Comparison of ribosomal DNA sequences of Lamellodiscus spp, (Monogenea, Diplectanidae) parasitising Pagellus (Sparidae, Teleostei) in the North Mediterranean Sea: species divergence and coevolutionary interactions

We sequenced DNA fragments from four monogenean species of the genus Lamell odiscus and their three fish host species from the genus Pagellus in the No rth Mediterranean Sea, in order to estimate the molecular divergence and th e coevolutionary interactions in this association. By comparing the ITS1 se quences of the parasites, we assessed their level of interspecific differen ces and tested the phylogenetic status of Lamellodiscus virgula and Lamello discus obeliae, formerly described as two different species. Moreover, we w anted to know if closely related parasites used closely related hosts, to i nvestigate the coevolutionary interactions in this complex. Phylogenetic re lationships among Lamellodiscus species were estimated with partial 18S rib osomal DNA sequences while mitochondrial cytochrome-b DNA sequences were us ed for their fish hosts. The ITS1 sequences appear to be highly variable am ong Lamellodiscus species, except L. virgula and L. obeliae, suggesting an old divergence time or a rapid molecular evolution within this genus. This fish-parasite association seems to exhibit coevolutionary interactions. L.v irgula and L. obeliae are proposed to be a single species on the basis of t heir almost identical ITS1 sequences. (C) 2000 Australian Society for Paras itology Inc. Published by Elsevier Science Ltd. All rights reserved.