Abstract
Deeply diverged yet hybridizing species provide a system to investigate the final stages of the speciation process. We study a hybridizing pair of salamander species—the morphologically and genetically drastically different newts Triturus cristatus and T. marmoratus—with a panel of 32 nuclear and mitochondrial genetic markers. Morphologically identified hybrids are mostly of the F1 generation and mothered by T. cristatus. The sex ratio of the F1 hybrid class is reciprocally skewed, with a preponderance of females in T. cristatus‐mothered hybrids and males in T. marmoratus‐mothered hybrids. This amounts to the Haldane effect operating in one direction of the cross. Deeper generation hybrids are occasionally produced, possibly including F1 hybrid × backcross hybrid offspring. Interspecific gene flow is low, yet skewed toward T. cristatus. This asymmetry may be caused by hybrid zone movement, with the superseding species being predisposed to introgression. The persisting gene flow between deeply differentiated species supports the notion that full genetic isolation may be selected against. Conversely, published morphological data suggest that introgressive hybridization is detrimental, with digital malformations occurring more frequently in the area of sympatry. Finally, to assist field identification, both within the area of natural range overlap and concerning anthropogenic introductions elsewhere, we document the phenotypical variation of two generations of hybrids compared with both parental species. We suggest that fluctuating range boundaries, ecological segregation, cytonuclear incompatibilities and hybrid breakdown through Bateson–Dobzhansky–Muller incompatibilities all contribute to species integrity, despite incomplete isolation during secondary contact.
Highlights
It has long been realized that new genetic variants introduced to populations through gene flow can improve a population's response to natural selection (Fisher, 1930; Fitzpatrick & Reid, 2019)
Tail tips for genetic analysis were collected from newts at breeding sites in the département Mayenne in the west of France, which is situated within the area of T. cristatus and T. marmoratus range overlap (Figure 1)
Alien alleles are defined as those found in T. marmoratus but that are diagnostic for T. cristatus, and vice versa
Summary
It has long been realized that new genetic variants introduced to populations through gene flow can improve a population's response to natural selection (Fisher, 1930; Fitzpatrick & Reid, 2019). One example of a hybridizing pair of species that represents the “far side of the speciation continuum” (Hendry et al, 2009; Shaw & Mullen, 2014) is that of the crested newt Triturus cristatus (Laurenti, 1768) and the marbled newt T. marmoratus (Latreille, 1800). These species are distinct from the perspective of morphology and genetics, and last shared a common ancestor ca. We document the phenotypes of various hybrid classes in comparison with those of both parental species, from photographs of the ventral sides of adults
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