The nearness of you: the effect of population structure on siring success in a gynodioecious species

Theoretically, both balancing selection and genetic drift can contribute to the maintenance of gender polymorphism within and/or among populations. However, if strong differences exist among genotypes in the quantity of viable gametes they produce, then it is expected that these differences will play an important role in determining the relative frequency of the genotypes and contribute to whether or not such polymorphism is maintained. In this issue, De Cauwer et al. (2010) describe an investigation of gynodioecious wild sea beet, which in addition to containing females, contain two types of hermaphrodites: restored hermaphrodites carrying a cytoplasm that causes pollen sterility and a nuclear gene that restores pollen fertility, and hermaphrodites without the sterilizing cytoplasm. The results show that restored hermaphrodites, who have relatively low pollen viability, achieve disproportionately high siring success simply because of where they are located in a patchy population ( Fig. 1 ). Notably, these individuals tend to be close to females because of the genetics of sex determination. These results indicate that population structure caused by drift processes can have an unexpectedly large effect on the fitness of these low quality hermaphrodites, thereby contributing in the short term to the maintenance of gynodioecy in this population. While these results indicate that population structure caused by drift processes can have a large effect on the relative fitness of genetic variants, whether these effects promote or discourage the maintenance of polymorphism in the long term is still up for debate.

Figure 1 Open in figure viewer PowerPoint A stretch of beach along which wild sea beet can be seen to be growing among the rocks above the splash zone. This linear arrangement enhances the potential for mating success to depend on proximity to other plants (Photo: J.‐F. Arnaud).