Cloning and sequence analysis of pituitary prolactin cDNA from the northern brown bandicoot (Isoodon macrourus).

The nucleotide sequence for pituitary prolactin cDNA from the marsupial bandicoot (Isoodon macrourus) was determined by reverse transcription-polymerase chain reaction and 5'/3' rapid amplification of cDNA ends. The deduced amino acid sequence showed high sequence identity with brushtail possum prolactin (95%) and all of the expected structural features of a quadruped prolactin. A prolactin gene tree was constructed and rates of evolution calculated for bandicoot, possum, opossum and several mammalian and non-mammalian prolactins. Bootstrap analysis provided strong support for marsupials as a sister group with eutherian mammals and weak support for opossum and bandicoot as an independent grouping from the brushtail possum. The rates of molecular evolution for marsupial prolactins were comparable to the slow rate seen in the majority of quadruped prolactins that have been sequenced.

The GH/IGF-I axis in the brushtail possum (Trichosurus vulpecula) pouch young.

Plasma and pituitary GH concentrations and liver GH receptor (GHR), IGF-I and IGF-binding protein-3 (IGFBP-3) mRNA expression were determined in brushtail possum (Trichosurus vulpecula) pouch young aged 12-150 days post-partum and in adults. Mean plasma GH concentrations were highest, measuring around 150 ng/ml, from 12 to 100 days post-partum, and thereafter declined so that by 150 days post-partum levels were not significantly different from those in adults (10.8+/-1.8 ng/ml (S.E.M.)). In contrast to plasma levels, pituitary GH content increased markedly throughout pouch life, with an 87-fold increase between 12 and 150 days post-partum. However, when expressed per gram body weight, pituitary content was relatively constant between 25 and 150 days post-partum, indicating that the decline in plasma GH after 100 days post-partum was not due to decreased synthesis and/or storage of GH in the pituitary gland. Expression of GHR, IGF-I and IGFBP-3 mRNAs was determined by semi-quantitative RT-PCR. Liver GHR and IGF-I mRNA expression were low at 12 and 25 days post-partum and did not show sustained and significant increases (P<0.05) until 125 and 150 days post-partum. IGFBP-3 expression was also low at 12 days post-partum but then increased rapidly to a maximum at 50 days post-partum and thereafter declined. For all three mRNAs, liver expression at day 150 was not significantly different from that in adults. These patterns of gene expression for GHR and IGF-I suggest that the possum liver is resistant to the high plasma GH concentrations during early pouch life and in this way is similar to the fetal liver of some eutherian mammals.

Plasma growth hormone and growth hormone-binding protein during development in the marsupial brushtail possum (Trichosurus vulpecula).

Plasma concentrations of growth hormone (GH) were measured in the brushtail possum (Trichosurus vulpecula) pouch young from 25 through to 198 days post-partum (n=71). GH concentrations were highest early in pouch life (around 100 ng/ml), and thereafter declined in an exponential fashion to reach adult concentrations (10.8+/-1.8 ng/ml; n=21) by approximately 121-145 days post-partum, one to two months before the young is weaned. Growth hormone-binding protein (GHBP), which has been shown to modify the cellular actions of GH in eutherian mammals, was identified for the first time in a marsupial. Based on size exclusion gel filtration, possum GHBP had an estimated molecular mass of approximately 65 kDa, similar to that identified in other mammalian species, and binding of (125)I-labelled human GH (hGH) was displaced by excess hGH (20 microg). An immunoprecipitation method, in which plasma GHBP was rendered polyethylene glycol precipitable with a monoclonal antibody to the rabbit GHBP/GH receptor (MAb 43) and labelled with (125)I-hGH, was used to quantitate plasma GHBP by Scatchard analysis in the developing (pooled plasma samples) and adult (individual animals) possums. Binding affinity (K(a)) values in pouch young aged between 45 and 54 and 144 and 153 days post-partum varied between 1.0 and 2.4 x 10(9)/M, which was slightly higher than that in adult plasma (0.96+/-0.2 x 10(9)/M, n=6). Binding capacity (B(max)) values increased from non-detectable levels in animals aged 25-38 days post-partum to reach concentrations around half that seen in the adult (1.4+/-0.2 x 10(-9) M) by about 117 days post-partum and remained at this level until 153 days post-partum. Therefore, in early pouch life when plasma GH concentrations are highest, the very low concentrations of GHBP are unlikely to be important in terms of competing with GH-receptor for ligand or altering the half-life of circulating GH.

