Low reproductive isolation and highly variable levels of gene flow reveal limited progress toward speciation between European river and brook lampreys

Ecologically based divergent selection is a factor that could drive reproductive isolation even in the presence of gene flow. Population pairs arrayed along a continuum of divergence provide a good opportunity to address this issue. Here we used a combination of mating trials, experimental crosses and population genetics analyses to investigate the evolution of reproductive isolation between two closely related species of lampreys with distinct life histories. We used microsatellite markers to genotype over 1000 individuals of the migratory parasitic river lamprey (Lampetra fluviatilis) and freshwater-resident nonparasitic brook lamprey (L. planeri) distributed in 10 sympatric and parapatric population pairs in France. Mating trials, parentage analyses and artificial fertilizations demonstrated a low level of reproductive isolation between species even though size assortative mating may contribute to isolation. Most parapatric population pairs were strongly differentiated due to the joi...