The balance between sex and asex: evolutionary genetic studies of reproductive variation in Allium vineale

Abstract: Evolutionary theory predicts a disadvantage to sexual reproduction. This manifests itself either by the higher growth rate of asexual females in a dioecious species, or by the higher transmission rate of a gene conferring asexual female function to its carriers in a hermaphrodite species. The disadvantage to sexual reproduction may be as high as two-fold provided all else is equal. This phenomenon is known as the 'two-fold cost of sex'. Since most higher organisms nevertheless rely on sexual reproduction, it has been presumed that sex has some advantage that outweighs its cost. However, despite much effort to identify such an advantage, no single hypothesis has received unequivocal empirical support. Moreover, most theories have largely ignored situations in which there is within-population genetic variation for the balance between sexual and asexual reproduction. Also, experimental studies have yet to determine to what extent genetic variation for the degree of sexuality and asexuality exists in natural populations, and the importance of the two reproductive modes in partly asexual organisms. The fact that most organisms capable of asexual reproduction simultaneously retain sexual capacity underscores the importance of these issues for understanding the evolutionary significance of sex. This thesis addresses some of these questions. A theoretical model was set up from which it was concluded that the maintenance of a genetically determined polymorphism for the proportion of sexually produced offspring is possible. It is dependent upon temporal fluctuations in the relative fitness of sexually and asexually produced propagules and/or in sexual versus asexual fecundity. A study of natural populations of the partly asexual plant Allium vineale (wild garlic) revealed that genetic variation for the allocation to sexual and asexual reproduction exists under field conditions. Moreover, considerable fluctuations in the proportions of sexually and asexually produced propagules occurred between years, a state of affairs likely to be instrumental in maintaining a balance between the two reproductive modes. An investigation of genetic variation in A. vineale populations across Europe showed asexual reproduction to be more important for the recruitment of offspring over the short term. Recombination turned out to have played a significant role in introducing new genotypes, but at a rate that may well be lower than one sexually produced offspring per generation. How a balanced reproductive system can be maintained when sexual recruitment occurs at such a low rate is at present unclear. In A. vineale a greenhouse study showed that the genetic correlation between the allocation to sexual and to asexual reproduction differed among sexually and asexually produced offspring. Thus, selecting for an increase in the allocation to one mode of reproduction may give a different response in the allocation to the other mode, depending on whether selection acts among sexually or asexually produced individuals. This may explain why the capacity of sexual reproduction has persisted in this species despite its apparently low 'effective' rate.