Mitochondrial ghost lineages blur phylogeography and taxonomy of Natrix helvetica and N. natrix in Italy and Corsica

Abstract

Grass snakes are widely distributed across the Western Palearctic. Recent phylogeographic studies provided evidence that three distinct parapatric species exist. Two of these occur in Italy, Natrix helvetica and N. natrix, and a contact zone between both taxa has been suggested for north‐eastern Italy. Moreover, previous investigations revealed for the Italian Peninsula a complex phylogeographic structure. Using mtDNA sequences and microsatellite loci, we examined the situation for mainland Italy, Sicily, Sardinia, and Corsica. Our study confirmed the occurrence of N. natrix in north‐eastern Italy. Cline analyses revealed limited gene flow between N. helvetica and N. natrix across a narrow hybrid zone. Within N. helvetica, conflicting patterns of mitochondrial and nuclear genomic differentiation were revealed. Three nuclear genomic clusters were found; one of them corresponded to no fewer than five distinct and, in part, deeply divergent and ancient mitochondrial lineages from mainland Italy and Sicily. This cluster was paraphyletic with respect to the two remaining mitochondrial lineages, each of which matched with another nuclear genomic cluster (one from Corsica plus Sardinia and another one from western Europe north of the Alps). This unexpected pattern most likely results from mainly male‐mediated gene flow and female philopatry combined with population‐density‐dependent processes such as ‘high‐density blocking’. With respect to taxonomy, we propose to synonymize N. h. lanzai Kramer, 1970 with N. h. sicula (Cuvier, 1829), acknowledging their lacking nuclear genomic differentiation. The studied hybrid zone of N. h. helvetica and N. h. sicula in Italy is wide, with a smooth cline for nuclear markers, supporting their subspecies status. We found no evidence for the distinctiveness of the two subspecies from Corsica (N. h. corsa) and Sardinia (N. h. cetti), suggesting their synonymy, but refrain from taxonomic conclusions because of small sample sizes and the endangered status of the Sardinian taxon.

Reconstructing hotspots of genetic diversity from glacial refugia and subsequent dispersal in Italian common toads (Bufo bufo)

“…This distribution closely matches phylogeographic patterns in co-distributed taxa (e.g. 20 , 30 – 34 ). However, the spatial distribution of genetic variation within the range of the southern lineage of B. bufo contrasts with that found in other species studied with enough sampling depth, typically comprising finer population structure and multiple population units arranged along the south-north axis 18 , 20 , 31 , 34 – 38 .…”

“…At first glance, the geographic distribution of the northern lineage, albeit incompletely captured by our sampling design (see 27 ), appears in concordance with the distribution of several phylogeographic lineages sandwiched between the Alps and the northern Apennines 34 – 36 , 44 . However, in B. bufo this probably involved a unique biogeographic route, encompassing two distinct colonisation events of the river Po plain, one from the east and one from the west.…”

“…The geographic distribution of the central lineage parallels the distribution of intraspecific lineages in other organisms, including reptiles 34 and amphibians 26 – 37 , as well as several cryptic endemic species 32 . In this case, however, concordance also involves the spatial and temporal features of phylogeographic reconstructions.…”

“…The phylogeographic boundaries between lineages, although sharper with mtDNA than with the microsatellite markers, coincide with two well-known suture zones of the Italian peninsula: the lower Volturno-Calore river basin and the northern Apennines 12 . The extent of phylogeographic concordance with previously studied organisms from the same area is substantial, in terms of the co-distribution of intraspecific lineages 20,[30][31][32][33][34][35][36][37] . However, the spatial pattern of genetic variation observed within B. bufo populations shows some remarkable features, that may well exemplify the disparity between co-distribution of phylogeographic lineages and correlated population histories.…”

“…20,[30][31][32][33][34] ). However, the spatial distribution of genetic variation within the range of the southern lineage of B. bufo contrasts with that found in other species studied with enough sampling depth, typically comprising finer population structure and multiple population units arranged along the south-north axis 18,20,31,[34][35][36][37][38] . The south of the Italian peninsula has been recurrently identified as a hotspot of intraspecific genetic diversity 14,18,31 .…”