MITOCHONDRIAL-DNA RATES AND BIOGEOGRAPHY IN EUROPEAN NEWTS (GENUS-EUPROCTUS)

Sequence divergence for segments of three mitochondrial DNA (mtDNA) ge nes encoding the 12S and 16S ribosomal RNA and cytochrome b was examin ed in newts belonging to the genus Euproctus (E. asper, E. montanus, E . platycephalus) and in three other species belonging to the same fami ly (Salamandridae), Triturus carnifex, T. vulgaris, and Pleurodeles wa ltl. The three Euproctus species occur (one species each) in Corsica, Sardinia, and the Pyrenees. This vicariant distribution is believed La have been determined by the disjunction and rotation of the Sardinia- Corsica microplate from the Pyreneean region. Because time estimates a re available for the tectonic events that led to the separation of the three landmasses, we used sequence data to estimate rates of evolutio n for the three gene fragments and investigated whether they conform t o the rate-constancy hypothesis, Dy the Tajima (1993, Genetics 135:599 -607) test, we could not detect rate heterogeneities for the ribosomal genes and for transversions in the cytochrome b gene. Assuming that t hese sites are evolving linearly over time and that cessation of gene flow occurred simultaneously with vicariant events, we compared the ti me of divergence estimated by molecular distances with the divergence times based on the geological estimates. Because we have two estimates of divergence time from the geological record, the split of Corsica/S ardinia from the Pyrenees and the split of Corsica from Sardinia, we c ould compare ratios of molecular divergence with the ratio of geologic al time divergence. The ratios are very similar, indicating that the m olecular clade hypothesis cannot be rejected. These geological events also allowed us to calculate absolute rates of evolution for ribosomal and cytochrome b genes and compare them to rates for the same regions in other salamandrids and other vertebrates. Ribosomal mtDNA rates ar e comparable to those reported for other vertebrates, bur cytochrome b rates are 3-7 times lower in salamanders than in other ectotherms. Fr om a phylogenetic perspective, our data suggest that the cladogenic Ev ents leading to species formation in Euproctus and Triturus occurred v ery closely in time, indicating that the two genera may not be monophy letic. A duplication of the cytochrome b gene in T. carnifex was found , and the implications of this finding for mtDNA phylogenetic studies are discussed.