Sympatric speciation in Oenanthe aquatica/conioides
Final Report Abstract
Oenanthe conioides is a narrow endemic of parts of the Elbe river above, in and below Hamburg (Germany). Its habitat is characterized by the unusual property of tidal movement of freshwater. Oenanthe conioides is most closely related to the geographically widespread Oe. aquatica from which it can be distinguished mainly by leaf shape and ecology. Different from the tidal freshwater habitat of Oe. conioides, Oe. aquatica is found mainly in standing or slow-flowing freshwater habitats such as the margin of ponds. The narrow distribution area of Oe. conioides is situated well within the wide distribution area of Oe. aquatica so that the sympatric distribution and ecogeographical differentiation of the two species suggest that Oe. conioides may have originated sympatrically through ecological speciation. Support for the ecological speciation hypothesis comes from both molecular and experimental data. Molecular markers have shown that the species, although clearly separated, are very closely related (progenitor-derivative species pair). Transplantation experiments have shown that the two species are less well adapted to the habitat of the other species. These experiments further suggested a selective role of insect herbivory in summer and exposure to wind and cold in the winter months. The present project investigates the genomic pattern of ecological differentiation in sympatry. A genome scan was performed using Amplified Fragment Length Polymorphisms (AFLP) in 193 individuals from three populations representing Oe. conioides and two representing Oe. aquatica. Simulations, using estimates of effective population size from our data, identified nine out of 552 AFLP loci (1.6%) as potentially under divergent selection. In addition to showing higher differentiation than expected, this diffentiation is mainly between the two species and their associated habitats. The nine loci thus are good candidates for being involved in the genetic and ecological differentiation seen between the two species. The proportion of outliers found is similar to other studies of non-allopatric differentiation and is in accordance with a process of species differentiation where a small number of loci is under divergent selection and the rest of the genome is permeable to gene flow. The construction of a genetic map showing the location of these outlier loci is still underway.
