Monotremes are a small but remarkable group of mammals that consists of a single species of duckbill platypus (Ornithorhynchus anatinus) found in Australia and three (perhaps four) species of spiny anteaters (echidnas) found in Australia and New Guinea.
These animals retain several traits of their therapsid ancestors including
- a cloaca - the final segment of the digestive tract into which both the urinary and reproductive tracts empty (monotreme = single hole);
- lay shelled eggs that undergo merobastic cleavage like that of reptiles (and birds) rather than the holoblastic cleavage of all other mammals.
Despite these reptilian features, the monotremes meet all the criteria of true mammals:
- milk secreted from mammary glands (but no nipples)
- teeth (only in the young; they are lost in adult montremes)
In the May 8 issue of Nature, a consortium of gene sequencers reported the results of sequencing the complete genome of the platypus. They identified 18,527 protein-coding genes distributed on 52 chromosomes.
The mix of mammalian and reptilian phenotypic features turns out to be reflected in the genome as well. Examples:
- The platypus has genes for egg yolk proteins that are also found in birds but not in therians.
- The gene content of their X chromosomes resembles that of the Z chromosome in birds, not the X chromosome of other mammals like us.
Other features of their genome reflect their unique biology:
- The platypus produces a venom with genes which in other mammals encode for antimicrobial peptides called defensins.
- The platypus has some 1000 genes for receptors in its vomeronasal organ — far more than found in other mammals. The platypus hunts for food underwater and probably uses these receptors to detect prey (as well as using its electroreceptors for this purpose).