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BMC Evolutionary Biology 2008, 8:56 http://www.biomedcentral.com/1471-2148/8/56

stitution rate, which ranged from 0.03–1% per lineage per gesting recent expansion, perhaps after its arrival on the
My (see refs. in the Discussion). The two markers, with mainland; however, a no growth scenario could not be
different substitution rates, yield similar 95% HPD time rejected statistically. Tajima's D did not support a demo-
intervals. graphic expansion in either of the balearicus groups sepa-
rately, or combined. The northeastern range limit of B.
Although the values reported should be regarded with the balearicus appears to be the Po River drainage, which may
caveats mentioned, it is highly improbable that the vicar- also be the southwestern boundary for B. viridis. Recent
iant event that separated the African and Sicilian green mapping of green toad distributions in Italy [38], how-
toads took place during the Messinian (5.3 Mya), that is, ever, shows an apparently continuous range from the
earlier than the range of dates estimated by this method. northeast (probable haplotype group of B. v. viridis) to the
Instead, it is very likely that the Africa-Sicily divergence is southwest (haplotype group of B. balearicus) across the Po
post-Messinian. In order to validate the hypothesis pro- drainage. It is likely that the groups are in contact in that
posed here and to test competing paleobiogeographic sce- area; discerning contacts and possible hybridization
narios, a formal comparative phylogeographic study dynamics represent challenging future research topics.
including more genetic markers and other terrestrial spe- Indeed, the Po drainage seems to be a biogeographic bor-
cies in Sicily and North Africa would be necessary. Several der and/or contact zone between variously related taxa of
new comparative phylogeographic approaches have been amphibians and reptiles (for overview [65]: Rana latastii/
proposed [55-57], each requiring the inclusion of more R. italica; Hyla arborea/H. intermedia [66]; Bombina varie-
data for a more reliable statistical inference to be gata/B. pachypus [67]; Rana lessonae/R. bergeri[68]) or, it is
obtained. considered a "source of genetic variability" [69].

Historical biogeography of Bufo balearicus Fossils demonstrate the presence of green toads on the
The second major result of our study is that green toads Balearic Islands from the Upper Pleistocene of Mallorca
from most of the Apennine Peninsula, Corsica, Sardinia [70], but early human introduction from Corsica was pro-
and the Balearic Islands represent a separate taxon, which posed as the source of green toads on these islands [35].
is different from other Eurasian green toads on the mito- Our data show toads from Mallorca and Menorca to be
chondrial and nuclear levels. Three B. balearicus subclades nested within the mitochondrial clades from the Apen-
emerged from our mtDNA control region data; the subc- nine Peninsula and Corsica/Sardinia, and thus corrobo-
lade on Corsica and Sardinia exhibited basal haplotypes rate serum albumin data on toads from Mallorca and
and the most substantial structure. These observations Corsica that prove them more closely related to each other
suggest speciation of B. balearicus on these islands around than to green toads from Palestine, Africa, and Greece
0.9 Mya to 1.8 Mya (Table 3). The earliest green toad fos- [35]. We cannot locate the exact geographic origin of Bal-
sils in Italy are known from the Late Miocene from the earic green toads with our current data; faster evolving
northeast (Ravenna Province) and from the Pliocene in nuclear markers and denser sampling in regions of possi-
the southeast (Gargano region) of the Apennine Penin- ble origin are required. Our data also corroborate provi-
sula [58]. They have also been identified from the Pleis- sional allozyme data (Lattes, A. 1997, Abstr. 3rd World
tocene of Corsica and Sardinia [59]. Corsica and Sardinia Congr. Herpetol., Prague: p. 123) that associated toads
had landbridges to (or were separated only by narrow from Corsica, Sardinia and NW-Italy (all from the B. bal-
marine straits from) the Apennine Peninsula during sev- earicus range) with a ~0.25 Nei's distance from toads from
eral geological periods (~18 Mya; 9 Mya; 5.3 Mya [60]). Vienna based on a UPGMA dendrogram (B. viridis; [71]
Although a narrow strait may have mostly separated Cor- quoting Lattes' abstr.).
sica/Sardinia from Tuscany [61], paleontological [62,63]
and phylogeographic data (Discoglossus [25], isopods Although Calabria consisted of islands until the Pleis-
[64]) suggest limited early Pleistocene faunal (perhaps tocene [72], episodic Upper Pleistocene faunal exchange
oversea) exchange between these islands and the main- across the Strait of Messina is well documented by the fos-
land. sil record [72]. While few amphibians crossed the Strait of
Messina [67], B. balearicus haplotypes have been detected
This same signature is suggested by our data. The baleari- in northeastern Sicily (loc. 20).
cus clade widespread on the Apennine Peninsula dis-
played a mismatch distribution visually, but not Biogeography of Bufo siculus n. sp
statistically fitting that expected for a population that Our phylogeographic findings agree with paleogeo-
underwent a sudden expansion (Figure 3). Similarly, the graphic and fossil data [72], which suggest a long Plio-
coalescent simulation program Fluctuate estimated a Pleistocene isolation of Sicily. After their "out of Africa"
growth parameter ten times higher for the peninsular origin, green toads apparently spread across most of Sic-
group than for the island balearicus clade (Table 2), sug- ily, where they are well known from the Pleistocene fossil

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