Role of PGF2alpha and oxytocin in parturition in the brushtail possum (Trichosurus vulpecula).

Maturation of the fetal pituitary and adrenal glands allows the secretion of cortisol, which in turn leads to an increase in prostaglandin and mesotocin production. The production of prostaglandin and mesotocin results in an increase in uterine contractions and initiates birth in marsupials. The major metabolite of PGF(2alpha), 13,14-dihydro-15-keto-prostaglandin F(2alpha) (PGFM), has been found in the plasma of the possum at the time of birth and administration of PGF(2alpha) to female possums induced the adoption of the birth position. Evidence that mesotocin is an integral hormone of birth in the tammar wallaby indicates that both PGF(2alpha) and mesotocin or oxytocin are required for marsupial birth. The presence of PGF(2alpha) receptors in the uterus and corpus luteum of the possum, and the in vitro uterine responsiveness to PGF(2alpha) or oxytocin, were examined. PGF(2alpha) receptors were not observed in possum uteri and the inability of PGF(2alpha) to cause contractions indicates that PGF(2alpha) is not involved directly in contraction of the uterus at parturition. The presence of oxytocin and mesotocin receptors in the uterus of possoms and the ability of oxytocin to induce uterine contraction in vitro supports the view that mesotocin is required for expulsion of the young from the uterus. Low numbers of PGF(2alpha) receptors were found in the possum corpus luteum at birth, indicating an involvement of PGF(2alpha) in regression of the corpus luteum.

Induction of birth in the bandicoot (Isoodon macrourus) with prostaglandin and oxytocin.

As in eutherians, maturation of the fetal pituitary and adrenal glands together with an increase in prostaglandin and mesotocin or oxytocin production initiates birth in marsupials. In this study, prostaglandin (Lutalyse) or oxytocin (Syntocinon) were administered to pregnant bandicoots at 05:00 h on the calculated day of birth and the resultant effects were filmed for analysis. The administration of prostaglandin caused the bandicoot to adopt the birth position several minutes after injection (n = 2). However, the bandicoot did not give birth for several hours. Birth occurred at a similar time of day to that observed for untreated bandicoots (n = 7), between 08:00 h and 12:00 h. After an injection of oxytocin, the bandicoot assumed the birth position and birth occurred within several minutes. The young were alive while still connected to their allantoic stalks. However, they were unable to attach to the teats and did not survive (n = 4). The induced young were the colour of venous blood and died soon after the umbilicus was separated, indicating that the cardiopulmonary system of these neonates was underdeveloped and inadequate to maintain life. The results from this study demonstrate that prostaglandin is required to prepare the bandicoot for birth, and mesotocin is required for contraction of the uterus and for birth to occur.

Insulin-like growth factor 2 cDNA cloning and ontogeny of gene expression in the liver of the marsupial brushtail possum (Trichosurus vulpecula).

The cDNA sequence for insulin-like growth factor 2 (IGF-2) was determined from the liver of the marsupial brushtail possum (Trichosurus vulpecula) using reverse transcription followed by polymerase chain reaction (RT-PCR) with gene-specific primers. The 359 bp of possum sequence encompassed the mature peptide, 27 bp of the signal peptide, and 125 bp of the E-peptide. Alignment of the deduced amino acid sequence with those from other species indicated that the mature peptide was 71 amino acids in length, 4 amino acids longer than most other mammals. At both the nucleotide and amino acid levels there was a high degree of sequence identity with IGF-2 from other mammalian and nonmammalian species. Amino acid identity ranged from 94.4% with a variant form of human IGF-2 to 80.3% with zebrafinch IGF-2. Northern analysis revealed that radiolabeled possum IGF-2 cDNA hybridized to multiple transcripts in the liver of both adult possums and 150-day-old pouch young and that the overall level of expression was greater in pouch young. Semiquantitative RT-PCR with total RNA from liver samples of pouch young aged 12 to 150 days postpartum and adults confirmed that IGF-2 gene expression was two to three times more abundant in pouch young than in adults but there was no significant change in the level of expression during pouch life. Unlike other mammalian species, in which there is a decline in levels of liver IGF-2 gene expression around the time of birth, levels in the marsupial brushtail possum remain elevated for at least 150 days after birth. This suggests that the decline in liver IGF-2 expression in marsupials and eutherians occurs at a similar stage of development and may reflect a role for this growth factor during the postnatal growth and development of the marsupial.

Experimental infection of Australian brushtail possums, Trichosurus vulpecula (Phalangeridae: Marsupialia), with Ross River and Barmah Forest viruses by use of a natural mosquito vector system.

