Ontogeny of androgen receptor-like immunoreactivity in the reproductive tract of male Monodelphis domestica

Ancestral androgenic differentiation pathways are repurposed during the evolution of adult sexual plasticity

Although early exposure to androgens is necessary to permanently organize male phenotype in many vertebrates, animals that exhibit adult sexual plasticity require mechanisms that prevent early fixation of genital morphology and allow for genital morphogenesis during adult transformation. In Lythrypnus dalli, a teleost fish that exhibits bi-directional sex change, adults display dimorphic genitalia morphology despite the absence of sex differences in the potent fish androgen 11-ketotestosterone. Based on conserved patterns of vertebrate development, two steroid-based mechanisms may regulate the early development and adult maintenance of dimorphic genitalia; local androgen receptor (AR) and steroidogenic enzyme expression. Consistent with the ancestral pattern of AR expression during the multipotential phase of differentiation, juvenile differentiation into either sex involved high mesenchymal AR expression. In adults, AR expression was high throughout the male genitalia, but low or absent in females. Consistent with the hypothesis that adult sexual plasticity repurposes pathways from primary differentiation, we show that adults with transitioning genitalia also exhibited higher AR expression relative to females. Local androgen biosynthesis may also participate in genitalia transformation, as transitioning adults had greater 11β-HSD-like immunoreactivity in the epithelial layer of the dorsal lumen compared to both sexes. By administering an AR antagonist to adult males, we show AR is necessary to maintain male-typical morphology. In a species that is resistant to early sexual canalization, early androgenic differentiation mechanisms are consistent with other vertebrates and the tissue-specific regulation of AR expression appears to be repurposed in adulthood to allow for transitions between sexual phenotypes.

The opossum genome: insights and opportunities from an alternative mammal

The strategic importance of the genome sequence of the gray, short-tailed opossum, Monodelphis domestica, accrues from both the unique phylogenetic position of metatherian (marsupial) mammals and the fundamental biologic characteristics of metatherians that distinguish them from other mammalian species. Metatherian and eutherian (placental) mammals are more closely related to one another than to other vertebrate groups, and owing to this close relationship they share fundamentally similar genetic structures and molecular processes. However, during their long evolutionary separation these alternative mammals have developed distinctive anatomical, physiologic, and genetic features that hold tremendous potential for examining relationships between the molecular structures of mammalian genomes and the functional attributes of their components. Comparative analyses using the opossum genome have already provided a wealth of new evidence regarding the importance of noncoding elements in the evolution of mammalian genomes, the role of transposable elements in driving genomic innovation, and the relationships between recombination rate, nucleotide composition, and the genomic distributions of repetitive elements. The genome sequence is also beginning to enlarge our understanding of the evolution and function of the vertebrate immune system, and it provides an alternative model for investigating mechanisms of genomic imprinting. Equally important, availability of the genome sequence is fostering the development of new research tools for physical and functional genomic analyses of M. domestica that are expanding its versatility as an experimental system for a broad range of research applications in basic biology and biomedically oriented research.

The role of androgens in development of the scrotum of the grey short-tailed Brazilian opossum ( Monodelphis domestica)

In eutherian mammals, sex differentiation is initiated by expression of the testis-determining gene on the Y chromosome. Subsequent phenotypic development of the reproductive tract and genitalia depends on the production of hormones by the differentiated testis. In marsupials the mechanisms of phenotypic development may vary from this pattern, as differentiation of the scrotal primordia has been shown to occur before that of the gonad. Thus, the development of the scrotum in the marsupial has been regarded as an androgen-independent process. We have sought to clarify the ontogeny of scrotal development and the appearance of androgen receptor immunoreactivity by examining Monodelphis domesticaembryos/pups from 1 day prior to birth until 2 days after birth. We have also used immunocytochemistry to determine the expression of the key steroidogenic enzyme 3beta-hydroxysteroid dehydrogenase as an indicator of when the developing gonad may be capable of synthesizing androgens. Expression of this enzyme was first detected in the gonads and adrenals of both sexes 1 day prior to birth and before the appearance of scrotal bulges. Androgen receptor immunoreactivity was detected in the scrotal anlagen of male opossum pups as early as 1 day following birth. This finding is significantly earlier than previous reports and coincides with the appearance 1 day after birth of distinct scrotal bulges. Androgen receptor immunoreactivity was also observed in the genital tubercles of male pups, but not female pups, 2 days after birth. These results suggest that androgens may play an important role in the development of the male genitalia at a much earlier stage than that indicated by previously published work and that scrotal development in this species may not be androgen-independent.

