The short-beaked echidna (Tachyglossus aculeatus) is one of only four extant species of egg-laying mammals (Monotremata: three echidnas and one platypus), but despite its common and ubiquitous distribution throughout Australia, information on its reproductive biology is limited. What is known is that the reproductive biology of the echidna is anatomically and behaviourally distinct in comparison with both marsupial and eutherian mammals. After a short gestation, the egg is incubated in the pouch and once hatched, the young puggle continues its development while residing in the pouch, sucking milk form the mammary patches.
In conjunction with Currumbin Wildlife Sanctuary (Queensland) we have begun to investigate echidna reproduction and development. The newly hatched puggle is developmentally remarkably similar to a marsupial neonate, but nothing at all is known of the genes and hormones that influence the growth and development of the embryo in utero or in ovo. Together with the platypus, the echidna has a unique sex chromosome make up (echidna 5Xs:4Ys; platypus 5Xs and 5Ys), but as yet there is no evidence of Sry. We have begun to examine the developmental anatomy of the echidna embryo and pouch young and to look at the expression of genes involved in sex determination pathways in the gonads of other mammals. The phallus already appears as a distinct structure, while the gonads have a typically mammalian embryonic appearance in that the ovaries remain elongated by day 3 post-hatching, whereas the testis has already begun to round up. Many of the common mammalian sex-determining genes are present in the gonads of the echidna at the RNA level and we have optimised antibodies for a number of these and will be examining their protein expression. Together, these results will provide the first insights into the evolution of sex-determining pathways between monotremes, marsupials and eutherians.