Identification and functional characterization of a novel monotreme-specific protein expressed during lactation

 

Monotremes are the only oviparous mammals and exhibit a fascinating combination of reptilian and mammalian characters. They represent a component of synapsidal reproduction by laying shelled eggs which are incubated outside the mother’s body. This is accompanied by a prototherian lactation process, marking them as representatives of early mammals. Platypus, a semi- aquatic carnivore and two species of Echidna, a terrestrial insectivore, are the only extant monotremes and are limited to Australia and New Guinea. Lactation in these animals is prolonged relative to gestation and incubation of eggs. Milk is the only source of protection and nutrition for the eggs and the subsequent hatchlings which are highly altricial and immunologically incompetent. Much of the development of monotreme young occurs in the non-sterile outside environment,after hatching and before weaning and the role of the milk in this perspective needs to be studied. Using a non-invasive cDNA sequencing approach from cells harvested from monotreme milk, we have identified a novel transcript that is confined to monotremes. Tissue expression profile revealed that this gene is highly expressed in the milk cells. Further investigation with in vitro assays have validated antibacterial potential of this protein against E. coli, Staphylococcus species, Salmonella enterica and Pseudomonas aeruginosa. No antibacterial activity was detected against Enterococcus faecalis which is known to be a harmless commensal gut flora. These results support the hypothesis of protective attributes of the process of lactation during evolution and also emphasize the independent selection on milk provision, which can be linked to the differential developmental strategy displayed by the monotremes.

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