Genetic and Morphological Differentiation of Common Toads in the Alps and the Apennines

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

“…In line with previous studies 25 , 26 , 28 , mitochondrial and microsatellite markers consistently identified three main lineages of Bufo bufo , ranging in southern, central and northern Italy. 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 spatial distribution of these clusters shows a strong geographical signal: one cluster is widespread from the Alps to the northern side of the northern Apennines; a second cluster spans from the northern to the central Apennines; a third one spans from the central to the southern Apennines and Sicily and a fourth cluster is restricted to the Provence and to the westernmost Ligurian populations. This cluster is strongly differentiated from the other clusters and represents populations of B. spinosus (see also 27 , 28 ). Bar-plots showing individual admixture proportions and pie-charts showing the average proportion of each cluster in each sampled population are presented in Fig.…”

“…Finally, the absence of two sub-groups of B. bufo in northern Italy in the nuclear dataset, and the occurrence of B. spinosus in the putative range of the e2 sub-clade (Fig. 3 ) might be explained by a late expansion of B. spinosus in the north-west, which then hybridised with local B. bufo populations picking up the e2 mtDNA lineage in the process 27 , 28 .…”

“…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. Indeed, this lineage is characterised by a single nuclear gene pool (see also 28 ), but two distinct mtDNA sub-lineages, with western and central—eastern European distributions, respectively, coincident with the e2 and e3 lineages of 25 . Arntzen et al 27 suggested that the two colonisation events occurred after the LGM, eastward from a western refugium, and westward from the northern Balkans.…”

“…Thus, the minimal level of genetic diversity observed in this area fits better with a scenario of early disappearance of-probably small and insular-population units prior to the most recent expansion event in southern Italy and Sicily. The geographic distribution of the central lineage parallels the distribution of intraspecific lineages in other organisms, including reptiles 34 and amphibians [26][27][28][29][30][31][32][33][34][35][36][37] , as well as several cryptic endemic species 32 . In this case, however, concordance also involves the spatial and temporal features of phylogeographic reconstructions.…”