Islet hormones from the African bullfrog Pyxicephalus adspersus (Anura : Ranidae): Structural characterization and phylogenetic implications

The African bullfrog Pyxicephalus adspersus is generally classified along w ith frogs of the genus Rana in the subfamily Raninae of the family Ranidae but precise phylogenetic relationships between species are unclear. Pancrea tic polypeptide (PP), insulin, and glucagon-like peptide (GLP-1) were isola ted from an extract of P. adspersus pancreas and characterized structurally . A comparison of the amino acid sequence of Pyxicephalus PP (APSEPQ-HPGG(1 0)QATPEQLAQY(20)YSDLYQYITF(30)ITRPRF . NH2) with those of the known amphibi an PP molecules in a maximum parsimony analysis generates a single phylogen etic tree in which Pyxicephalus is the sister to the clade comprising the m embers of the genus Rana. The three orders of living amphibians form discre te clades with the representative of the Gymnophiona appearing as sister to the Caudata-Anura. In contrast, Pyxicephalus insulin (A chain, GIVEQC-CHSA (10) (CSLYDLENYCN)-N-20; B-chain, LANQHLCGSH(10)- LVEALYMVCG(10)ERGFFYYKS(3 0)) and and GLP-1 (HAEGTFTSDM(10)TSYLEEKAAK(20)EFVDWLIKGR(30)PK) resemble m ore closely the corresponding peptides from the cane toad Bufo marinus than the peptides from any species of Rana. Cladistic analysis based upon the a mino acid sequences of insulin produced a polyphyletic assemblage with the Gymnophiona nesting within an unresolved clade containing the non-ranid fro gs. The data support the assertion that the amino acid sequence of PP, but not those of the other islet hormones, is of value as a molecular marker fo r inferring phylogenetic relationships between early tetrapod species. (C) 2000 Academic Press.