The University of Edinburgh and Department of Zoology, Oxford Unversity, Oxford OX1 3PS, UNITED KINGDOM.
In sexually reproducing organisms, the allocation of resources to male and female reproduction can have direct and considerable effects on an organism’s fitness. Consequently, females are expected to allocate their resources to the production of sons and daughters (sex allocation) in such a way as to maximise their fitness. The field of sex allocation consists of a large body of theoretical and empirical research. This has resulted in sex ratio evolution becoming one of the most well understood areas of evolutionary biology, providing some of the clearest evidence to support evolution by natural selection. In addition, the success of sex allocation theory and the ease of collecting data (it is often fairly easy to count and sex offspring), has allowed evolutionary biologists to use it as a tool to answer more general questions. This approach can be extrapolated to answer a number of questions in any sexually reproducing organism, as the same general principles underlie sex allocation in dioecious and hermaphroditic organisms throughout animal, plant and protozoan taxa. I have investigated these principles in parasitoid wasps, sea turtles and malaria parasites. Experiments to test whether females of the gregarious parasitoid wasp, Nasonia vitripennis, adjust their offspring sex ratio in response to whether they mate with a sibling or a non-relative reveals that they are unable to discriminate kin. Field studies provide the first set of comprehensive data concerning the field sex ratios of 2 species of sea turtle (Chelonia mydas and Caretta caretta) nesting in the Mediterranean and an additional assessment of using indirect methods to measure sex ratios in the field. These observations are extended to test whether the differential fitness theory of environmental sex determination applies to Caretta caretta. Lab experiments using the rodent malaria (Plasmodium chabuadi) investigate: (1) facultative sex allocation in malaria parasites where parasites respond to host anaemia, (2) methods to estimate the sex ratio of malaria parasites, (3) whether the assumptions of sex allocation theory are appropriate to malaria parasites. Theory developed shows that malaria parasites should alter their sex allocation in response to intrinsic and host factors that could impair fertilisation in the mosquito.