X-Chromosome Inactivation in Marsupials

Marsupial (metatherian) mammals resemble their eutherian ('placental') counterparts in having inacti- vation of one of the two X chromosomes in the soma and premeiotic germ cells of their females. The marsupial X-inactivation system differs from the eutherian system in two respects: firstly, inactivation occurs for the paternally derived allele, i.e. it is not random, and secondly it is often incomplete. Data are available for four X-linked loci, all controlling enzyme structure: glucose-6- phosphate dehydrogenase (G6PD), phosphoglycerate kinase 1 (PGKl), alpha-galactosidase (GLA) and hypoxanthine phosphoribosyl transferase (HPRT). Both the G6PD and PGKl loci exhibit incomplete X-chromosome inactivation. The pattern of partial expression differs from tissue to tissue and from species to species. One of the two X chromosomes exhibits late replication, even in cells where a paternally derived gene is partly active, showing that late replication and absence of transcription are not completely correlated. Sex chromatin bodies are not as easily found as in some eutherians. In marsupials they are most clearly demonstrable in species with small Y chromosomes. Investigations into X-inactivation in early development have just begun. Absence of inactivation at the G6PD locus in yolk sac of a kangaroo has been observed. All other tissues exhibited complete paternal X-inacti- vation for G6PD. In a dasyurid, GLA showed complete paternal X-inactivation in all embryonic and extra-embryonic tissues. The role, if any, of methylation of cytosine residues in CpG pairs in the maintenance of X-inactivation in marsupials is unclear. Preliminary evidence indicates that sex-specific differences in methylation of sex linked genes do exist in marsupials.

DNA methylation in the developing marsupial embryo

Marsupial development differs from early development of placental mammals in that the blastocyst is unilaminar, so that both embryonic and extraembryonic cells are derived from a single layer of cells (protoderm) which faces the blastocyst cavity. Also, all cells in female marsupial conceptuses so far examined show preferential paternal X-inactivation. To test for a possible correlation between cell position, paternal X-inactivation and DNA hypomethylation, marsupial DNA preparations from three regions, embryo, vascular yolk sac and avascular yolk sac, were digested with methyl-specific restriction endonucleases, separated on agarose gels and end-labelled with 32P-dCTP. The size distribution of the fragments obtained indicated three levels of methylation: high methylation of embryonic DNA, intermediate levels of methylation of vascular yolk sac DNA and hypomethylation of avascular yolk sac DNA. The degree of methylation of repeat sequences, observed as discrete bands in end-labelled HpaII digests, was correlated with the overall methylation of tissue DNA. Thus, the difference in methylation in embryonic and extraembryonic DNA was similar to that described for the mouse conceptus, and the outside cell position of marsupial fetal precursor cells did not correlate with hypomethylation. HpaII tiny fragments, which indicate the presence of CpG-rich islands of DNA, were evident in the marsupial digests. In the mouse DNA, these islands are associated with gene transcription and provide one route to cloning of unique gene sequences.

X chromosome inactivation in female embryos of a marsupial mouse (Antechinus stuartii)

Blastocysts and late gestation stages of the marsupial mouse, Antechinus stuartii, were examined cytologically and electrophoretically to investigate X chromosome activity during embryogenesis. A late replicating X chromosome was identified in the protoderm cells of female unilaminar blastocysts and in the cells of embryonic and extra-embryonic regions of older blastocysts. Sex chromatin bodies were also observed in female bilaminar and trilaminar blastocysts. The X linked enzyme alpha-galactosidase showed no evidence of paternal allele expression in the extra-embryonic region of bilaminar blastocysts or in the yolk sac and embryonic tissue of known heterozygotes. It is concluded that the late replicating X chromosome is paternal in origin and that unlike the laboratory mouse, X inactivation is not correlated with cell differentiation in Antechinus.

Lack of Correlation between Gpd Expression and X Chromosome Late Replication in Cultured Fibroblasts of the Kangaroo Macropus robustus

Cultured fibroblasts and lymphocytes from M. robustus females heterozygous for the X-linked Gpd gene were examined electrophoretically and cytologically. Gpd expression in lymphocytes was restricted to the maternal allele while in fibroblasts there was also partial expression of the paternal allele. The Gpd gene is thought to be located on the long arm of the X chromosome. However, in fibroblasts the long arm of the paternal X chromosome showed no indication of an early replicating segment.

