Testing models of sex allocation in subdivided populations
How should individuals divide their limited reproductive resources between the production and dispersal of pollen versus seeds when they occur in a fragmented population? We are currently addressing this question from a theoretical perspective through the construction of evolutionarily stable strategy (ESS) and quantitative genetic models of sex allocation for metapopulations, in which local populations are subject to dramatic fluctuations in their size, including extinction and colonisation events. The predictions made by this theory are being tested using the plant Mercurialis annua as a model system. This species shows remarkable variation in its sexual system in Europe, especially in the , where dioecious (the co-occurrence of males and females), monoecious (functional hermaphrodites) and androdioecious populations (the co-occurrence of males with hermaphrodites) are all widespread in different regions.
A spatially hierarchical analysis of the role of colonisation in plant metapopulations
Annual plant populations are notoriously patchy. Whether these patches should usefully be regarded as subpopulations or not is a largely operational question. However, the way in which patches become established, become extinct, or coalesce into a continuous population by expansion into one another can have a profound influence on the amount and patterns of genetic diversity maintained within a species. We are currently assessing the extent to which patches within spatially extended populations are spatially coherent, particularly with a view towards understanding why many self-fertilising annual plant populations maintain much less variation than one would expect on the basis of species-wide diversity.
The genetic and ecological significance of cryptic females in dioecious species
In dioecious species, it is usually only the males that produce pollen. However, several species in a range of different families are known in which males and ‘hermaphrodites’ occur at a ratio of 1:1. Species with even sex ratios must be functionally dioecious, and simple theory tells us that the apparent pollen producing ‘hermaphrodites’ are functioning only as females. We are interested in the functional significance of pollen produced by functional females in the Mediterranean species Fraxinus ornus. Collaborator Miguel Verdú, Valencia, Spain.
The evolution of local adaptation in colonising plants
Colonising plants, like all organisms, are expected to become locally adapted to their environments as a response to natural selection. They are particularly interesting for at least two reasons. First, colonising species that are new arrivals in a region will be potentially subject to strong selection in their new environment and might be expected to show rapid responses to this selection. And second, their morphology, life history and physiology is expected to respond to natural selection not only locally, but also to selective forces acting at regional level during the process of colonising itself. Selection at the regional or metapopulation level is thus particularly likely to shape dispersal-related traits, including the mating system.
The evolution and ecology of secondary sexual dimorphism in plants
Many dioecious species show dimorphism not only in their sex allocation and floral structures, but also in vegetative traits, with males and females often differing in their morphology and architecture, physiology and life history. Secondary sexual divergence has been much studied in animals, but our understanding of the underlying selective forces in plants are poorly understood. We are interested in how much of this variation is the result of sexual selection (or selective agents analogous to sexual selection), and how much a result of selection on the different reproductive roles carried out by males and females, with their divergent costs and benefits?
The evolution of plant-plant signalling strategies for gender determination
Because plants are sessile, their success depends critically on an ability to respond plastically to changes in their environment. For example, they respond to light quality, day length, soil nutrient availability, herbivory, etc. Such responses typically maximise growth and survivorship in a given context, but to what extent is phenotypic plasticity used to maximise mating and reproductive success? In many animal species, individuals are known to modify their morphology and behaviour in response to local mating opportunities. We are considering the possibility plants, too, might have evolved a similar ability to switch gender in response to perceived opportunities, given that mating success in plants also depends on local mate availability.
The evolution of sex-determination mechanisms
The gender of organisms with separate sexes can be determined be a range of environmental and genetic mechanisms. We are interested in why, and to what degree, plants have evolved aspects of environmental sex determination and environment-dependent sex allocation. We are also interested in the implications of genetic sex determination for the molecular evolution of the underlying loci, and those loci linked to them. Here, we are using as empirical models both the clade of annual Mercurialis species, with its remarkable diversity of sexual systems in which separate sexes have evolved more than once (collaborator: Dmitry Filatov), as well the freshwater crustacean Eulimnadia texana, in which self-fertilization by hermaphrodites evidently produces genotypes that express deleterious alleles linked to the sex-determining locus.