Résumé
The ant Petalomyrmexphylax is a protective mutualistic ant associated with Leonardoxa a. africana, a small tree of coastal rainforests of Cameroon. The association has expanded southwards during the last few centuries. Northern colonies of the ant are secondarily polygynous while in southern populations, colonies are strictly monogynous. This shift in social behavior seems to be associated with selection for dispersal along a colonization front. The Lobe river seems to constitute a geographic barrier for the system. In all populations north of the river, many polygynous colonies are observed, while mostly monogynous ones are observed south of the river. As the river flows towards the north-west, populations located close to the coast, but just south of the river, present a social structure mainly observed in more southern populations. We investigated, using microsatellite markers, whether the river constituted a genetic barrier thus explaining the rupture in social structure. For both plant and ant, the river did not appear to be an obstacle. These observations suggest that the distribution of social structure in the ant is not explained by physical obstacles. This distribution may rather reflect the historical dynamics of colonization. We showed also that the pattern of genetic structuring was the same for both plant and ant at a scale of a few hundred meters.