Brushtail possums, Trichosurus vulpecula Kerr, were experimentally infected with Ross River (RR) or Barmah Forest (BF) virus by Aedes vigilax (Skuse) mosquitoes. Eight of 10 animals exposed to RR virus developed neutralizing antibody, and 3 possums developed high viremia for < 48 hr after infection, sufficient to infect recipient mosquitoes. Two of 10 animals exposed to BF virus developed neutralizing antibody. Both infected possums maintained detectable neutralizing antibody to BF for at least 45 days after infection (log neutralization index > 2.0 at 45 days). Eight possums did not develop neutralizing antibody to BF despite exposure to infected mosquitoes. These results suggest that T. vulpecula may potentially act as a reservoir species for RR in urban areas. However, T. vulpecula infected with BF do not develop viremia sufficient to infect mosquitoes and are unlikely to be important hosts for BF.

Utilisation of aliphatic compounds by acidophilic heterotrophic bacteria. The potential for bioremediation of acidic wastewaters contaminated with toxic organic compounds and heavy metals.

Acidophilic, heterotrophic bacteria isolated from acidic mine effluent metabolised a range of aliphatic compounds. Aliphatic acids, which are normally thought to be toxic to acidophiles, were utilised as substrates for energy and growth by these bacteria. This biodegradative ability, concomitant with their tolerance of heavy metals, has demonstrated the potential for using these organisms for the bioremediation of multiply contaminated acidic wastewaters.

Sexual development in marsupials: genetic characterization of bandicoot siblings with scrotal and testicular maldevelopment.

In marsupials testis determination requires the presence of a Y chromosome. The sex determining region on the Y gene (SRY) is necessary for testicular development in eutherians and it is assumed to play a similar role in marsupials. Relatively few studies have investigated the genetic basis of sexual development, and as yet there is no direct evidence that SRY is required for testis development in marsupials. Studies on intersexual marsupials have revealed a fundamental difference between marsupial and eutherian sex determination. The scrotum of marsupials is analogous, not homologous, to the eutherian scrotum and is under the control of X-linked genes not androgens. The current study describes two bandicoot (Isoodon macrourus) siblings. Both siblings had underdeveloped male reproductive tracts and testicular dysgenesis, one was ascrotal and the other had a diminutive scrotum. Their karyotypes were normal for this species which eliminates the Y chromosome from some somatic tissues. SRY was detected by Southern blotting. SRY, ubiquitin activating enzyme-1 on the Y (UBE1Y) and glucose 6-phosphate dehydrogenase (G6PD) gene expression were examined. UBE1Y was widely expressed in many tissues. SRY gene expression was much lower than normal in the abnormal siblings and may be responsible for their failure of testicular and epididymal development. The cause of their scrotal abnormalities is unknown. It is possible that the separate defects of scrotal and testis development in the two siblings, which had normal relatives, were due to a mutation in a gene common to both developmental pathways.

Plasma concentrations of thyroxine and growth hormone in the developing marsupial bandicoot, Isoodon macrourus.

Plasma concentrations of thyroxine (T4) and growth hormone (GH) were measured in the developing bandicoot from 21 through to 69 days post partum. Thyroxine concentrations increased from 7 ng mL(-1) at 21 days post partum to reach a maximum of around 40 ng mL(-1) approximately 40 days post partum. After this time, plasma T4 concentrations decreased until about 50 days post partum, when levels were not different from those in the adult (9.2 +/- 0.7 ng mL(-1), n = 10). In contrast, GH concentrations were greatest early in pouch life, with a maximum concentration of 88.2 ng mL(-1) at 24 days post partum, and thereafter declined to adult levels (4.9 +/- 0.9 ng mL(-1), n = 7) by about 60 days post partum. The temporal relationship between T4 and GH in the developing bandicoot is similar to that seen in developing eutherian mammals, but in the latter species, peak plasma T4 and the decline in GH occur before or soon after parturition, whereas in the bandicoot these events occur more than one month post partum. This comparison between eutherian mammals and a marsupial indicates that the timing of these endocrine changes correlates with key developmental or maturational changes rather than the time of parturition.

Ontogeny of the immune system of the brushtail possum, Trichosurus vulpecula.

