Orthologs of magainin, PGLa, procaerulein-derived, and proxenopsin-derived peptides from skin secretions of the octoploid frog Xenopus amieti (Pipidae)

The Volcano clawed frog Xenopus amieti Kobel, du Pasquier, Fischberg, and Gloor, 1980, with a chromosome number of 2n = 72, is believed to have undergone two rounds of genome duplication since evolving from a diploid ancestor. Nine peptides with differential antimicrobial activity against Escherichia coli and Staphylococcus aureus were isolated from norepinephrine-stimulated skin secretions of X. amieti that showed structural similarity to peptides previously isolated from the tetraploid frog X. laevis (2n = 36) and the diploid frog Silurana (formerly Xenopus) tropicalis (2n = 20). Two peptides (magainin-AM1 and -AM2) are othologous to the magainins, two peptides (PGLa-AM1 and -AM2) orthologous to peptide glycine-leucine-amide, four peptides (CPF-AM1, -AM2, -AM3, -AM4) orthologous to caerulein-precursor fragments, and one peptide (XPF-AM1) structurally similar to xenopsin-precursor fragments were characterized. CFP-AM1 (GLGSVLGKALKIGANLL.NH₂) was the most potent peptide present in the secretions and magainin-AM2 (GVSKILHSAGKFGKAFLGEIMKS) was the most abundant. The data indicate that nonfunctionalization has been the most common fate of duplicated antimicrobial peptide genes following the polyploidization events in the X. amieti lineage. However, the very low antimicrobial activity of the magainin-AM1 and PGLa-AM2 paralogs suggests the possibility that certain peptides may have evolved toward a new, as yet undetermined, function (neofunctionalization).