Combined-data phylogenetics and character evolution of Clitellata (Annelida) using 18S rDNA and morphology

This is the first phylogenetic analysis of Clitellata using 18S rDNA in combination with morphological data of a selection of species representing Hirudinida, Acanthobdellida, Branchiobdellida, and eight oligochaetous families. The morphological data set includes 48 somatic (light-microscopical) and 34 spermatozoal (ultrastructural) characters. Eight new sperm models belonging to Lumbriculidae (two), Enchytraeidae (two), Phreodrilidae (one), and Tubificidae (three) are compared with the spermatozoal pattern already described among Clitellata. Somatic characters for each species are extracted from both general literature and the original species description. One new 18S sequence of Lumbriculidae and two of Tubificidae are reported, and are aligned together with corresponding sequences of 36 previously studied clitellate taxa. Two polychaete species are used as outgroups. The phylogenetic trees recovered using parsimony and Bayesian inference as optimization criteria of both individual and combined data sets yield largely consistent results. Our combined-data phylogenetic analysis is congruent with recent molecular studies. Somatic and spermatozoal characters contribute to the 18S rDNA phylogeny under both optimization criteria: in resolving the 18S topology, in adding new nodes, and in increasing the support for many groups. Morphological characters in combination with 18S rDNA suggest the following sister-group relationships: (1) between Acanthobdella and Hirudinida, with Branchiobdellida as their plesiomorphic sister group, and (2) between enchytraeids and Propappus, with both taxa grouping at the base of a large assemblage containing Lumbricidae, Lumbriculidae, Branchiobdellida, Acanthobdella, and Hirudinida. Maximum parsimony and maximum likelihood ancestral character state reconstructions on the combined-data tree indicate a new set of somatic and spermatozoal autapomorphies, and propose new evolutionary trends of somatic and spermatological characters. The observed complexity of the spermatozoal characters patterns among oligochaetous clitellates is discussed. This analysis supports a trend from primarily aquatic forms, with bifid chaetae indefinite in number, towards a more terrestrial mode of life leading to a simplification of the chaetae, thus supporting the hypothesis that the first clitellate was an aquatic form.