Spotlight on islands: on the origin and diversification of a new lineage of the Italian wall lizardPodarcis siculusin the western Pontine Islands

Abstract

Groups of proximate continental islands may conceal more tangled phylogeographic patterns than oceanic archipelagos as a consequence of repeated sea level changes, which allow populations to experience gene flow during periods of low sea level stands and isolation by vicariant mechanisms during periods of high sea level stands. Here, we describe for the first time an ancient and diverging lineage of the Italian wall lizard Podarcis siculus from the western Pontine Islands. We used nuclear and mitochondrial DNA sequences of 156 individuals with the aim of unraveling their phylogenetic position, while microsatellite loci were used to compare several a priori insular biogeographic models of migration with empirical data. Our results suggest that the western Pontine populations colonized the islands early during their Pliocene volcanic formation, while populations from the eastern Pontine Islands seem to have been introduced recently. The inter-island genetic makeup indicates an important role of historical migration, probably due to glacial land bridges connecting islands followed by a recent vicariant mechanism of isolation. Moreover, the most supported migration model predicted higher gene flow among islands sharing a longitudinal arrangement. Considering the threatened status of small insular endemic populations, we suggest this new evolutionarily independent unit be given priority in conservation efforts.

A study in scarlet: incipient speciation, phenotypic differentiation and conservation implications of the Podarcis lizards of the western Pontine Islands, Italy

“…The genetic distances found for the western Pontine with respect to the mainland lineages are comparable to those observed in other Podarcis species and even twice as much as, for example, between Podarcis waglerianus and its sister species, the endemic Aeolian wall lizard (Podarcis raffoneae) (Harris et al, 2005;Poulakakis et al, 2005;Lymberakis & Poulakakis, 2010, Psonis et al, 2017. The molecular clock was in line with the dating of the principal episodes of volcanic eruptions that led to the formation of the islands (Cadoux et al, 2005), suggesting an early Pliocene colonization of the palaeo-archipelago followed by a mid-Pleistocene make-up of the current genetic variation among the islands (Senczuk et al, 2018).…”

“…1). These populations were selected taking into account both the geography and the phylogenetic relationship assessed in previous studies (Podnar et al, 2005;Senczuk et al, 2017Senczuk et al, , 2018.…”

“…The latter is genetically close to the eastern Pontine group. Finally, the 'Sicilian' population (Palermo and Siracusa) belongs to a phylogeographical unit well differentiated from the continental and the Pontine populations (Podnar et al, 2005;Senczuk et al, 2017Senczuk et al, , 2018. A pairwise Procrustes ANOVA (RRPP; 1000 permutations) was then computed to assess the degree of shape divergence among phylogeographical units after having controlled for sexual dimorphism using the function advanced.procD.lm.…”

“…In addition, such genetic investigations underlined the inconsistency of almost all the subspecies described on the basis of morphological traits. The latest study, based on mitochondrial and nuclear sequences, described a new diverging lineage for the western Pontine Islands (Ponza, Zannone, Palmarola and Gavi islands), whereas populations from the eastern Pontine (Ventotene and Santo Stefano islands) were found to be genetically indistinguishable from the mainland populations (Biaggini et al, 2009;Senczuk et al, 2018). The genetic distances found for the western Pontine with respect to the mainland lineages are comparable to those observed in other Podarcis species and even twice as much as, for example, between Podarcis waglerianus and its sister species, the endemic Aeolian wall lizard (Podarcis raffoneae) (Harris et al, 2005;Poulakakis et al, 2005;Lymberakis & Poulakakis, 2010, Psonis et al, 2017.…”

“…First, we tested a phylogeographical hypothesis considering that the populations sampled belonged to five separate phylogeographical units. Indeed, according to the literature (Biaggini et al, 2009;Senczuk et al, 2018), the western Pontine Islands (Ponza, Zannone and Palmarola) belong to an ancient lineage distinct from the lineage inhabiting the eastern Pontine Islands (Ventotene and Santo Stefano), which is strictly related to one of the continental lineages. Moreover, Sicilian and continental populations belong to three distinct lineages (Tyrrhenian, Adriatic and Sicilian).…”