Molecular analysis of an extensive specimen collection of morphologically distinct tapeworms of the genus Caryophyllaeus Gmelin, 1790 (Cestoda: Caryophyllidea), parasites of cyprinid fishes in the Palaearctic Region, brought evidence of host-related plasticity in critical morphological characters widely used for species circumscription and classification of these tapeworms. The results obtained here do not support the traditionally used morphological concept of species-defining characters of the order Caryophyllidea Carus, 1863, especially due to high morphological plasticity of the scolex and the anterior end of the body. Several morphotypes within both Caryophyllaeus laticeps (Pallas, 1781) and Caryophyllaeus brachycollis Janiszewska, 1953 generally corresponding to different hosts and geographical regions, were recognised likely suggesting host- and geography-related intraspecific morphological variability. Genetic data confirmed euryxenous host specificity of both species, most profoundly C. laticeps, but did not support the existence of cryptic species. In fact, some of the fish hosts may harbour both of the congeneric species including several of their respective morphotypes. The pattern of morphological and genetic divergence observed in both cestode species studied indicates a scenario of possible host range extensions and subsequent parasite diversification. If molecular sequence variability and host-related morphological polymorphism are confirmed in other groups of monozoic cestodes, it may complicate species identification and straightforward classification of these parasites.
There has been a long-standing debate as to whether Sympetrum striolatum (Charpentier, 1840) and the darker northern form, S. nigrescens (Lucas, 1912) should be recognised as separate species of dragonfly. Here we address this question using genetic analysis based on AFLP markers and samples collected from sites across the species' United Kingdom range. The program STRUCTURE finds no support for specific status. Instead, it reveals strong patterns of divergence between populations sampled from Scottish islands and those on the mainland, suggesting that salt water is a major barrier to gene flow. Thus, the dark form is quite likely to reflect a beneficial polymorphism that allows individuals to take advantage of short periods of warmer weather. Our AFLP markers appear to be very rapidly evolving, showing little or no overlap between congeneric species, and hence are ideally suited to answering questions relating to the levels of gene flow among populations within species.