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Virilization of the Male Pouch Young of the Tammar Wallaby Does Not Appear to be Mediated by Plasma Testosterone or Dihydrotestosterone¹

^a Department of Internal Medicine, The University of Texas Southwestern Medical Center, Dallas, Texas 75235 ^b Department of Zoology, University of Melbourne, Parkville, Victoria 3052, Australia

Virilization of the male urogenital tract of all mammals, including marsupials, is mediated by androgenic hormones secreted by the testes. We have previously demonstrated profound sexual dimorphism in the concentrations of gonadal androgens in pouch young of the tammar wallaby Macropus eugenii during the interval when the urogenital sinus virilizes. To provide insight into the mechanisms by which androgens are transported from the testes to the target tissues, we measured testosterone and dihydrotestosterone in plasma pools from tammar pouch young from the day of birth to Day 150. Plasma testosterone levels were measurable (0.5–2 ng/ml) at all times studied, but there were no differences between males and females. These low concentrations of plasma testosterone appear to be derived from the adrenal glands and not the testes. Plasma dihydrotestosterone levels in plasma pools from these animals were also low and not sexually dimorphic.

We conclude that virilization of the male urogenital tract cannot be explained by the usual transport of testosterone or dihydrotestosterone in plasma but may be mediated by the direct delivery of androgens to the urogenital tract via the Wolffian ducts. Alternatively, circulating prohormones may be converted to androgens in target tissues.

¹ Aided in part by grant DK03892 from the National Institutes of Health, by grant 980779 from the National Health & Medical Research Council of Australia, and by a grant from the Perot Family Foundation.

² Correspondence: Jean D. Wilson, Department of Internal Medicine, University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas TX 75235–8857. TEL: 214 648 3685; FAX: 214 648 8917;jwils1{at}mednet.swmed.edu

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