A landscape genetics perspective on the spatial dynamics of hybridization between two species of wall lizards

Understanding reproductive isolation and barriers to gene flow is essential to understand speciation. As such, hybrid zones, areas where genetically differentiated species come into contact and interbreed to some extent, allow the study of processes involved in the emergence of reproductive barriers, allowing to identify potential barriers to gene flow, to study the genomic patterns of selection and introgression, and to identify the phenotypic characters under selection. Podarcis bocagei and P. carbonelli are two lacertid lizards, endemic to the Iberian Peninsula. They provide a good model system to study speciation because they are known to interbreed in the wild to some extent, but also to maintain their genetic and morphological identities in their small contact area. In this work we studied the hybrid zone between these two species using a multidisciplinary approach. Specifically, we combined population genetics and morphological analyses with a fine-scale geographic mapping of individuals to understand the spatial dynamics of this hybrid zone. For molecular analyses we used mitochondrial DNA and a combination of microsatellites to evaluate the level of admixture of each individual. These molecular markers revealed a strongly bimodal hybrid zone and allowed the identification of 144 individuals of P. carbonelli and 33 of P. bocagei as well as 18 hybrids, resulting in a proportion of hybrids of 9.2%. Among these, we found hybrids of multiple classes (F1, F2, backcrosses with each parental form and other types of hybrids). Moreover, despite the presence of more P. carbonelli individuals in our sample, we found that the majority of hybrids had a P. bocagei maternal origin, indicating a directionality of crosses where females of P. bocagei breed with males of P. carbonelli and hybrid males. Our results provide some support to Haldane’s rule suggesting the reduced hybrid female fertility, although not complete unviability. These results suggest strong barriers to gene flow in the hybrid zone. Further, we used a combination of pholidotic (scalation) traits to examine the effects of hybridization on the morphology of the species by exploring the morphological properties of hybrids. Morphologically, P. bocagei and P. carbonelli are more similar in sympatric than in allopatric areas, allowing us to reject reinforcement acting at these morphological traits. The higher similarity between the two species at the contact zone can be related with introgression due to long-term hybridization. Hybrid individuals do not differ in morphology from the parental forms while no morphologically intermediate forms were identified. No relation was detected between genetic and morphological assignments of hybrid individuals. Therefore, morphological FCUP A landscape genetics perspective on the spatial dynamics of hybridization between two species of wall lizards vi traits do not follow genetic admixture patterns. Also, we examined the level of asymmetry of several bilateral traits to test whether hybridization could be associated to increased developmental instability with potential effects on the fitness of hybrids. In this case, no differences in developmental stability were detected between hybrids and parental forms. Together with a previous study, our results suggest that although rare, hybridization has been regularly occurring at this hybrid zone. As strong barriers to gene flow were suggested, we studied the spatial dynamics of the hybrid zone at the microgeographic scale, in order to obtain more insights on the factors limiting gene flow between these two wall lizards. A steep transition across the landscape was detected in genetic traits trough clinal analysis. On the other hand, morphological traits show a mosaic distribution across the area, possibly due to the high intra and interspecific morphological variability of the model species. Our results suggest a strong influence of a geographic barrier that seems to limit the contact between the two species. However, the low number of individuals with intermediate genotypes when compared to those with carrying pure genotypes, strongly suggest the existence of reproductive barriers between the two species. Further studies are required to identify these barriers and study its nature and its role impeding gene flow between these species. In the present study, the combination of genetic, morphological and spatial evidence allowed to characterise the features of this interesting hybrid zone providing important information that allowed us to better understand how geographic and reproductive barriers interplay and limit gene flow between these two Podarcis species. This study also highlights the importance of multidisciplinary approaches in hybrid zone research and how these approaches allow to investigate how species integrity is maintained, to enhance biological interpretation of the results, eventually leading to a better understanding of the processes driving speciation.