Defensins and the convergent evolution of platypus and reptile venom genes

Abstract

When the platypus (Ornithorhynchus anatinus) was first discovered, it was thought to be a taxidermist's hoax, as it has a blend of mammalian and reptilian features. It is a most remarkable mammal, not only because it lays eggs but also because it is venomous. Rather than delivering venom through a bite, as do snakes and shrews, male platypuses have venomous spurs on each hind leg. The platypus genome sequence provides a unique opportunity to unravel the evolutionary history of many of these interesting features. While searching the platypus genome for the sequences of antimicrobial defensin genes, we identified three Ornithorhynchus venom defensin-like peptide (OvDLP) genes, which produce the major components of platypus venom. We show that gene duplication and subsequent functional diversification of beta-defensins gave rise to these platypus OvDLPs. The OvDLP genes are located adjacent to the beta-defensins and share similar gene organization and peptide structures. Intriguingly, some species of snakes and lizards also produce venoms containing similar molecules called crotamines and crotamine-like peptides. This led us to trace the evolutionary origins of other components of platypus and reptile venom. Here we show that several venom components have evolved separately in the platypus and reptiles. Convergent evolution has repeatedly selected genes coding for proteins containing specific structural motifs as templates for venom molecules.

Strain-Specific Polymorphisms in Paneth Cell α-Defensins of C57BL/6 Mice and Evidence of Vestigial Myeloid α-Defensin Pseudogenes

“…The different orientations of several marker genes, including Defcr5, ribo L21, and several of the DefmaN-ps genes, further illustrate the divergence of the locus in these inbred strains. In contrast to the expansion and diversification of the ␣-defensin proteomes, the organization of ␤-defensin gene clusters is much more highly conserved and the primary structures of certain ␤-defensin peptides exhibit similarities across phylogenetic lines (41,42,57,65). The molecular basis of this disparity remains unknown, but perhaps it relates to receptor-mediated roles that have been identified for members of the ␤-defensin peptide family (4,7,15,49,(62)(63)(64).…”

“…Although ␣-defensin genes are absent in the cattle and dog genomes (13,42), the peptides are expressed in distant mammalian species, including humans and primates as well as additional Euarchontoglires, such as horses (5,6), elephants, opossum, tenrecs (3,30), and the platypus (65,66).…”

Discovery and Characterization of Novel Cyclotides Originated from Chimeric Precursors Consisting of Albumin-1 Chain a and Cyclotide Domains in the Fabaceae Family

“…However, for this to happen, it would require massive mutations and substitutions for complete conversion from A1bs to cyclotides. An interesting example of convergent evolution has recently been reported for the platypus (54). The toxin genes identified from this animal show striking homology to those found in other venomous species such as snakes, starfish, and sea anemones.…”

Genome analysis of the platypus reveals unique signatures of evolution

“…Notably, duplications in each of the b-defensin, C-type natriuretic peptide and nerve growth factor gene families have also occurred independently in reptiles during the evolution of their venom 47 . Convergent evolution has thus clearly occurred during the independent evolution of reptilian and monotreme venom 48 . Immunity.…”

“…However, a direct test The diagram illustrates separate gene duplications in different parts of the phylogeny for platypus venom defensin-like peptides (vDLPs), for lizard venom crotamine-like peptides (vCLPs) and for snake venom crotamines. These venom proteins have thus been co-opted from pre-existing non-toxin homologues independently in platypus and in lizards and snakes 48 .…”