The unusual nature of the symptoms of platypus envenomation may be due to the complex mixture of many different peptides and proteins in the venom, some of which have unknown function ( de Plater et al. Injection of venom into humans results in immediate and acute pain and swelling, which is not alleviated by normal first aid techniques or drugs such as morphine ( Fenner et al. The crural system can also be used as a defensive weapon, and the occasional unwary fisherman, zookeeper, or biologist may be spurred when handling male platypuses during the breeding season ( Grant 1995). Evidence from field observations, including increased spurring behavior and male-directed aggression in the breeding season, a high incidence of spur marks on males ( Temple-Smith 1973), and the development of the crural system only in the male, suggest that it has evolved primarily as an offensive weapon for use in conspecific aggression to assert dominance over other male platypuses ( Temple-Smith 1973). In the nonbreeding season the platypus crural system is not functional: The venom gland regresses, its secretory epithelium becomes inactive, and males show little spurring activity. The venom has mild proteolytic activity and, in laboratory experiments, causes cutaneous anaphylaxis in rabbits and rapid death in mice after intravenous doses of 75–90 mg/kg ( Temple-Smith 1973). The platypus venom gland is seasonally active and secretes venom only during the short annual breeding season ( Temple-Smith 1973). Collectively, this is known as the crural system.
Male platypuses possess a hollow spur on each hind leg, which is connected via a duct to a venom gland ( Grant 1995). This same pattern of cysteine interconnections is observed in some components of platypus venom called the OvDLPs ( Ornithorhynchus venom defensin- like peptides). Animal defensins possess a distinctive pattern of six cysteine residues, and are categorized into alpha-, beta-, and theta-defensins according to the particular arrangement of disulfide bonds between cysteine residues ( Lehrer and Ganz 2002). Defensins are cysteine-rich peptides with antimicrobial properties that have wide distribution, having been identified in plants, insects, and vertebrates ( Patil et al. To gain more insight into the basic biology of the platypus immune system, we searched the platypus genome for defensin genes that may play a role in protecting altricial platypus young.
comm.) and could also protect platypus hatchlings. Milk-derived antimicrobial peptides play an important role in immune protection of marsupial young (B.G. Hatchlings remain in the burrow for about 130 d ( Holland and Jackson 2002), during which time they suckle from maternal “milk patches” ( Grant 1995). 1998) and comparable in development to a newborn marsupial ( Manger et al. The incubation period for eggs is thought to be only 6–10 d ( Grant 1995), so at hatching, the nestlings are only about 15 mm long ( Grant 1995 Manger et al. The gestation period of the platypus probably lasts between 15 and 21 d ( Holland and Jackson 2002). The monotremes have features that place them definitively within the class Mammalia but are unique in laying eggs rather than giving birth to live young. Convergent evolution has repeatedly selected genes coding for proteins containing specific structural motifs as templates for venom molecules. Here we show that several venom components have evolved separately in the platypus and reptiles. This led us to trace the evolutionary origins of other components of platypus and reptile venom. Intriguingly, some species of snakes and lizards also produce venoms containing similar molecules called crotamines and crotamine-like peptides.
The OvDLP genes are located adjacent to the beta-defensins and share similar gene organization and peptide structures.
We show that gene duplication and subsequent functional diversification of beta-defensins gave rise to these platypus OvDLPs. 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. The platypus genome sequence provides a unique opportunity to unravel the evolutionary history of many of these interesting features. Rather than delivering venom through a bite, as do snakes and shrews, male platypuses have venomous spurs on each hind leg. It is a most remarkable mammal, not only because it lays eggs but also because it is venomous. 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.
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