Anomalous rates of evolution of pancreatic polypeptide and peptide tyrosine-tyrosine (PYY) in a tetraploid frog, Xenopus laevis (Anura:Pipidae)

The South African clawed frog Xenopus laevis is believed to have arisen as a result of a tetraploidization event occurring approximately 30 million years ago. Two molecular forms of pancreatic polypeptide (PP) have been isolated from an extract of the pancreas of this species and two molecular forms of peptide tyrosine-tyrosine (PYY) from the intestine. Despite the fact that the amino acid sequence of PP has, in general, been very poorly conserved during the evolution of tetrapods (only Pro⁵, Pro⁸, Gly⁹, Ala¹², Tyr²⁷, Arg³³ and Arg³⁵ are invariant among species studied so far), the two Xenopus PPs differ by only a single amino acid substition (Asp²²→Glu). In contrast the two molecular forms of PYY differ by six amino acid substitutions (Glu¹⁵→Gln, Thr¹⁸→Ala, Leu²¹→Met, Ile²²→Thr, Ile²⁸→Val, Val³¹→Ile). The data imply that strong evolutionary pressure is acting to conserve the functional domain in both genes encoding PP and so suggest that PP may have an important physiological role in amphibians (although the nature of this role has yet to be determined). The more rapid mutation of the functional domain in the genes encoding PYY, a peptide whose amino acid sequence has been quite well conserved in tetrapods and whose physiological significance is well established, suggests that one of the PYY genes may be evolving towards a new function or towards becoming a pseudogene.