Groupe Goudet
Mating Systems and Population Genetics

The main focus of the group concerns the understanding of the interplay of population structure, mating systems and selection. For this, we use different approaches, from theory and the development of statistical tools to field observations via experimental work. The main biological models are different species of freshwater snails (Galba truncatula, Physa acuta, Radix peregra... and the gynodioecious perenial plant, Silene vulgaris. On the theoretical side, we investigate the dynamics of multilocus genetic system under the influence of selection and drift, and develop statistical methods to infer mating systems and population structure.

Collaborators: Samuel Neuenschwander , Frédéric Hospital, François Balloux (Cambridge, UK), Andrea Manica (Cambridge, UK), Nicolas Perrin, Peter Waser (Purdue, USA), Guillaume Evanno

Molecular markers are plentifull nowadays, and of different types. The type of information that these markers can provide for understanding different aspects of the ecology of species is enormous, but largely unexplored. For instance, we were recently able to show that sex-biased dispersal can be detected using genetic markers. Statistical tools for analysing genetic data are often complex, and require specific packages. To this end, I developed the package FSTAT, which estimates and tests gene diversities and F-statistics, among other things. Another package on which I am working is PCAGEN , which carries out Multivariate Analysis on genetic data. These softwares are under constant development.

When applied to individuals, multivariate analysis allows the identification of clusters of individuals. I am currently investigating how a coupling of Multivariate analysis and clustering can help in identifying clusters of individuals.

Once the tools have been developed, they need testing. We are also investigating using computer simulations how these tools perform

Collaborators: Guillaume Martin, Yamama Naciri-Graven (Geneva) and Alex Ding.

Both theoretical and empirical studies have pointed to the puzzling result that while bottlenecks (the reduction in size of a population) diminish genetic variability, in some cases they lead to an increase in additive genetic variance. Epistasis (the non-additive action of genes at different loci on one character) seems to play a major role. Using multilocus models, we are trying to understand
- What type of genotypic values lead to stable polymorphic equilibrium?
- Can multilocus epistasis help in maintaining additive genetic variance following bottlenecks ?

We've answered partially the last question (yes,it can!), but we need to add selection to the model. Also, matrices of genotypic values are generated at random for the moment, but eventually, we want to link our work with metabolic control theory.

Evolutionary biologists struggle to explain why so many species reproduce sexually. Among these sexually reproducing organisms, many reproductive systems exist, ranging from separate sex to full hermaphroditism via some intermediate stages, where some individuals harbor the two sexes while others are unisexual. One characteristic of reproductive systems where at least some individuals harbor simultaneously the two sexes is the ability to self-fertilize. Selfing can be advantageous, since it alleviates the cost of producing males and of finding a mate, provides a reproductive insurance, and could preserve successful genotypes. But it can also be detrimental since one of the consequences of selfing is a higher homozygosity, that can translate into inbreeding depression. Our goal is to gain a better understanding of the factors promoting selfing in natural populations. Species where the proportion of selfing varies among individuals and populations are particularly well suited to study the relative importance of these different factors.

But this goal can only be attained if we have access to sufficient genetic information. The increasing availability of genetic markers, inherited either uniparentally (e.g. the cytoplasmic genome), or biparentally (most nuclear DNA), has opened new avenues to the evolutionary ecologist. Not only critical parameters such as relatedness, coancestry or selfing levels can be readily estimated from field data, but models of population structure have been greatly refined since the insightful ideas of Wright for partitioning genetic variation among components reflecting population structure. In particular equations for gene correlations have been derived, that include different elements of the social structure found in vertebrates (polygamy, multiple mating, preferential dispersal of one sex, variance in reproductive success). One goal of the research group is to adapt this model to understand the fine scale population structure of plants and hermaphroditic animals.

Galba truncatula

Collaborators: Sandrine Trouvé, Elodie Chapuis, Loïc Degen and François Renaud (Montpellier, France) .

Lymnaea truncatul a is a self-fertile pulmonate snail found on most continents. It is an intermediate host of many trematode parasites that castrate infested individuals. We are investigating the population structure of this snail in Switzerland, using microsatellite markers. We are in particular interested in the following questions:
· Does parasite prevalence affect the level of selfing in populations?
· Is selfing rate affected by the temporal stability of the habitat, or its size?
· How much genetic exchange is there between the populations?

Silene vulgaris

Collaborators: Mélanie Glaettli, Daniel Croll, Nicolas Juillet, Luca Pescatore.

Silene vulgaris is a common weed that originates from Eurasia. It is classified as a poor competitor, which might be unable to resist invasion by species coming later in the succession. It has been classified as gynodioecious (some plants are hermaphrodites, other are females) by most authors, although a small percentage of gynomonoecy (hermaphrodite and female flowers on the same individuals) has been found. It is allogamous, but autocompatible and the many flowers per plant allow for some selfing. In large populations, the proportion of hermaphrodite is high, while smaller populations could be mainly females. Mode of inheritance of sex is complex with one cytoplasmic and up to four nuclear genes involved. Genetic structure in S. vulgaris have been investigated using nuclear allozyme markers, which prove polymorphic and inherited in a Mendelian fashion, and cytoplasmic chloroplastic markers, which also proved to be polymorphic. The project goal is to identify how at a very fine geographical scale, the different forces involved in shaping the genetic structure of S. vulgaris patches interact. To this end, we will use both field studies (2 populations with contrasted sex-ratio in each of 3 valleys), green-house experiments and modeling.