How evolution proceeds from sexual to asexual reproduction and back again is poorly understood. In many stick insects, females can reproduce both sexually and asexually. If they mate, they produce both sons and daughters, but if they don’t, they produce only daughters. This plastic reproductive strategy, called facultative parthenogenesis, is extremely useful for understanding why sex is maintained in some populations but lost in others. One hypothesis is that sex might fail to reinvade once it is lost because females in asexual populations are better off reproducing on their own. This leads to the prediction that females of asexual origin might gain no reproductive benefit from mating and so mate less frequently. Alternatively, females may benefit by mating and producing offspring with shuffled genes because such offspring will have fewer of the deformities and developmental problems that can result from asexual reproduction.
To test these predictions, we performed an experiment on the facultatively parthenogenetic spiny leaf stick insect, Extatosoma tiaratum. Starting with a sexual population, we obtained females from fertilized and unfertilized eggs, and then either allowed them to mate or not. We assessed their body size, wing deformities, leg asymmetry, and mating behaviour, and counted the number of offspring they produced. We found that females of asexual origin were smaller than females of sexual origin and had more deformed wings, but not more asymmetrical legs. Since females cannot fly, these morphological differences are unlikely to greatly affect female performance. Females of asexual origin received fewer mating attempts, mated less frequently, and produced fewer offspring if they mated than if they didn’t mate. By contrast, females of sexual origin produced more offspring if they did mate. These results provide intriguing insights into how all-female populations might withstand the re-introduction of sex: if females of asexual origin are less attractive, mate less frequently, and produce fewer offspring when they happen to mate, then sex may have a very difficult time coming back once it is lost.