Projekte

The central issue of this thesis is the understanding of evolutionary mechanisms, that maintain and drive the divergence of populations and can lead to sympatric speciation. A former study showed that two populations of perch (Perca fluviatilis L.) co-exist in Lake Constance. For the first time I provide empirical evidence, that socially mediated divergence (kin- and population preference) in combination with ecological factors (difference in spawning times) could explain the origin and persistence of the perch subpopulations. Divergence between perch populations could be reinforced by reduced hybrid fitness.
My results support that both populations originated from a single source population because analysis of the variability of mitochondrial DNA (mtDNA D-loop sequencing) showed no difference in haplotype frequencies between the eastern and western subpopulations of perch in Lake Constance. Moreover, perch originated most likely from the Danube River because Danube and Lake Constance perch share the same distinct haplotype, whereas perch from the Rhine (the second possible refuge) lack this special haplotype. This is consistent with the geological history of Lake Constance with a known connection between the lake and the Danube during lake formation in the Pleistocene (app. 15 000 years ago). Subsequent upstream colonization from the Rhine is unlikely because there has always been a impassable waterfall.
A microsatellite analysis on the genetic structure of larval perch in the pelagic zone of Lake Constance showed that perch form shoals of closely related conspecifics. To test if this behaviour is due to a preference for members of their own subpopulation, including kin, I investigated active choice for olfactory cues in laboratory reared perch. Juvenile perch were tested in a two-channel flume for preference of odours from different conspecifics of known relatedness. Three different choice experiments were conducted: holding water from kin (three different sibling groups were tested: full-sibs, maternal half sibs or paternal half sibs) was always tested vs. holding water of non-kin belonging to the same subpopulation. Perch significantly preferred kin over non kin, there was no difference between the different kinship tests. In a second experiment population recognition was tested. Juvenile perch significantly preferred unrelated members of their own versus members of the foreign population. From these experiments I conclude that kin and population coherence can be an active choice based on olfactory preference, which could easily lead to assortative mating.
An important constraint on disruptive population divergence is selection against hybrids. Therefore I measured the fertilization and hatching success of F1 hybrids of the two populations compared to those within each parental population. Two different F1 hybrids (mother eastern population, father western population and vice versa) were produced by artificial fertilization. Compared to the eastern and western populations, fitness was significantly lower for both hybrids. These laboratory measures of hybrid fitness suggest that some genetic incompatibility has already accumulated in the parental populations. Therefore, genetic divergence between the two perch populations in Lake Constance seems to be underway and is reinforced by reduced hybrid fitness.
Differences in lake basin morphology could lead to ecological separation of the two subpopulations based on asynchrony in spawning time/location. In a preliminary morphological analysis slight but significant differences could be found between the two subpopulations. Differences in morphological traits did not affect characters, that are commonly driven by ecological speciation but reflect an accumulation of neutral genetic differences between reproductively isolated subpopulations.
To conclude, I provide empirical evidence for a socially mediated divergence, that may drive sympatric speciation of perch: asynchrony in spawning time and location cause ecological separation of perch into subpopulations. The resulting disruptive population divergence is enhanced by olfactory preference for kin and conspecifics of the same population, and is reinforced by selection against hybrids.