In this paper, we review briefly the current state of knowledge about sexual differentiation in eutherian mammals, and then describe the situation in detail in two marsupial species: the North American opossum and the tammar wallaby. The conventional explanation for the genesis of all male somatic sexual dimorphisms in mammals is that they are a consequence of the systemic action of testicular hormones. In the absence of testes, the embryo will develop a female phenotype. We present evidence for the tammar wallaby that calls into question the universal applicability of this hormonal theory of mammalian sexual differentiation. We have shown that extensive somatic sexual dimorphisms precede by many days the first morphological evidence of testicular formation, which does not occur until around the third day of pouch life. Male foetuses, and pouch young on the day of birth, already have a well-developed gubernaculum and processus vaginalis, paired scrotal anlagen, and a complete absence of mammary anlagen, whereas female foetuses and newborn pouch young have a poorly developed gubernaculum and processus vaginalis, no scrotal anlagen, and well-developed mammary anlagen. Because it seems unlikely that the male gonad could begin hormone secretion until after the Sertoli and Leydig cells are developed, our results strongly suggest that some sexually dimorphic somatic characteristics develop autonomously, depending on their genotype rather than the hormonal environment to which they are exposed. We have been able to confirm the hormonal independence of the scrotum, pouch and mammary gland by administering testosterone propionate daily by mouth to female pouch young from the day of birth; although the Wolffian duct was hyperstimulated, there was no sign of scrotal development, or pouch or mammary inhibition. When male pouch young were treated with oestradiol benzoate in a similar fashion, there was hyperstimulation of the Müllerian duct and inhibition or pouch or testicular migration and development, but no sign of scrotal inhibition or pouch or mammary development. Our results in the tammar wallaby are consistent with the earlier studies on the opossum, whose significance was not appreciated at the time. Further evidence in support of this hormonal independence comes from earlier studies of spontaneously occurring intersexes in several species of marsupial, including the opossum and the tammar wallaby. An XXY individual had intra-abdominal testes and complete masculinization of the male reproductive tract internally, but externally there was a pouch and mammary glands and no scrotum. A similar picture was found in two XY individuals. On the other hand, an XO individual had hypoplastic ovaries, normal development of the female reproductive tract internally, and an empty scrotum. Thus the scrotum can develop in the absence of a testis, whereas the pouch and mammary glands can develop in the presence of one. These results suggest a fundamental dichotomy between marsupials and eutherians in their sex-determining mechanisms. Although both subclasses probably require a Y-linked gene or genes for testis determination, marsupials appear to use other X-linked genes to control the development of structures such as the scrotum, pouch and mammary glands. In eutherians, on the other hand, scrotal and mammary development appears to be entirely under hormonal control. The lack of any genetic interchange between the X and the Y during meiosis in marsupials has presumably resulted in a much greater degree of genetic isolation of one sex chromosome from the other than is the case in eutherians, and the small size of the marsupial Y suggests that marsupials may have progressed further than eutherians in capture of genetic material by the X from the ancestral Y. Marsupials seem destined to play a vital role in the years to come in the mapping of sex-linked genes and determining their modes of action. Clearly they have much to tell us about the evolution of sex-determining mechanisms in all mammals.
Sex Maintenance in Mammals
