Phylogeny of vertebrate ADHs. Seven amphibian classes of ADH can be differentiated phylogenetically, where branches of each class are shown in a different color. The reliability of the Neighbour-joining (NJ) tree was tested by bootstrap analysis (1000 replicates). Within each class, branches were collapsed when bootstrap values were <80 with the exception of X. tropicalis ADH7. A second tree constructed following the Maximum-likelihood (ML) method (500 replicates) produced a similar topology. Figures at nodes are the scores from bootstrap resampling of the data, NJ values are in bold and ML values in italics. ADH sequences from X. laevis, X. tropicalis, A. carolinensis and P. sinensis are those described in the present manuscript and their accession numbers are provided in Tables 3 and 4. Accession numbers of other ADH sequences are compiled in Table 2. Alignment of all vertebrate ADHs included in the phylogenetic tree is presented in Additional file 15. Scale-bar represents substitutions per nucleotide.