Post-natal imprinting: evidence from marsupials.

Genomic imprinting has been identified in therian (eutherian and marsupial) mammals but not in prototherian (monotreme) mammals. Imprinting has an important role in optimising pre-natal nutrition and growth, and most imprinted genes are expressed and imprinted in the placenta and developing fetus. In marsupials, however, the placental attachment is short-lived, and most growth and development occurs post-natally, supported by a changing milk composition tailor-made for each stage of development. Therefore there is a much greater demand on marsupial females during post-natal lactation than during pre-natal placentation, so there may be greater selection for genomic imprinting in the mammary gland than in the short-lived placenta. Recent studies in the tammar wallaby confirm the presence of genomic imprinting in nutrient-regulatory genes in the adult mammary gland. This suggests that imprinting may influence infant post-natal growth via the mammary gland as it does pre-natally via the placenta. Similarly, an increasing number of imprinted genes have been implicated in regulating feeding and nurturing behaviour in both the adult and the developing neonate/offspring in mice. Together these studies provide evidence that genomic imprinting is critical for regulating growth and subsequently the survival of offspring not only pre-natally but also post-natally.

A dual role for SHH during phallus development in a marsupial.

The mammalian phallus arises from identical primordia in both sexes and is patterned in part by the key morphogen Sonic hedgehog (SHH). We have investigated SHH and other morphogens during phallus development in the tammar wallaby. In this marsupial, testis differentiation and androgen production occurs just after birth, but it takes a further 50-60 days before the phallus becomes sexually dimorphic. One day before birth, SHH was expressed in both sexes in the urethral epithelium. In males, there was a marked upregulation of SHH, GLI2, and AR at day 50 postpartum, a time when testicular androgen production falls. SHH, GLI2, and AR were downregulated in female pouch young treated with androstanediol from days 24-50, but not when treatments were begun at day 29, suggesting an early window of androgen sensitivity. SHH, GLI2, and AR expression in the phallus of males castrated at day 23 did not differ from controls, but there was an increase in SHH and GLI2 and a decrease in FGF8 and BMP4 expression when the animals were castrated at day 29. These results suggest that the early patterning by SHH is androgen-independent followed by an androgen-dependent window of sensitivity and a sharp rise in SHH expression after androgen withdrawal at day 50.

Fibroblast growth factor-9 in marsupial testicular development.

FGF9 is a member of the fibroblast growth factor (FGF) family and is critical for early testicular development and germ cell survival in the mouse. Fgf9 reinforces the testis determinant Sox9 and antagonizes Wnt4, an ovarian factor. To determine whether FGF9 has a conserved role in the mammalian gonad, we examined its expression in the gonads of a marsupial, the tammar wallaby Macropus eugenii, and compared it to WNT4 expression. Marsupial FGF9 is highly conserved with orthologues from eutherian mammals, including humans. FGF9 protein was detected in both the testis and ovary before sexual differentiation, but it subsequently became sexually dimorphic during the period of testicular differentiation. The protein was specifically enriched in the seminiferous cords of the developing testis in the Sertoli and germ cells. FGF9 mRNA expression was upregulated in the tammar testis at the time of seminiferous cord formation and downregulated in the developing ovary in an opposite profile to that of marsupial WNT4. These observations suggest that FGF9 promotes male fate in the early gonad of marsupials through an antagonistic relationship with WNT4 as it does in eutherian mammals.

Early expression of the androgen receptor in the Sertoli cells of a marsupial coincides with downregulation of anti-Müllerian hormone at the time of urogenital virilization.

Anti-Müllerian hormone (AMH), responsible for the regression of Müllerian ducts, is strongly expressed by eutherian fetal and postnatal Sertoli cells. Both AMH and testosterone levels are high during the period of fetal reproductive tract virilization which occurs largely in utero in eutherian mammals. Taking advantage of the fact that differentiation of the urogenital tract occurs after birth in marsupials, we studied the ontogeny and regulation of AMH in the tammar wallaby testis and related it to the expression of the androgen receptor in Sertoli cells. Testicular AMH expression was high between days 10-30 post partum, then fell to basal levels by day 60 and remained low until day 90, the oldest age examined. AMH expression was repressed by treatment of male pouch young with the potent androgen androstanediol. Thus, in the tammar, AMH expression decreases in response to androgen at the time of initial urogenital masculinization, in contrast to the situation in humans in which AMH is repressed by testosterone only at the time of puberty. The difference might be explained by the timing of androgen receptor expression which appears in tammar Sertoli cells at around day 40 of pouch life but only at a later developmental stage in eutherians.

