A molecular phylogeny of ‘true’ salamanders (family Salamandridae) and the evolution of terrestriality of reproductive modes

Abstract

Key innovations enable species to conquer new habitats. Within the family Salamandridae, particular adaptations to terrestrial life, such as the anatomy and physiology of the feeding apparatus, courtship behaviour and in some cases viviparity, allowed the ‘true’salamanders (genera Chioglossa, Mertensiella, Salamandra) to shift from a semi-aquatic to a more terrestrial life cycle. We sequenced 423 base pairs of the 16S RNA gene of the mitochondrial DNA for all species of the ‘true’salamanders. Based on the resulting phylogeny we discuss the evolution of terrestrial reproductive modes within this species group. We especially tested two hypotheses of monophyletic origin of specific adaptations to terrestriality within the ‘true’salamanders: Mertensiella caucasica caucasica/Mertensiella luschani, whose congeneric relationship has already been questioned on the basis of morphological, behavioural and molecular data, and Salamandra atra atra/Salamandra lanzai, the two species of Alpine salamanders, which are characterized by melanism and matrotrophic viviparity. We tested alternative tree topologies and included additional published and unpublished data on morphology, allozyme polymorphisms, and further mtDNA sequences. Maximum parsimony and maximum likelihood analyses always rejected the hypothesis of monophyly of the two Mertensiella species. Although data on courtship behaviour of ‘true’salamanders indicate the loss of a symplesiomorphic tail projection in Salamandra and Chioglossa, the most parsimonious explanation may at present be a convergent evolution of the projection as indicated by recent histological studies. Although our DNA sequence and additional allozyme data suggest that S. atra and S. lanzai do not form a monophyletic group despite their geographic proximity and shared traits, we cannot reject their monophyly. Using the flooding of the Strait of Gibraltar five million years ago for the separation of African and European species, a molecular 16S RNA clock was calibrated with 0.7% total sequence divergence per million years. Estimated times of divergence for further evolutionary splits within ‘true’salamanders coincide with paleogeographical data.