Influence of the immature testis on sexual differentiation in the tammar wallaby, Macropus eugenii (Macropodidae: Marsupialia)

Reproduction in the tammar wallaby, Macropus eugenii (Desmarest), is highly seasonal in the females but not the males. This study was designed to determine whether the difference is established during early life as a result of exposure to the developing testes. At day 10 after birth, when the sex can be distinguished externally, testes were removed from males and placed under the flank skin of females, while other groups of males and females were subjected to surgery without interfering with the gonads. The testis grafts remained palpable for 3-6 months. Sex-chromosome constitution was confirmed by karyotyping. At 3 years of age, the body weights and dimensions of the grafted females were not significantly different from those of the sham-operated females, whereas those of the castrated males were significantly larger and were equal to those of the sham-operated males, indicating that there is genetical control of growth independent of the testis in this species of marsupial. During 5 years of observations, none of the grafted females ever produced young, whereas all of the sham-operated females produced young each year from the second year. The grafted females had a mixture of male and female reproductive structures. The pouch and mammary glands developed normally, as did the Mullerian duct derivatives, the vaginal complex, the uteri and the oviducts. The ovaries were either devoid of oocytes and follicles or had reduced numbers, the Wolffian ducts were retained to varying degrees, the urogenital strand had developed into a prostate indistinguishable in size and structure from that of intact males, and the genital tubercle had developed into a normal-sized penis with a crus penis and Cowper's glands. In the castrated males, the scrotum developed normally and contained the gubernaculum and vas deferens. There was no evidence of Mullerian duct derivatives, and the urogenital strand was a simple canal, as in females. There were no Cowper's glands and no penis or erectile tissue. In one hemicastrated male, there was no development of the penis, although the remaining testis occupied the scrotum and showed compensatory hypertrophy. These findings indicate that the testis, at day 10, has a profound influence on the early differentiation of the Wolffian ducts, prostate and penis but cannot influence the differentiation of the Mullerian duct derivatives. The testis does not have any effect on the development of the pouch, mammary glands or scrotum or on somatic growth, all of which are apparently under independent genetical control.