Sexual selection and sex allocation in a simultaneous hermaphrodite: examining phenotypic and genetic influences

The evolution of anisogamy resulted in a cascade of unique phenomena and evolutionary consequences. Among those phenomena are sexual selection, the problem of sex allocation and genomic conflict over sex allocation. During my PhD project I studied aspects of these evolutionary consequences of anisogamy using the reciprocally copulating, simultaneously hermaphroditic flatworm Macrostomum lignano.
Sexual reproduction, especially when it involves reciprocal copulation and internal fertilization, requires close interactions of at least two mating partners. In Chapter 2 I report an experiment testing for effects of sperm donor genotype by sperm recipient genotype interactions on i) mating behaviors and ii) pre- as well as postcopulatory fitness components. Two mating behaviors, but not the pre- and postcopulatory fitness components were affected by such genotype-by-genotype interactions, while almost all variables were influenced by the genotype of the donor. The sperm donor by sperm recipient genotype interactions on mating behaviors reveal that there is genetic variation for both sexual selection and selection arising from sexual conflict to act on during this precopulatory stage. The lack of these interaction effects on the pre- and postcopulatory fitness components could indicate that sexual conflict and sexual selection is shifted towards later stages, namely the stage between sperm storage in the recipient, and the fertilization of eggs. This conclusion may not only hold for M. lignano but possibly also more generally for other reciprocally copulating hermaphrodites.
The local sperm competition model predicts not only the selection for a more female-biased sex allocation due to competition between related sperm. It also specifies the mechanism by which this change in sex allocation is selected, namely diminishing fitness returns for investment into the male function, due to competition between related sperm. I present results in Chapter 3 that confirm a positive relationship between testis investment and paternity success. However, the predicted diminishing fitness returns for testis investment in smaller group sizes, i.e. group sizes which should have resulted in strong local sperm competition, could not be confirmed. Since there are no other, more plausible hypotheses to explain the phenotypically plastic shifts in sex allocation, I conclude that the local sperm competition model could still be valid, but that an improved experimental design, increasing the range of local sperm competition, may be used in future studies.
Nuclear genes and cytoplasmic genetic factors are not equally transmitted via eggs and sperm, potentially leading to cytonuclear conflict over the optimal sex allocation. Cytonuclear conflict involving mitochondria can therefore be expected to be widespread, but it has mainly been documented in plants rather than animals. In Chapter 4 I report the results from a quantitative genetic breeding experiment testing for cytotype effects on sex allocation traits, as predicted under an ongoing cytonuclear conflict over sex allocation. Contrary to this prediction, we did not find evidence for strong cytonuclear conflict over sex allocation. I propose two possible explanations: namely i) that the nuclear genome in animals ‘won’ the coevolutionary arms race and ‘domesticated’ the mitochondrion during the course of coevolution or ii) that the studied population was not polymorphic for loci involved in cytonuclear conflict.
The different aspects of the male-female phenomenon, which I studied during my PhD are quite diverse but interconnected. Sexual selection, sex allocation and genomic conflict over sex allocation all influence each other, because ultimately they are all consequences of the evolution of anisogamy. I therefore suggest that it may often be necessary to study how these different aspects of the male-female phenomenon are connected, rather than focusing on them in isolation.