Sperm models in European Plecoptera

Systematic issues regarding Plecoptera are still debated, and the molecular data seem to be unable to definitively clarify the relationships within the order. Spermatozoa are under constant evolutionary pressure, and comparative spermatology can be useful in carrying systematic and phylogenetic information. In the present paper we describe the sperm structure, using light, scanning and transmission electron and immunofluorescence microscopy, of six Euholognatha species belonging to genera not analyzed in our previous studies, i.e. Capnopsis, Amphinemura, Rhabdiopteryx, Tyrrhenoleuctra, Zwicknia and Protonemura. The spermatozoa of all the species examined are fîliform and have a flagellum characterized by an axoneme with 9 + 9+2 pattern and two mitochondrial derivatives. Their ultrastructure shows a degree of heterogeneity within the order. On the contrary, morphological features of sperm are well conserved inside a single Euholognathan family, and the species share a general family sperm model, even if different interspecific or intergeneric characters can be identified and used for systematic inferences. Among Nemouroidea, Taeniopterygidae, showing a peculiar sperm model, seems to have an isolated phylogenetic position. Nemouridae, with a mono-layered acrosome, are isolated among the remaining families, while we can hypothesize a sister taxa relationship between Leuctridae and Capniidae. As regards Perloidea, the sperm characters suggest a closer relationship between Chloroperlidae and Perlodidae, rather than between Perlidae and Perlodidae, as commonly hypothesized.