Sibling species in interstitial flatworms: a case study using<Emphasis Type="Italic"> Monocelis lineata</Emphasis> (Proseriata: Monocelididae)

The genetic relationships between morphologically indistinguishable marine and brackish-water populations of Monocelis lineata (O.F. Müller, 1774) (Proseriata: Monocelididae) were analysed by means of allozyme electrophoresis. Fifteen samples of M. lineata (13 from the Mediterranean and two from the Atlantic) from coastal marine and brackish-water habitats were examined for variation at 18 loci. Eleven loci were polymorphic in at least one population of M. lineata. Low levels of within-population genetic variability were found, with average observed and expected heterozygosity values ranging from H_o=0.015±0.015 to 0.113±0.044, and from H_e=0.028±0.028 to 0.138±0.054, respectively. The occurrence of a number of private alleles indicated a marked genetic divergence among populations of M. lineata, with Rogers genetic distances ranging from D_R=0.003 to 0.676 and a highly significant F_ST value (0.918±0.012, P<0.001). UPGMA (unweighted pair-group method using arithmetic average) cluster analysis and multidimensional scaling showed a clear genetic divergence between marine and brackish-water populations. Moreover, Atlantic and Mediterranean populations were sharply separated. Our results suggest that M. lineata is a complex of sibling species.Communicated by R. Cattaneo-Vietti, Genova