Hybridization and introgression in deeply differentiated salamander species – molecular genetics and a reappraisal of Dr. Louis Vallée’s osteological data

Abstract

Abstract Two morphologically and genetically distinct salamander species (the crested newt, Triturus cristatus and the marbled newt, T. marmoratus) engage over a large area in the west of France where they hybridize at ca. 4%. The species interaction is characterized by ecological differentiation and limited gene flow beyond the F1 hybrid generation. Incompletely isolated species like these allow to investigate the genetic mechanisms and evolutionary forces that maintain their identity in the face of ongoing gene flow, to which a large mosaic hybrid zone provides excellent opportunities. A reanalysis of published morphological data supports the partial breakdown of the species barrier whereas extensive genetic data show that introgression is low yet asymmetric, in line with the dynamics of species replacement inferred earlier. The current work, with seven diagnostic nuclear markers studied for a large sample, revises the estimates of introgression in both T. cristatus (to 0.24%) and T. marmoratus (to 0.11%). Difficulties remain in the recognition of potential triploid hybrids versus backcross hybrids. Haldane’s rule is partially supported, but deeper analyses require the use of a molecular marker for sex that is not yet available.