Inferences on the phylogenetic relationships of Succineidae (Mollusca, Pulmonata) based on 18S rRNA gene

The systematic position of Succineidae and its relationship with Athoracophoridae and other pulmonate mollusks is controversial. In this paper, we attempted to study their relationships in the light of the 18S rRNA gene. We determined complete sequences from the 18S rRNA gene for Omalonyx matheroni (Potiez & Michaud, 1835), a closely related species to Succinea putris, the type-species of the family Succineidae (Tillier, 1981), and Athoracophorus bitentaculatus (Quoy & Gaimard, 1832), the type-species of the family Athoracophoridae. All complete sequences of closely related pulmonate molluscs available from GenBank were included in the phylogenetic analysis in an attempt to find the positions of these taxa in the pulmonate tree. Comparative sequence analysis and base stacking energy methods were used to predict the secondary structure of the 18S rRNA gene. The secondary structure was used to resolve ambiguous nucleotide positions for our alignment. Stems E10-1, E23-1, 43 and 49 of the 18S rRNA gene were highly variable and the suggested secondary structure was presented for 21 mollusks. We used maximum parsimony and maximum likelihood methods for assessment of the relationships of the Succineidae. The 18S rRNA data indicate Succineidae and Athoracophoridae taxa form a monophyletic clade, the Elasmognatha. The Elasmognatha is positioned among the stylommatophoran molluscs. The order Stylommatophora is monophyletic. The subclass Pulmonata is not supported. Siphonariids are an early diverging lineage to all taxa or positioned as a sister taxon to the pulmonates. Furthermore this study indicated that siphonariids are not part of or even closely related to the main lineage of basommatophorans represented here by Planorbidae and Lymnaeidae.