Geography and host specificity: two forces behind the genetic structure of the freshwater fish parasite Ligula intestinalis (Cestoda: Diphyllobothriidae).

Parasite species with global distributions and complex life cycles offer a rare opportunity to study alternative mechanisms of speciation and evolution in a single model. Here, genealogy and genetic structure, with respect to geography and fish host preference, have been analyzed for Ligula intestinalis, a tapeworm affecting freshwater fish. The data analyzed consisted of 109 tapeworms sampled from 13 fish host species in 18 different localities on a macrogeographic scale. Two mitochondrial genes, cytochrome oxidase subunit I and cytochrome B, and the nuclear sequence of intergenic transcribed spacer 2 (ITS2) were used for the genetic reconstruction. Different evolutionary patterns were found at the local and at the global geographic scales. On a local scale, the flat genetic structure was mainly attributed to contiguous range expansion. Migrating birds are the most likely cause of the homogenisation of the whole population, preventing the creation of significant genetic barriers. By contrast, on a global scale, genetically distant and well-separated clusters are present in different geographic areas. Reproductive isolation was found even between clades living in sympatry and infecting the same definitive host, suggesting the existence of efficient biologically determined genetic barriers, and thus possibly separate species. Although the ITS2 sequences were found to display considerable intragenomic variability, their relationships were generally in good agreement with the topology derived from mitochondrial genes.