Background: Monotremes (echidna and platypus) are egg-laying mammals. One of their most unique characteristics is that males have crural glands that are seasonally active. Male platypuses produce venom in significant quantities during the breeding season, presumably to aid in competition against conspecifics. Platypus envenomation of humans causes excruciating pain and prolonged swelling that is unresponsive to conventional painkillers. The fact that male echidnas also produce secretions during the breeding season was only discovered recently. Given that echidnas are not able to erect their spurs, the composition and purpose of these secretions remains to be determined.
Methods: We used Illumina sequencing to identify highly expressed genes in the crural gland of both species.
Results: Peptides identified which are likely to be responsible for the unique symptoms of platypus envenomation included serine proteases, metalloproteinases, antimicrobials, cytokines and protease inhibitors. The echidna crural gland transcriptome appeared markedly different to the platypus with no correlation between the top 50 most highly expressed genes between the datasets. Gene ontology terms associated with the top 100 most highly expressed genes in echidna, but not platypus, showed functional terms associated with steroidal and fatty acid production.
Conclusions: The non-aggressive behavior of the animal during the breeding season, the structure of the spur, along with the histology of the echidna venom gland suggests that echidna venom has a vastly different purpose to platypus venom. Our early results support that echidna crural gland proteins may function in chemical communication consistent with a role in reproduction and that over evolutionary time the echidna crural gland must have lost its venomous capabilities.