The numbers and distribution of T and B cells in the thoracic thymus, spleen and intestinal tissue and the proliferation of T lymphocytes were examined during pouch life and in the adult to determine when the developing brushtail possum reaches immunological maturity. CD3-positive cells were observed in the thoracic thymus at day 2 post-partum indicating that the thymus produces T lymphocytes at or soon after birth. By day 25 the thymus was fully populated with CD3-positive T lymphocytes and they were observed in distinct regions of the cortex and medulla. By day 48 post-partum, B and T lymphocytes were identified in the follicles and parafollicular areas of the spleen. Although the numbers of T and B cells in the spleen increased significantly from day 25 to day 100 post-partum (P < 0.005), fewer cells were present at day 150 post-partum than in the adult (P < 0.05). Peyer's patches were not observed in the intestines up to day 73 post-partum. However, both T and B cells were observed in the intestinal lymph nodes. Although the T lymphocytes at weaning showed a proliferative response, the response was not as great as that observed in the adult possum. Thus, the immune system of the possum is not fully developed at weaning but continues its development after pouch life.

Physiological changes in the brushtail possum (Trichosurus vulpecula) following relocation from Armidale to Brisbane, Australia.

To determine the effect of relocation on the health of possums the body weights and hormone and immune responses of six male and nine female brushtail possums were monitored for 20 weeks following transfer from the environs of Armidale into enclosures in Brisbane. Over the first 6 weeks of captivity, male possums lost 11.0% of their original body weight and females lost 16.8%. The mean concentrations of plasma cortisol in the male and female possums were 14.5 and 29.4 ng/mL, respectively, and did not change over the 20-week period. Male and female possums displayed a similar pattern of thyroxine secretion over the 20 weeks, with low concentrations up to week seven (2.1 and 2.7 ng/mL, respectively) increasing to 6.9 and 5.8 ng/mL in weeks 7-12 (P < 0.005). This increase in the concentration of thyroxine corresponded with the increase in body weight. The number of white blood cells (WBCs) and the percentage of neutrophils increased from the capture to week 6-10. However, during the last 10 weeks of captivity the number of WBCs and the percentage of neutrophils decreased, indicating recovery of the immune system. This was in accord with the proliferative response of lymphocytes to the T cell mitogen PHA that increased from weeks 11-15 to weeks 16-20 in both male and female possums. The results above suggest that the Armidale possums, like the Brisbane possums, were stressed following their relocation; however, their immune systems were able to gradually recover as they adjusted to their new environment in Brisbane. The death rate of pouch young and of adult female possums after relocation was considerably higher in the Armidale possums compared to Brisbane possums. The mortality rate of Brisbane possums over the first 20 weeks of captivity was 8.3% and 19.6% for male and female possums, respectively, and for Armidale possums 16.6% and 47.1%, respectively. The possums transferred from the environs of Armidale into captivity in Brisbane were under greater stress than possums captured in Brisbane and placed in captivity in Brisbane.

Proposal of a new halobacterial genus Natrinema gen. nov., with two species Natrinema pellirubrum nom. nov. and Natrinema pallidum nom. nov.

A phylogenetic analysis of 69 halobacterial 16S rRNA gene sequences has been carried out, integrating data from new isolates, previously described halobacteria and cloned sequences from uncultivated halobacteria. Halobacterium halobium NCIMB 777, Halobacterium trapanicum NCIMB 784 and Halobacterium salinarium NCIMB 786, together with several other strains (strains T5.7, L11 and Halobacterium trapanicum NCIMB 767) constitute a distinct lineage with at least 98.2% sequence similarity. These strains have been incorrectly assigned to the genus Halobacterium. Therefore, based on a variety of taxonomic criteria, it is proposed that Halobacterium salinarium NCIMB 786 is renamed as Natrinema pellirubrum nom. nov., the type species of the new genus Natrinema gen. nov., and that Halobacterium halobium NCIMB 777 and Halobacterium trapanicum NCIMB 784 are renamed as a single species, Natrinema pallidum nom. nov. It was notable that halobacteria closely related to the proposed new genus have been isolated from relatively low-salt environments.

Halobacteria: the evidence for longevity.

