Triploidy in a pair of hybridizing salamanders at the far end of the speciation continuum

Ecological and genetic interactions of species at the far end of the speciation continuum can often be studied in sympatry. Triturus cristatus (C) and T. marmoratus (M) are two deeply differentiated yet hybridizing salamander species that engage in a mosaic distribution over a wide zone of range overlap in the west of France. Interspecies hybrids are easy to distinguish from both parentals and occur at ca. 3% of the total breeding population. Most hybrids are thought to be F1 diploids (CM). We aim to identify triploid F1 hybrids with single nucleotide polymorphism (SNP) data and determine their frequency. Special attention to the genus Triturus is warranted because triploidy might in principle constitute an escape route to the enigmatic chromosome-1 syndrome that constitutes a long-term 50% genetic load. Species-specific signals for a panel of 30 SNPs, analysed with metric multidimensional scaling, suggest that triploid frequency is one out of 26 (3.8%), although a frequence of 18% was found in a larger sampling (N=100) that paid particular attention to aberrant hybrid phenotypes. Triploids with a CMM genetic configuration appear more frequent than the CCM counterpart. The triploid status could be confirmed for two CMM females, either by target capture or cellular morphometric data. One triploid individual relocated in the field showed a hybrid phenotype leaning towards T. marmoratus, an exceptional life span (17+ yrs), length (187 mm) and the frequent skipping of annual breeding opportunities. Future research on the T. cristatus - T. marmoratus system should consider the different classes of interspecies hybrids.