Coexistence of two newt species in a transition zone of range overlap

Theory suggests that spatial segregation of similar, co-occurring species may be driven by alternative innate life history and dispersal strategies, and that it operates through catastrophic events. An inventory of the evolutionary closely related small-bodied newts Lissotriton helveticus and L. vulgaris in the northwest of France demonstrated the species’ spatial partitioning, with L. vulgaris dominating in two pond-rich and historically disturbed coastal areas and L. helveticus prevailing inland where ponds are sparser. Population numbers were followed over several decades (1975–2021) in a pond within the narrow (ca. 2000 m wide) species transition zone. Early in the temporal survey (1986) a massive die-off was observed of two-third of the L. helveticus breeding population from a late frost event. Yet, the contribution of L. helveticus to the newt assemblage was more or less stable around 60%, even though the total population size fluctuated by an order of magnitude. Lissotriton vulgaris and a third species, Ichthyosaura alpestris, made up ca. 30% and 10% of the total till 1993, after which date their relative contributions reversed. These data suggest that a state shift may have occurred among the latter two species and that the assumed two-species dynamics of Lissotriton underlying the study has been an oversimplification. The local decline of L. vulgaris is paralleled by the loss of well-vegetated ponds from the wider agricultural terrain that affects this species more than L. helveticus and I. alpestris.

A lot of convergence, a bit of divergence: environment and interspecific interactions shape body color patterns in Lissotriton newts

Coexistence with related species poses evolutionary challenges to which populations may react in diverse ways. When exposed to similar environments, sympatric populations of two species may adopt similar phenotypic trait values. However, selection may also favor trait divergence as a way to reduce competition for resources or mates. The characteristics of external body parts, such as coloration and external morphology, are involved to varying degrees in intraspecific signaling as well as in the adaptation to the environment, and consequently may be diversely affected by interspecific interactions in sympatry. Here, we studied the effect of sympatry on various color and morphological traits in males and females of two related newt species Lissotriton helveticus and L. vulgaris. Importantly, we did not only estimate how raw trait differences between species respond to sympatry, but also the marginal responses after controlling for environmental variation. We found that dorsal and caudal coloration converged in sympatry, likely reflecting their role in adaptation to local environments, especially concealment from predators. In contrast, aspects of male and female ventral coloration, which harbours sexual signals in both species, diverged in sympatry. This divergence may reduce opportunities for interspecific sexual interactions and the associated loss of energy, suggesting reproductive character displacement (RCD). Our study emphasizes the contrasting patterns of traits involved in different functions and calls for the need to consider this diversity in evolutionary studies.

Divergence of species-specific protein sex pheromone blends in two related, nonhybridizing newts (Salamandridae)

In animals that use chemical communication during courtship and reproduction, speciation is often associated with divergence of their sex pheromones. In multicomponent pheromone systems, divergence can be obtained either by adding or deleting components, or by altering the relative contribution of individual components to the mixture. Protein pheromone systems can additionally evolve by amino acid sequence divergence to produce pheromones with a species-specific effect. The sodefrin precursor-like factor (SPF) pheromone system, a blend of proteins that essentially enhances receptivity in salamanders, has had a long and dynamic evolution of gene duplications, but the mechanisms that govern interspecific divergence and the role they play in reproductive isolation remain elusive. Here, we use transcriptomics and proteomics to characterize the SPF protein repertoire of the alpine newt (Ichthyosaura alpestris), and compare it to the previously identified repertoire of SPF proteins of the palmate newt (Lissotriton helveticus), a related but nonhybridizing species. Subsequent phylogenetic analyses indicate that, despite the availability of multiple SPF gene copies, both species predominantly express the same subset of orthologs. Our study demonstrates that species specificity in the SPF protein pheromone system can be established by gradual sequence divergence of the same set of proteins alone.

Widespread introgression does not leak into allotopy in a broad sympatric zone

Species that overlap over a large part of their range and habitat requirements are challenging for the study of speciation and hybridization. In this respect, the study of broadscale introgressive hybridization has raised recent interest. Here we studied hybridization between two closely related amphibians Lissotriton helveticus and Lissotriton vulgaris that reproduce over a wide sympatric zone. We used mitochondrial and microsatellite markers on 1272 individuals in 37 sites over Europe to detect hybrids at the individual-level and to analyse Hardy-Weinberg and linkage disequilibria at the population-level. Morphological traits showed a strong bimodal distribution. Consistently, hybrid frequency was low (1.7%). We found asymmetric introgression with five times more hybrids in L. vulgaris than in L. helveticus, a pattern probably explained by an unequal effective population size in a study part wherein L. helveticus numerically predominates. Strikingly, significant levels of introgression were detected in 73% of sites shared by both species. Our study showed that introgression is widespread but remains confined to the sites where the two species reproduce at the same time. This pattern may explain why these species remain genetically distinct over a broad sympatric zone.