The comparative genomics of monotremes, marsupials, and pinnipeds: models to examine the functions of milk proteins

Milk has many functions, ranging from the provision of factors crucial to the operation of the mammary gland and the development of the suckled young through to their protection from infection. Lactation evolved about 200 million years ago with the aplacental, egg-laying monotremes, but since that time there has been extensive adaptation to reproduction, including a large repertoire of lactation strategies. This chapter discusses three animal models with extreme adaptation to lactation, and examines the option of exploiting their comparative biology to identify milk protein bioactives that may have potential in functional foods or pharmaceuticals. The echidna (Tachyglossus aculeatus, a monotreme) has a fascinating combination of reptilian and mammalian characteristics. It retains a primitive component of reptilian reproduction in the form of laying shelled eggs, but it also has a prototherian lactation process. The tammar wallaby (Macropus eugenii, a marsupial) has adopted a reproductive strategy that includes a short gestation (26.5 days), birth of an immature young, and a relatively long lactation (300 days). The composition of milk changes progressively during the lactation cycle, and these changes in milk composition subsequently control development of the young. The tammar can also practice concurrent asynchronous lactation; the mother provides a concentrated milk for an older animal that is out of the pouch, and a dilute milk from an adjacent mammary gland for a newborn pouch young. The third study species, the Cape fur seal (Arctocephalus pusilluspusillus, a eutherian), has a lactation characterized by a repeated cycle of long at-sea foraging trips (up to 23 days) alternating with short suckling periods of 2–3 days ashore. Lactation almost ceases while the seal is offshore, but the mammary gland does not progress to involution and apoptosis.Technology platforms using genomics, proteomics, and bioinformatics have been used to exploit these models to identify milk bioactives. In addition, the availability of sequenced marsupial, dog, platypus, and bovine genomes permits rapid transfer of information to the cow to provide outcomes for the dairy industry.