Abstract
The monotremes (platypuses and echidnas) represent one of only four extant venomous mammalian lineages. Until recently, monotreme venom was poorly understood. However, the availability of the platypus genome and increasingly sophisticated genomic tools has allowed us to characterize platypus toxins, and provides a means of reconstructing the evolutionary history of monotreme venom. Here we review the physiology of platypus and echidna crural (venom) systems as well as pharmacological and genomic studies of monotreme toxins. Further, we synthesize current ideas about the evolution of the venom system, which in the platypus is likely to have been retained from a venomous ancestor, whilst being lost in the echidnas. We also outline several research directions and outstanding questions that would be productive to address in future research. An improved characterization of mammalian venoms will not only yield new toxins with potential therapeutic uses, but will also aid in our understanding of the way that this unusual trait evolves.
Highlights
Mammalian VenomVenom has evolved independently across multiple vertebrate and invertebrate lineages [1]
Toxins 2014, 6 disruptive substance that is secreted from a specialized gland and delivered via a specialized delivery system [1], there are four lineages of venomous mammals
Platypuses have up to 4 mL of venom available to be injected at any one time [17], in practice, the small diameter of the spur aperture (~0.2 mm) and associated high pressure required for injection mean that injected venom volumes around 100 μL are predicted [5]
Summary
Venom has evolved independently across multiple vertebrate and invertebrate lineages [1]. The venomous mammalian lineages have been largely unstudied until recently. Toxins 2014, 6 disruptive substance that is secreted from a specialized gland and delivered via a specialized delivery system [1], there are four lineages of venomous mammals. The increasing availability of genomic techniques has resulted in renewed possibilities for the characterization of mammalian venom. This review aims to highlight the current state of knowledge and propose hypotheses of venom evolution in one mammal lineage, the monotremes
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