A biogeographic and ecological perspective to the evolution of reproductive behaviour in the family Salamandridae

Amphibians have a complex reproductive behaviour, which shows the highest diversity among tetrapodes. The family Salamandridae, distributed across the entire Holarctic, is one of the most diverse groups of extant salamanders comprising 114 species in 21 genera. The family has a remarkable diversity of courtship modes, amplexus and sperm transfer. It is often hypothesised that this diversity has evolved in adaptation to a specific mating and/ or breeding habitat. We test this hypothesis based upon a phylogenetic reconstruction using the complete mitochondrial genome sequences of 45 Salamandridae species, representing all existing genera. We used ancestral character state reconstruction methods and geographic range models and applied relaxed Bayesian molecular clock models to discuss the results in a temporal framework of Salamandridae evolution. Our results show that the family Salamandridae started to diversify in the Late Cretaceous (ca. 87 mya) and is of Western Palearctic origin. Ancestral character state reconstruction predicts that its common ancestor was oviparous, mated on land without amplexus and probably showed a pin wheel spermatophore transfer, which is still found in the Italian endemic Salamandrina terdigidata. Our results suggest that several colonization of continents with subsequent radiations took place, once to the Nearctic and twice into Eastern Asian realms. However, these events were only in one case associated with a change in mating behaviour (dorsal amplexus in Nearctic newts). Around the Cretaceous-Paleogene boundary (K-Pg boundary) several Salamandridae lineages further diverged, again with no obvious changes in mating behaviour. Overall, there is no significant signal for mating character evolution being caused by changes in habitat type, with only a slight tendency that changes in mating habitat might have led to changes in the type of sperm transfer which in turn was associated with changes in the presence or absence of amplexus.