Subterranean salt deposits are the remains of ancient hypersaline waters that presumably supported dense populations of halophilic microorganisms including representatives of the haloarchaea (halobacteria). Ancient subterranean salt deposits (evaporites) are common throughout the world, and the majority sampled to date appear to support diverse populations of halobacteria. The inaccessibility of deep subsurface deposits, and the special requirements of these organisms for survival, make contamination by halobacteria from surface sites unlikely. It is conceivable that these subterranean halobacteria are autochthonous, presumably relict populations derived from ancient hypersaline seas that have been revived from a state of dormancy. One would predict that halobacteria that have been insulated and isolated inside ancient evaporites would be different from comparable bacteria from surface environments, and that it might be possible to use a molecular chronometer to establish if the evolutionary position of the subsurface isolates correlated with the geological age of the evaporite. Extensive comparisons have been made between the 16S rRNA genes of surface and subsurface halobacteria without showing any conclusive differences between the two groups. A further phylogenetic comparison exploits an unusual feature of one particular group of halobacteria that possess at least two heterogeneous copies of the 16S rRNA gene, the sequences of which may have been converging or diverging over geological time. However, results to date have yet to show any gene sequence differences between surface and evaporite-derived halobacteria that might arguably be an indication of long-term dormancy.

Physiological changes in brushtail possums, Trichosurus vulpecula, transferred from the wild to captivity.

To determine the effect of relocation on the health of possums, the body weights and hormone and immune responses of 11 male and 9 female brushtail possums were monitored following transfer from the environs of Brisbane into an established breeding colony in Brisbane. The possums were monitored weekly for the first 20 weeks of captivity, and their immune responses assessed again 12 months after capture. Over the first 5 weeks of captivity, male possums lost a mean of 8.8% of their original body weight, and females lost 15.3% over the first 6 weeks. Variation between individual possums was evident, and the 11 male possums could be divided into two groups, those that gained weight (number of animals, N = 4) and those that lost weight (N = 7) in captivity. Four males gained weight following capture, and their body weight after 20 weeks of captivity was higher than at capture. The remaining seven males lost weight over the 20 weeks following introduction into captivity, resulting in a lower weight at week 20 than at capture. All of the nine female possums lost weight and were slower to regain weight compared to the males. Plasma cortisol concentrations did not vary greatly over the 20 weeks in male possums, and the mean plasma concentration of cortisol for the 11 male possums was 7.8 ng/ml (number of samples, n = 220). The female possums showed a different pattern. The concentration of cortisol for the nine female possums at week 1 was 34.0 ng/ml, which was significantly higher than 13.3 ng/ml at week 20 (P < 0.016). No significant variation in the mean concentration of plasma thyroxine of 5.7 ng/ml occurred in the 11 male possums over the 20-week period (n = 220). The plasma concentration of thyroxine for the nine female possums was 2.5 ng/ml (n = 54) for the first 6 weeks. At week 6, an increase in the concentration of thyroxine occurred, and a peak concentration of 6.9 ng/ml was reached at week 13. This increase correlated with the females regaining body weight. A low concentration of thyroxine is often associated with stress, thus an increase in the concentration of this hormone, combined with an increase in body weight, may indicate that these females had begun to adjust to their new environment. The seven male possums that lost weight following introduction into captivity displayed a significantly higher concentration of cortisol (9.1 compared with 5.3 ng/ml P < 0.01), and a lower concentration of thyroxine compared to the four males that gained weight following capture (4.7 compared with 7.3 ng/ml, P < 0.005). Over the 20-week period, the total number of white blood cells increased, and the number of neutrophils increased in both males and females. The proliferative response of lymphocytes from male possums to the T-cell mitogen, phytohaemagglutin (PHA) decreased significantly over the 20-week period (P < 0.002). In females an initial decrease in the reactivity of lymphocytes observed over the first 10 weeks was followed by an increase in this response over the remaining 10-week period. Twelve months following capture, the white blood cell parameters of both males and females had returned to similar levels to those of the first 1-5 weeks. The reactivity of lymphocytes from male possums that had been in captivity for 12 months was significantly higher than that of the first 20 weeks of captivity (P < 0.005). Females that had been in captivity for 12 months displayed lymphocyte responses similar to those observed at weeks 16-20. The body weight and hormonal results would suggest that possums undergo a more severe stress response than males immediately following their capture. In contrast, the immune response of males is lower than females and is depressed for a longer period following capture.

The role of maternal thyroid hormones on the development of the brushtail possum, Trichosurus vulpecula.

