Mitogenomic Perspectives on the Origin and Phylogeny of Living Amphibians

Abstract

Establishing the relationships among modern amphibians (lissamphibians) and their ancient relatives is necessary for our understanding of early tetrapod evolution. However, the phylogeny is still intractable because of the highly specialized anatomy and poor fossil record of lissamphibians. Paleobiologists are still not sure whether lissamphibians are monophyletic or polyphyletic, and which ancient group (temnospondyls or lepospondyls) is most closely related to them. In an attempt to address these problems, eight mitochondrial genomes of living amphibians were determined and compared with previously published amphibian sequences. A comprehensive molecular phylogenetic analysis of nucleotide sequences yields a highly resolved tree congruent with the traditional hypotheses (Batrachia). By using a molecular clock-independent approach for inferring dating information from molecular phylogenies, we present here the first molecular timescale for lissamphibian evolution, which suggests that lissamphibians first emerged about 330 million years ago. By observing the fit between molecular and fossil times, we suggest that the temnospondyl-origin hypothesis for lissamphibians is more credible than other hypotheses. Moreover, under this timescale, the potential geographic origins of the main living amphibian groups are discussed: (i) advanced frogs (neobatrachians) may possess an Africa-India origin; (ii) salamanders may have originated in east Asia; (iii) the tropic forest of the Triassic Pangaea may be the place of origin for the ancient caecilians. An accurate phylogeny with divergence times can be also helpful to direct the search for "missing" fossils, and can benefit comparative studies of amphibian evolution.

Global patterns of diversification in the history of modern amphibians

“…In contrast, we find several major clades to be considerably younger than previously estimated by using large mitochondrial data sets. Zhang et al (21) (22), based on complete mitochondrial sequences, inferred a late Jurassic/early Cretaceous [129 (109-152) Mya] age for the earliest plethodontid divergences. Our younger age estimates cannot be explained by differences in calibration point selection alone, because the mentioned studies either included one or few minimum time constraints or added maximum time constraints.…”

“…Recent analyses of a single-gene data set (19) have resulted in the first timetree for amphibian evolution but provided relatively broad confidence intervals for divergence times. Other molecular clock analyses have been focused on specific parts of the amphibian tree, such as the basal splits among and within the three orders (20,21) or the origin of single taxa (22)(23)(24)(25)(26). To obtain a more precise and comprehensive overview of amphibian net diversification through the Mesozoic and early Tertiary, we constructed an evolutionary timetree based on a 3.75-kb data set, combining one mitochondrial and four nuclear gene fragments for 171 amphibians.…”

A Bayesian approach on molecules and behavior: reconsidering phylogenetic and evolutionary patterns of the Salamandridae with emphasis on Triturus newts

“…These two calibration points are likely to be better estimators of the age of their nodes given that they produced older age estimates than the ones produced by the C3 and C5 calibration points (see methods and results sections). Interestingly, a dating for the Salamandridae root as old as ours C2-C4-based one has already been reported by Zhang et al (2005). Using fossil calibration points at the base of Tetrapoda, they proposed a divergence for Salamandridae of 139 mya (CI 119-160) on the basis of full mtDNA data for multiple amphibian species, but including only two Salamandridae.…”

Two New Species of Pachytriton from Anhui and Guangxi, China (Amphibia: Urodela: Salamandridae)

“…For comparisons, published DNA sequence data for 37 specimens of Pachytriton (Wu et al, 2010) and a single specimen of Par. hongkongensis (Zhang et al, 2005) were obtained from GenBank and added to our analyses (Table 1, Fig. 1, Appendix 1).…”