Marsupials are often neglected, and although they comprise 6% of living mammal species they have unique characteristics that have taught us so much about the control of mammalian reproduction and development. Marsupials give birth to tiny highly altricial young after a relatively short gestation period supported by a chorio-vitelline placenta. At under 5 mg, the honey possum newborn is the smallest neonate of any mammal. Marsupials complete much of their development within the pouch, dependent on a long and highly sophisticated lactation. They can even produce milk of totally different compositions from adjacent mammary glands. The changing milk composition exquisitely regulates each stage of development: marsupials have effectively replaced their short-lived placenta with the long-lived mammary gland. Unlike all other mammals, their blastocysts have no inner cell mass, so the enigma of which cells are stem cells remains. Contrary to the dogma that marsupials are aplacental, they have a fully functional placenta providing nutritional exchange, respiratory exchange and hormonal regulation with genomic imprinting.
A major difference is the timing of gonadal differentiation which takes place post-natally after delivery of their altricial young, allowing easy treatments whilst the young is in the pouch. We discovered some unexpected findings that overturned the powerful Jost paradigm when we demonstrated a number of hormone-independent sexual dimorphisms before testicular differentiation. Because of their post-natal gonadal development, we achieved testicular, prostatic and phallus sex reversal after treatment with oestrogen in vivo and in vitro. We discovered an alternate pathway of androgen metabolism that we now know underpins human congenital adrenal hyperplasia and polyendocrine metabolic ovarian syndrome. Most recently, we have been focusing on the differences in gene expression and differential responses of the testis and ovary in the critical first weeks after birth, and responses to exposure to the EEDs atrazine and bisphenol A.