Mapping platypus SOX genes; autosomal location of SOX9 excludes it from sex determining role.

In the absence of an SRY orthologue the platypus sex determining gene is unknown, so genes in the human testis determining pathway are of particular interest as candidates. SOX9 is an attractive choice because SOX9 deletions cause male-to-female sex reversal in humans and mice, and SOX9 duplications cause female-to-male sex reversal. We have localized platypus SOX9, as well as the related SOX10, to platypus chromosomes 15 and 10, respectively, the first assignments to these platypus chromosomes, and the first comparative mapping markers from human chromosomes 17 and 22. The autosomal localization of platypus SOX9 in this study contradicts the hypothesis that SOX9 acts as the sex determining switch in platypus.

Sex determination in platypus and echidna: autosomal location of SOX3 confirms the absence of SRY from monotremes.

In eutherian ('placental') mammals, sex is determined by the presence or absence of the Y chromosome-borne gene SRY, which triggers testis determination. Marsupials also have a Y-borne SRY gene, implying that this mechanism is ancestral to therians, the SRY gene having diverged from its X-borne homologue SOX3 at least 180 million years ago. The rare exceptions have clearly lost and replaced the SRY mechanism recently. Other vertebrate classes have a variety of sex-determining mechanisms, but none shares the therian SRY-driven XX female:XY male system. In monotreme mammals (platypus and echidna), which branched from the therian lineage 210 million years ago, no orthologue of SRY has been found. In this study we show that its partner SOX3 is autosomal in platypus and echidna, mapping among human X chromosome orthologues to platypus chromosome 6, and to the homologous chromosome 16 in echidna. The autosomal localization of SOX3 in monotreme mammals, as well as non-mammal vertebrates, implies that SRY is absent in Prototheria and evolved later in the therian lineage 210-180 million years ago. Sex determination in platypus and echidna must therefore depend on another male-determining gene(s) on the Y chromosomes, or on the different dosage of a gene(s) on the X chromosomes.

Marsupial WT1 has a novel isoform and is expressed in both somatic and germ cells in the developing ovary and testis.

The Wilms' tumour 1 gene is essential for the formation of the mouse and human urogenital systems. We characterised this gene and examined its expression throughout gonadal development in a marsupial, the tammar wallaby. WT1 protein was detected in the Sertoli and granulosa cells of the developing testis and ovary, respectively. There was also strong immunostaining in the germ cells of both males and females at all stages of gonadal development. In the adult gonads WT1 appears to be dynamically regulated during spermatogenesis and oogenesis. Tammar WT1 has a novel isoform in which a portion of exon 1 is removed, partially deleting the RNA recognition motif (RRM). Despite its removal, WT1 still localised to RNA rich regions of the oocyte including speckled bodies within the nucleus, in the nucleolus and the perinucleolar compartment. This suggests that the RRM is not required for WT1 co-localisation with RNA. This is also the first report of WT1 in association with the perinucleolar compartment, important for RNA metabolism. Our data suggest that WT1 has a conserved function in both the somatic and germ cell lineages of the gonads of marsupials.

Expression of DMRT1 in the mammalian ovary and testis--from marsupials to mice.

Doublesex and mab3 related transcript (DMRT1) was identified as a candidate gene for human 9p24.3 associated sex reversal. DMRT1 orthologues have highly conserved roles in sexual differentiation from flies and worms to humans. A DMRT1 orthologue was isolated from a marsupial, the tammar wallaby Macropus eugenii. The wallaby gene is highly conserved with other vertebrate DMRT1 genes, especially within the P/S and DM domains. It is expressed in the differentiating testis from the late fetus, during pouch life and in the adult. As in eutherian mammals, DMRT1 protein was localized in the germ cells and the Sertoli cells of the testis, but in addition it was detected in the Leydig cells, peri-tubular myoid cells and within the acrosome of the sperm heads. DMRT1 protein was also detected in the fetal and adult ovary pre-granulosa, granulosa and germ cells. Similarly, we also detected DMRT1 in the granulosa cells of all developing follicles in the adult mouse ovary. This is the first report of DMRT1 expression in the adult mammalian ovary, and suggests a wider role for this gene in mammals, in both the testis and ovarian function.

Structure and expression of the follicle-stimulating hormone receptor gene in a marsupial, Macropus eugenii.

