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Sexual system and genetic diversity in Mercurialis annua: a patch-level test of the metapopulation model

Final Activity Report Summary - GENEDIVPATCH (Sexual system and genetic diversity in Mercurialis annua: a patch-level test of the metapopulation model)

Plants show a wide variety of sexual systems. Part of this variation can be explained in terms of colonisation history and population dynamics. During colonisation, the need for reproductive assurance selects for self-compatible monoecy (system with only hermaphroditic plants present in a population). When population size increases, factors like trade-offs between investment in male and female function start to play a role, the selfing rate decreases and males can invade the population. In large, stable populations, dioecy (a system with separate female and male plants) is considered to be an evolutionary stable strategy. Such changes in sexual system will also have consequences for genetic diversity: with the lower level of inbreeding, within-population genetic diversity increases and the genetic differentiation among the populations decreases. Although, these predictions are implicit from the many theoretical studies, empirical evidence for them is still very poor.

This project aimed at testing the predicted association between the sexual system and genetic diversity. As a study species, Mercurialis annua was used because it shows a unique variety of sexual systems, from monoecy to androdioecy (where males coexist with hermaphroditic plants) and finally to dioecy. Androdioecious populations differ widely with the percentage of male individuals. We hypothesised that such fine-scale patterns of sex-ratio variation in M. annua are due to colonisation and patch dynamics at a small scale within sampling localities, and that these processes will be evident as signatures in the patterns of neutral genetic diversity.

We developed microsatellite markers, which provide high resolution, to test the above hypothesis, as they appear to be polymorphic even within a sample of plants from a single 2 x 2 meter patch. In particular, we used a novel approach by developing diploid, codominant markers for allohexploid species; this simplifies data collection and broadens the spectrum of analyses that can be subsequently used. Patch-level analysis of genetic diversity and differentiation showed, as predicted, that patches within monoecious populations have lower genetic diversity and higher differentiation compared to patches within androdioecious populations.