Interpopulation genetic divergence in European grayling (Thymallus thymallus, Salmonidae) at a microgeographic scale: implications for conservation

In order to provide guidelines for conservation and management of the severely declining Lake Saimaa (eastern Finland) European grayling (Thymallus thymallus, Salmonidae), we investigated the microgeographic genetic structure of three populations from the water system using 17 microsatellite loci and mitochondrial DNA polymerase chain reaction-restriction fragment length polymorphism analysis. Microsatellites revealed low levels of intrapopulation genetic diversity and substantial divergence between populations sampled from spawning sites separated by as little as 55 kilometers. Mitochondrial analysis indicated the occurrence of two composite haplotypes within Lake Saimaa. The nucleotide substitution estimates between the haplotypes suggested their separation to markedly predate the late Pleistocene period. The populations exhibited marked frequency differences for the two mitochondrial haplotypes, reinforcing the view of limited interpopulation gene flow within Lake Saimaa. An individual based microsatellite Neighbor-Joining dendrogram demonstrated clustering of all the specimens according to their sampling origin. Individual assignment tests revealed 100% assignment success of individuals into their population of origin and 100% exclusion (p < 0.05) from all alternative reference populations. These findings exemplify that T. thymallus populations may be genetically highly structured over small geographical scales and provide a good starting point for the development of appropriate conservation strategies for Lake Saimaa grayling.