Androgens and the development of the vagina

Today it is generally held that the vagina develops from sinovaginal bulbs and that the lower third of the definitive vagina is derived from the urogenital sinus. Here we show that the entire vagina arises by downward growth of Wolffian and Müllerian ducts, that the sinovaginal bulbs are in fact the caudal ends of the Wolffian ducts, and that vaginal development is under negative control of androgens. We designed a genetic experiment in which the androgen receptor defect in the Tfm mouse was used to examine the effects of androgens. Vaginal development was studied by 3D reconstruction in androgen-treated female embryos and in complete androgen-insensitive littermates. In androgen-treated females, descent of the genital ducts was inhibited, and a vagina formed in androgen-insensitive Tfm embryos as it does in normal females. By immmunohistochemical localization of the androgen receptor in normal mouse embryos, we demonstrated that the androgen receptor was expressed in Wolffian duct and urogenital sinus-derived structures, and was entirely absent in the Müllerian duct derivatives. We conclude that the Wolffian ducts are instrumental in conveying the negative control by androgens on vaginal development. The results are discussed under evolutionary aspects at the transition from marsupial to eutherian mammals.

Sexual dimorphism in the central nervous system of marsupials

It is now evident that gonadal steroids, acting within a limited critical period during fetal or neonatal life, bring about sexual differentiation of both the reproductive tract and the central nervous system (CNS) in eutherians. This results in structural dimorphism in several regions of the brain and spinal cord and the programming of future patterns of adult reproductive behavior. At birth the CNS of marsupials is very underdeveloped and debate continues as to the importance of hormones in its sexual differentiation. Nevertheless, some sexually dimorphic regions have been identified, including the lateral septal nucleus in the hypothalamus and the spinal nucleus of the bulbocavernosus and dorsolateral nucleus in the spinal cord, but interestingly not the cremasteric nucleus, which is dimorphic in eutherians. To date, no apparent sex differences in estrogen and androgen receptor-immunoreactive structures have been detected in the marsupial brain; however, higher levels of aromatase activity during early development in male opossums have been reported. Sex differences have been identified in the localization of cholecystokinin-immunoreactive structures in the marsupial brain indicating that the expression of this neuropeptide is differentially regulated in each sex. A sex difference also exists in the density of arginine vasopressin-immunoreactive fibers. Arguments continue as to whether sexually dimorphic behavior in marsupials, as in eutherians, is largely predetermined by hormones acting on the CNS early in development or if it is entirely dependent on the adult steroid hormonal environment.

Developmental expression of the androgen receptor during virilization of the urogenital system of a marsupial

In the marsupial tammar wallaby, virilization begins approximately 3 wk after the onset of testosterone synthesis. In the eutherian mammal, in contrast, the onset of virilization of the male urogenital tract occurs shortly after the onset of androgen synthesis. Androgen action requires the presence of the androgen receptor to mediate a response in target tissues. We therefore investigated the developmental expression of the androgen receptor (AR) in both sexes of the tammar wallaby. AR gene transcript was detected in fetal gonad and brain as early as Day 19 of the 26.5-day gestation, 7 days earlier than the first rise in testicular testosterone (Days 0-5 postpartum [p.p.]). Immunoreactive AR was identified in the male urogenital sinus (UGS) 2 days before birth and in the female UGS and mammary glands by the day of birth. AR was present in the UGS, vagina, and prostate until Day 152 p.p., the oldest age examined. AR was identified in the gubernaculum testis at Day 2 p.p. and became more abundant by Day 32. In the phallus of both sexes, AR was identified by Day 4 p.p. and until Day 157, the oldest age examined. AR was not detected in the scrotum at any age from the day of birth to Day 157. Maturation of the phallus, wolffian duct, and epididymis was marked by appearance of epithelial immunostaining. AR was localized in the epithelium of the UGS in females by Day 50 p.p. but was not found in the epithelium of the male UGS up to Day 152 p.p., the oldest examined. AR were found in the mesenchyme of the UGS of male and female tammars 3-4 wk before virilization is first evident in the male at Day 25 p.p. We conclude that the presence of AR is not the initiating signal for virilization of the UGS in this marsupial male.