Sexual selection in a simultaneous hermaphrodite : mate choice and sperm utilization patterns

Sexual selection processes, in general, are distinguished in precopulatory and postcopulatory mechanisms. In the animal kingdom, precopulatory mechanisms are related to mate competition and mate choice before the copulation. Postcopulatory mechanisms like sperm competition and cryptic female choice occur inside the female reproductive organs and can influence the reproductive outcome due to differences in sperm utilization for the fertilization of the ova. In simultaneous hermaphrodites, both sexes are present in an individual, therefore sexual selection processes are influenced by the interests of both genders.
In the present thesis, different approaches including behavioural studies, evaluations of life-history traits and genetic parentage analyses were combined to broaden our view on the sexual selection, particularly in mate choice and sperm utilization patterns, in the model organism Arianta arbustorum, a simultaneously hermaphroditic land snail. Furthermore, an immunocytochemical approach was further developed to allow studies relating to postcopulatory selection mechanisms in the female reproductive organs of land snails.
In Chapter 1, we assessed derminants of mating success and female and male reproductive success in individuals of A. arbustorum using video-records of the behaviour of individually-tagged snails kept in groups of six animals over one reproductive period and assigned the genotyped hatchlings to the female and male function of individual parents. Mating success, which is equal to the female and male function in simultaneous hermaphrodites with reciprocal copulation, was mainly determined by the activity of an individual. We found that female reproductive was positively correlated with male reproductive success and that both were determined by the individual's activity. These results contrast the existing sex allocation theory.
In Chapter 2, mating frequency and the sperm utilization patterns in the wild were assessed using microsatellite DNA markers and paternity reconstruction of 1088 offspring from 26 mother snails. Overall, multiple paternity was detected in every mother-progeny array ranging from two to six fathers. Moreover, depending on the population density, the four populations examined differed in the level of multiple paternity. The results revealed also highly skewed paternity patterns in the progeny of 57.7 % of the mother snails, i.e. the number and identity of fathers siring the offspring of single mothers also varied among successive clutches. Furthermore, genetic analyses indicate a low level of self-fertilization in one of the four populations.
In species with multiple mating and long-term sperm storage males are expected to show a preference for mating with virgin and young females to reduce the risk of sperm competition. To examine whether sperm transfer and sperm utilization of virgin mating partner are postcopulatory preferred over those of an experienced mate, polymorphic microsatellite markers were applied to assess sperm utilization and last mate sperm precedence in hatchlings of copulating pairs from the wild consisting either of two adults, two subadults or of one adult and one subadult (Chapter 3). The results showed that adult snails used sperm received from subadult mating partners for egg fertilization in the same frequency as sperm from adult partners, indicating that subadult and adult snails do not differ in male function. Furthermore, in 35% of the subadult individuals sperm stored from previous mating(s) was utilized for the fertilization of the ova. However, compared with adults, these young individuals exhibited a lower risk of sperm competition to their partners, indicated by a higher last mate sperm precedence. On the other hand, subadult snails produced fewer eggs than adult snails, which may counteract the evolutionary advantage of preferring a young partner with low sperm competition risk.
Sexual selection goes hand-in-hand with reproductive isolation and both can influence gene flow, genetic diversity and biodiversity. Different mechanisms of reproductive isolation and sexual selection were examined in individuals of A. arbustorum (Chapter 4). Snails from two geographically isolated populations in the Swiss Alps were allowed to copulate with both a homotypic (individual from the same population) and a heterotypic (individual from the other population) partner (in half of the pairs in reversed order). In the first mating, successful copulations occurred in a lower frequency in heterotypic pairs (55.6%) than in homotypic pairs (82.9%). Heterotypic pairs that eventually copulated showed more breaks during courtship than homotypic pairs. However, neither the number of eggs produced nor their hatching success was influenced by the type of mating partner. In the second mating, the sequence of different partners had an effect on the proportion of successful copulations in snails from one geographical population. Paternity analyses of progeny of snails that mated twice indicate no influence of the origin of mating partner. These findings indicate the presence of partial precopulatory isolation between two distant snail populations, although reproductive compatibility is still maintained.
To understand the patterns of sexual selection after copulation, the mechanism of sperm storage, utilization and digestion are of relevant importance. In Chapter 5, an immunocytochemical technique to track the fate of labelled sperm in the female reproductive organs of invertebrate species was further developed and tested on individuals of A. arbustorum.6