Follicle stimulating hormone (FSH) is essential for folliculogenesis. The function of FSH is mediated through its receptor (FSHr) and modulation of the receptor is thought to be the mechanism by which the responsiveness of follicles to gonadotrophins is regulated. FSHr is alternatively spliced to produce several transcripts in all eutherian species studied. However, controversy exists over the significance of alternatively spliced transcripts. In this study, we cloned and characterised the tammar wallaby (Macropus eugenii) FSHr gene and examined its expression. Comparison of gene structure and function between marsupials and eutherians enables identification of conserved features that are likely to be of functional significance. Tammar FSHr shares 94% amino acid similarity with human FSHr and is expressed in both the adult testis and ovary suggesting a similar function for this gene in both marsupials and eutherians. Tammar FSHr undergoes alternate splicing to produce four transcripts consistent with the splice variants seen in eutherians. These results strongly suggest that alternate splicing is of functional significance in the ovary since it has remained a highly conserved character of this gene for over 100 million years of divergent evolution.

Characterization of steroidogenic factor 1 during sexual differentiation in a marsupial.

In eutherian mammals, such as mice and humans, steroidogenic factor 1 (SF1) plays important roles in the development of the gonad and in its steroidogenic activity. Marsupial and eutherian mammals have been evolving independently for at least 100 million years and so we were interested in comparing SF1 of a marsupial with that of eutherians. To this end, we have cloned SF1 from an Australian marsupial, the tammar wallaby. Although the amino acid sequence of SF1 is highly conserved among vertebrate species, tammar SF1 appears to have diverged less from the ancestral SF1 than have eutherian SF1 proteins. Tammar SF1 is expressed by both ovaries and testes on the day of birth, just prior to the onset of testicular differentiation, until at least 8 days after birth by which time the ovary also has begun to sexually differentiate. SF1 transcripts are localized predominantly to the pre-granulosa and Sertoli cells of the ovary and testis, respectively. In the testis SF1 transcripts are also present in the interstitial cells, although at a lower level than that which is observed in the Sertoli cells. SF1 is also transcribed in adult testis and ovary. In the adult ovary SF1 is expressed in the interstitial gland, and in the granulosa cells and theca interna of small to medium-sized antral follicles, but is not expressed in large antral follicles. Thus, although the structure of tammar SF1 is divergent from that of eutherians, its expression profile is similar, supporting a conserved role in gonadal development and steroidogenesis.

Sex down under: the differentiation of sexual dimorphisms during marsupial development.

Marsupials have many characteristic features that make them ideal models to study the control of sexual differentiation and development. They are distinguished from eutherian mammals in their mode of reproduction and their greater dependence on the teat and mammary gland than on the placenta for development. They give birth to a highly altricial young which completes its development while firmly attached to a teat, usually within the confines of a pouch. At birth, the marsupial neonate has a well-developed digestive, respiratory and circulatory system, but retains its fetal excretory system with a fully functional mesonephric kidney and undifferentiated gonads and genitalia.

Absence of SOX3 in the developing marsupial gonad is not consistent with a conserved role in mammalian sex determination.

Expression of Sox3 has been detected in the testes of humans and of developing and adult mice at the same time as Sox9 and Sry. The co-expression of these three related Sox genes in the mouse indifferent gonadal ridge led to the hypothesis that these three genes, encoding transcription factors with similar DNA target binding sites, may interact with each other in initiating testis differentiation. The location of SOX3 on the marsupial Dunnart X chromosome also makes it a candidate for the marsupial X-linked gene responsible for the SRY- and hormone-independent initiation of scrotum or mammary gland development. Here we show that although marsupial SOX3 is highly conserved at the genetic level and appears to have a conserved role in CNS development, its expression during sexual differentiation differs from that of mice and humans. SOX3 expression is absent from the developing marsupial genital ridge and from the scrotal and mammary primordia during the critical time of differentiation and throughout the time that SRY is expressed. The absence of expression in the developing gonad strongly suggests that SOX3 does not have a conserved role in mammalian sexual determination or differentiation.

Urinary calculi associated with portosystemic shunts in six dogs.

Portosystemic shunts were diagnosed in 6 dogs with urinary calculi and signs of neurologic disease. Five of the dogs were initially examined because of neurologic signs, and 1 was seen because of urinary tract problems. The composition of the calculi was variable, but in all 13 episodes of calculi formation, the calculi contained an ammonium or uric acid component. One dog is being controlled under medical management; 4 of the dogs died, and 1 was euthanatized. The interpretation of pathogenesis was that decreased delivery of blood to the liver due to shunting or decreased liver function can potentially result in high blood concentrations of ammonia and uric acid, both of which are metabolic end products cleared by the kidneys; thus, the urinary tract would be an excellent environment for the precipitation of their salts.