Inbreeding depression and the genetic architecture of fitness in natural populations

Individuals within natural populations often exhibit genetic variation in fitness-associated traits, but the genetic basis of this variation is largely unknown. By studying the genetic basis of inbreeding depression, inferences can be made about the number and effects (e.g., selection and dominance coefficients) of loci that contribute to fitness variation. We study a Daphnia magna metapopulation, in which local populations suffer from a high genetic load due to inbreeding. Previously, we have found that the genetic load in this metapopulation consists of many alleles of small to intermediate effects, many of which show overdominance or apparent overdominance. The next steps will be to experimentally distinguish between true and apparent overdominance, and to test whether – as predicted by theory – the number and effects of the contributing alleles vary with the degree of population structure. This work, which takes advantage of several specific assets of the Daphnia system, will hopefully lead to a more quantitative understanding of the nature of genetic variation in fitness and will help us to understand the effects of population structure and small effective population size on evolutionary processes.

Evolutionary genetics of sexual reproduction in Daphnia

The wide-spread occurrence of sexual reproduction remains one of the great enigmas of evolutionary biology, but recent theoretical work suggests that population subdivision and genetic drift are likely to play a key role. In addition, the effects of population structure and genetic drift may also help to explain why the geographic distribution of asexual species often differs from that of closely related sexual species (“geographic parthenogenesis”). We study sexual and asexual Daphnia pulex at a field site in southern Finland, where both reproductive forms co-occur. Using these, we investigate in a common environment how population structure affects the relative fitness of sexual and asexual forms. We also study whether sexuals and asexuals differ in their ecological adaptations, how asexuality affects genetic diversity, and how the reproductive mode is genetically determined.

Balancing selection

Recent quantitative genetic evidence suggests a substantial contribution of balancing selection to the maintenance of genetic variation natural populations. However, to date, only a few examples of genes under balancing selection have been identified. In collaboration with Mathieu Joron, we analyze DNA sequence variation in the butterfly Heliconius numata, concentrating on the superene P, which determines wing-pattern polymorphism (mimicry types), and which is likely under balancing selection in this species. An earlier project (in collaboration with the group of Ilkka Hanski) has  investigated Pgi in the Glanville fritillary butterfly. We found that variation at this gene influences dispersal behavior, and that this likely leads to contemporary balancing selection through heterozygote advantage. The DNA sequence variation suggest that balancing selection at Pgi has maintained alleles for extended evolutionary times.

Genetic variation in endangered bird species

The advent of non-invasive DNA sampling methods has allowed us to improve our understanding of the ecology of endangered species that are otherwise difficult to study. We use non-invasive DNA sampling methods to study Capercaillie, Hazel grouse, and Fish otter. The collected data allows us to estimate population sizes and breeding success, and to gain a better understanding of the factors that lead to a decline (grouse species) or recent expansion (Fish otter) in population sizes and distribution ranges.