Characterisation of nine European wall lizard (Podarcis muralis) microsatellite loci of utility across sub-species

No evidence for differential sociosexual behavior and space use in the color morphs of the European common wall lizard (Podarcis muralis)

Explaining the evolutionary origin and maintenance of color polymorphisms is a major challenge in evolutionary biology. Such polymorphisms are commonly thought to reflect the existence of alternative behavioral or life-history strategies under negative frequency-dependent selection. The European common wall lizard Podarcis muralis exhibits a striking ventral color polymorphism that has been intensely studied and is often assumed to reflect alternative reproductive strategies, similar to the iconic "rock-paper-scissors" system described in the North American lizard Uta stansburiana. However, available studies so far have ignored central aspects in the behavioral ecology of this species that are crucial to assess the existence of alternative reproductive strategies. Here, we try to fill this gap by studying the social behavior, space use, and reproductive performance of lizards showing different color morphs, both in a free-ranging population from the eastern Pyrenees and in ten experimental mesocosm enclosures. In the natural population, we found no differences between morphs in site fidelity, space use, or male-female spatial overlap. Likewise, color morph was irrelevant to sociosexual behavior, space use, and reproductive success within experimental enclosures. Our results contradict the commonly held hypothesis that P. muralis morphs reflect alternative behavioral strategies, and suggest that we should instead turn our attention to alternative functional explanations.

Spatial variation in gene flow across a hybrid zone reveals causes of reproductive isolation and asymmetric introgression in wall lizards

Hybrid zones provide insights into the evolution of reproductive isolation. Sexual selection can contribute to the evolution of reproductive barriers, but it remains poorly understood how sexual traits impact gene flow in secondary contact. Here, we show that a recently evolved suite of sexual traits that function in male-male competition mediates gene flow between two lineages of wall lizards (Podarcis muralis). Gene flow was relatively low and asymmetric in the presence of exaggerated male morphology and coloration compared to when the lineages share the ancestral phenotype. Putative barrier loci were enriched in genomic regions that were highly differentiated between the two lineages and showed low concordance between the transects. The exception was a consistently low genetic exchange around ATXN1, a gene that modulates social behavior. We suggest that this gene may contribute to the male mate preferences that are known to cause lineage-assortative mating in this species. Although female choice modulates the degree of reproductive isolation in a variety of taxa, wall lizards demonstrate that both male-male competition and male mate choice can contribute to the extent of gene flow between lineages.

Genetic differentiation predicts body size divergence between island and mainland populations of common wall lizards (Podarcis muralis)

Small-bodied vertebrates sometimes evolve gigantism on islands, but there is a lack of consistent association with ecological factors or island characteristics. One possible reason is that, even if the ecological conditions are right, body size might fail to diverge on islands that were isolated recently or if there is gene flow between islands and the mainland. We studied body size, ventral colour polymorphism and genetic structure across nine islands and adjacent mainland populations of common wall lizards (Podarcis muralis) off the western coast of France. Population genetic data suggested that island populations might have maintained gene flow after their geographical isolation from the mainland. Island lizards were larger and heavier than mainland lizards on average, but the extent of gigantism varied substantially between islands. Island size and distance from the mainland were poor predictors of body size, but lizards from populations that were highly genetically differentiated from the mainland were larger than lizards from less differentiated populations. Colour morphs that were rare on the mainland tended to be more common on islands. We propose that genetic isolation or bottlenecks promote body size evolution in island lizards, which makes it challenging to identify ecological causes of island gigantism without complementary genetic information.

Male behaviour drives assortative reproduction during the initial stage of secondary contact

Phenotypic divergence in allopatry can facilitate speciation by reducing the likelihood that individuals of different lineages hybridize during secondary contact. However, few studies have established the causes of reproductive isolation in the crucial early stages of secondary contact. Here, we establish behavioural causes of assortative reproduction between two phenotypically divergent lineages of the European wall lizard (Podarcis muralis), which have recently come into secondary contact. Parentage was highly assortative in experimental contact zones. However, despite pronounced divergence in male phenotypes, including chemical and visual sexual signals, there was no evidence that females discriminated between males of the two lineages in staged interactions or under naturalistic free-ranging conditions. Instead, assortative reproduction was driven by male mate preferences and, to a lesser extent, male-male competition. The effects were more pronounced when the habitat structure promoted high lizard densities. These results emphasize that assortative reproduction can occur in the absence of female choice and that male behaviour may play an important role in limiting hybridization during the initial stages of secondary contact.

Widespread primary, but geographically restricted secondary, human introductions of wall lizards, Podarcis muralis

Establishing the introduction pathways of alien species is a fundamental task in invasion biology. The common wall lizard, Podarcis muralis, has been widely introduced outside of its native range in both Europe and North America, primarily through escaped pets or deliberate release of animals from captive or wild populations. Here, we use Bayesian clustering, approximate Bayesian computation (ABC) methods and network analyses to reconstruct the origin and colonization history of 23 non-native populations of wall lizards in England. Our analyses show that established populations in southern England originate from at least nine separate sources of animals from native populations in France and Italy. Secondary introductions from previously established non-native populations were supported for eleven (47%) populations. In contrast to the primary introductions, secondary introductions were highly restricted geographically and appear to have occurred within a limited time frame rather than being increasingly common. Together, these data suggest that extant wall lizard populations in England are the result of isolated accidental and deliberate releases of imported animals since the 1970s, with only local translocation of animals from established non-native populations. Given that populations introduced as recently as 25 years ago show evidence of having adapted to cool climate, discouraging further translocations may be important to prevent more extensive establishment on the south coast of England.

Phylogeography and conservation genetics of the common wall lizard, Podarcis muralis, on islands at its northern range

Populations at range limits are often characterized by lower genetic diversity, increased genetic isolation and differentiation relative to populations at the core of geographical ranges. Furthermore, it is increasingly recognized that populations situated at range limits might be the result of human introductions rather than natural dispersal. It is therefore important to document the origin and genetic diversity of marginal populations to establish conservation priorities. In this study, we investigate the phylogeography and genetic structure of peripheral populations of the common European wall lizard, Podarcis muralis, on Jersey (Channel Islands, UK) and in the Chausey archipelago. We sequenced a fragment of the mitochondrial cytochrome b gene in 200 individuals of P. muralis to infer the phylogeography of the island populations using Bayesian approaches. We also genotyped 484 individuals from 21 populations at 10 polymorphic microsatellite loci to evaluate the genetic structure and diversity of island and mainland (Western France) populations. We detected four unique haplotypes in the island populations that formed a sub-clade within the Western France clade. There was a significant reduction in genetic diversity (HO, HE and AR) of the island populations in relation to the mainland. The small fragmented island populations at the northern range margin of the common wall lizard distribution are most likely native, with genetic differentiation reflecting isolation following sea level increase approximately 7000 BP. Genetic diversity is lower on islands than in marginal populations on the mainland, potentially as a result of early founder effects or long-term isolation. The combination of restriction to specific localities and an inability to expand their range into adjacent suitable locations might make the island populations more vulnerable to extinction.