Thyroxine (T4) is a vital hormone for the development of mammals. To determine the role of maternal thyroid hormones on organ development, methimazole, an inhibitor of T4, was first administered via a minipump to 13 mothers with pouch young between days 10 and 80 post partum for 28 days. Three young survived and 10 of the young died at 104.0 +/- 10.8 days post partum (mean, SEM). Methimazole was then administered for 28 days to 6 lactating adult possums with pouch young at day 20 post partum. The effects of this treatment on the maternal plasma concentrations of T4 were monitored and the development of the lung, kidney and brain of the young were examined at day 90 post partum. There was no difference in the morphology of the lung, kidney and brain of pouch young at day 90 post partum whose mothers were treated with methimazole or saline. Thus methimazole administered to lactating possums for a short period early in lactation can cause the demise of the young about day 100 post partum although the cause of death is unknown. It is possible that the development of the central nervous system or some other vital organ has been inhibited, this altered state not being apparent morphologically. Nevertheless the marsupial appears to be similar to the eutherian in its requirement for thyroxine for normal development. However whereas this requirement is apparent during fetal development and around the time of birth in eutherians thyroxine is of importance during pouch development in marsupials.

Ontogeny of thyroid hormone receptors in the brushtail possum (Trichosurus vulpecula).

Newborn marsupials do not have a thyroid gland at birth. The gland develops while the young marsupial is in the mother's pouch. The young brushtail possum initiates secretion of thyroid hormones from its own thyroid at about Day 65 post partum. However, during the first three weeks of pouch life thyroxine is passed from the mother to the young via the milk. To determine if this maternal thyroxine can effect organ development in the young possum before it initiates secretion of thyroxine from its own thyroid, the ontogeny of thyroid hormone receptors was determined in nuclear extracts of lung, liver and kidney by radioreceptor assay, using (125)I-labelled tri-iodothyronine as ligand. Receptor density was calculated for tissues removed from young possums at Days 25 (n = 5), 50 (n = 4), 100 (n = 3) and 150 (n = 4) and from adults (n = 5). Receptors were found in possums of all age groups, including the small 25-day pouch young. Significant differences were not found in the receptor density between different tissues or at various ages. The association constant Ka (4.0+/-2.6 L nmol[-1] for lung) was similar in different tissues and at the various ages examined. The passage of thyroid hormones from the mother to the developing marsupial via the milk may have a role in the slow development of organ systems early in pouch life by acting on thyroid receptors in the pouch young. However, the functional maturation of the thyroid gland of the young possum, not an increase in receptors, appears to coincide with the rapid increase in the rate of growth and development which occurs in later pouch life.

Development of lung, kidney and skin in the brushtail possum, Trichosurus vulpecula.

The thyroid gland is not present at birth in marsupials and thyroid function begins during the latter half of pouch life. The hormonal output of the thyroid gland is important for normal development. In this study the structure of the lung, kidney and skin of the developing possum was examined and the structural development of these three organs was described. The lung of the newborn brushtail possum was functional and continued to develop during pouch life, alveolar formation beginning at day 39 and concluding at day 113 postpartum. The mesonephric kidneys of the newborn possum degenerated and were replaced by the metanephric kidneys, the nephrogenic zone of the metanephric kidney being present from 35 to 96 days postpartum. No new nephrons were formed after day 96. After the completion of nephrogenesis, the kidney increases in size through glomerular and tubular growth. The sequence of steps in the development of the possum skin was identical to that observed in other marsupials. The epidermis of the possum was thickest at 60 microns on about day 28 postpartum and as development proceeded the epidermis gradually decreased in thickness. A sparse number of primordia of hair follicles were observed at day 10 and the possum had a good covering of hair by day 129 postpartum. Correlation between the development of the lung, kidney and skin with the previously published plasma thyroxine concentration in the young possum suggests that thyroxine from the mother and from the young is important in the development of these three organs.

The effect of methimazole on the growth of the developing brushtail possum, Trichosurus vulpecula.

The brushtail possum, Trichosurus vulpecula, produces a single offspring 200 mg in body weight. The young grow slowly during the first half of pouch of life at approximately 1g/day. At day 96 post partum the growth rate increases dramatically and it has been suggested that this change in growth rate may result from the initiation of thyroid function in the young possum. To determine the role of the thyroid gland of the possum on growth, the production of hormones from the thyroid was inhibited by administration of the drug, methimazole. The drug was administered to adult and young possums at day 140 post partum for 28 days and to young at day 100 post partum for 42 days and the effects of this treatment on plasma concentrations of thyroxine and on growth were monitored. Methimazole was found to inhibit thyroxine production in 4 of the 6 adult possums although it had no effect on body weight. Similar treatment administered to nine young possums inhibited growth. The three surviving methimazole possum young treated at 100 days post partum weighed less than control possums after day 163 post partum. The four young treated at 140 days post partum also showed depression in growth rate. However, the depression was not as marked as that of the 100 day treated possums. It is highly likely therefore that the surge in growth rate in the possum at day 96 post partum is initiated by thyroxine produced by the thyroid gland of the young.