Influence of dietary tryptophan on the induction of gamma-glutamyltranspeptidase-positive foci in the livers of rats treated with hepatocarcinogen

The ingestion of an elevated level (2%) of L-tryptophan (TRP) in a purified diet was investigated to determine whether it would influence the induction of gamma-glutamyltranspeptidase (GGT)-positive foci in the livers of rats exposed to a hepatocarcinogen. Subtotal hepatectomies were performed, and 18 h later, the rats were given injections i.p. of diethylnitrosamine (30 mg/kg). Ten days later, groups of male rats were placed on choline-supplemented (CS), CS + TRP, choline-deficient (CD), or CD + TRP diets for 10 wk. In two separate experiments, the rats fed the CS + TRP diet or the CD diet developed more and larger GGT + foci than did rats fed the CS diet. Rats fed the CD + TRP diet revealed similar changes to those found in rats fed the CD diet. The liver weights of the rats fed the CD or the CD + TRP diet were greater than those of rats fed the CS or the CS + TRP diet. Hepatic GGT activity was somewhat elevated in rats fed the CS + TRP diet and markedly elevated in rats fed the CD or the CD + TRP diet. Hepatic ornithine decarboxylase activity was increased in rats fed the CD + TRP diet. The results suggest that increased dietary tryptophan has a promoting effect on liver carcinogenesis as measured by the induction of GGT + foci in the livers of rats exposed to diethylnitrosamine. A potentiating effect by tryptophan was not observed in the livers of rats fed a CD diet.

Glucose-6-phosphate dehydrogenase expression in heterozygous kangaroo embryos and extra-embryonic membranes

The electrophoretic expression of the X-linked enzyme glucose-6-phosphate dehydrogenase was examined in heterozygous Macropus robustus embryos and their extra-embryonic membranes. The amnion and allantois, like the somatic tissues of the embryo proper, showed paternal X inactivation while the avascular and vascular yolk sac cells showed evidence of activity of both maternal and paternal alleles.

Dictyate Oocytes of a Kangaroo (Macropus robustus) Show Paternal Inactivation at the X-linked Gpd Locus

Purified samples of large numbers of dictyate oocytes from 13 M. robustus pouch young heterozygous for glucose-6-phosphate dehydrogenase type and six homozygous controls were examined electrophoretically to determine activity states at the Gpd locus. Like somatic cortical and medullary cells, oocytes expressed only the maternal phenotype irrespective of the direction of the cross. No evidence was found of reactivation of the inactive (paternal) allele or inactivation of both maternal and paternal alleles. It was therefore concluded that unlike eutherian dictyate oocytes, only a single (maternal) allele is active in each dictyate oocyte in M. robustus. The stage of reactivation of the paternal allele remains to be determined.

Germ cell development in female pouch young of the tammar wallaby (Macropus eugenii)

The cytology, timing, numbers and arrangement of germ cells in gonads of 35 female tammar wallaby pouch young aged from newborn to 210 days post partum were determined from serial sections. Germ cell cytology was essentially similar to that in eutherians. The total number of germ cells per ovary reached a peak of about 4.5 X 10(5) at 50 days and was reduced to about 25% of the maximum value by 210 days. The first oogonia developed from primordial germ cells soon after birth; the first meiotic oocytes were formed between 24 and 30 days; follicularization of diplotene oocytes began at about 50 days, and by 210 days normal, prefollicular germ cell development had ended. Successive stages of germ cells were initiated at the inner margin of the cortical region of the gonad and spread as a series of waves to the tunica albuginea. At all stages of oogonial and oocyte development many cells showed characteristic degenerative changes leading to atresia. The timing of female germ cell development in relation to gestation in tammars and humans is comparable if tammar gestation is taken to include pouch life.

Glucose-6-phosphate dehydrogenase and lactate dehydrogenase activity in kangaroo and mouse oocytes

Glucose-6-phosphate dehydrogenase and lactate dehydrogenase activity in oocytes and follicle cells of the Australian marsupial, Macropus eugenii (the tammar), and the mouse were determined by a simplified microelectrophoresis method. Mouse oocytes have approx 285 times more G6PD activity per picolitre of cytoplasm than tammar oocytes and about 10 times more LDH. The ratio of LDH to G6PD in mouse follicle cells is close to 3 whereas in the tammar it approaches unity. The very low levels of activity of G6PD in tammar oocytes may be due to transcriptional, translational or metabolic differences compared with the mouse.

Polyploidy in the australian leptodactylid frog genus Neobatrachus

Karyotypic analysis of six species of the Australian leptodactylid frog genus Neobatrachus showed that N. pictus, N. centralis, N. pelobatoides and N. wilsmorei are diploid (2n = 24) while N. sudelli and N. sutor are tetraploid (4n = 48). Polyploidy has not been reported previously among Australian anurans. Idiograms of the six species indicate that they are similar to the other Australian leptodactylids so far discribed. DNA values of the tetraploids are approximately double the values for diploids. Tetraploid nuclear and cell sizes are greater compared with diploids but total body size shows no increase. At diakinesis in primary spermatocytes of tetraploids, mainly tetravalents together with a few bivalents are present. Silver straining of metaphase spreads clearly demonstrates the location of NORs at the secondary constrictions and their frequent association in the tetraploid N. sutor. Nucleolar number in interphase nuclei provides a reliable guide for distinguishing tetraploid from diploid frogs in the absence of chromosome analysis and can be determined for both living and preserved specimens. The possible origins and relationships of the tetraploid species are discussed.

Ocean Tide and Waves Beneath the Ross Ice Shelf, Antarctica

The ocean tide in the southern Ross Sea is principally diurnal. The tropic tide range (double amplitude) is between 1 and 2 meters, depending on the location, and is closely related to the local water-layer thickness. The range of the tropic tide is more than three times the range of the equatorial tide. Cotidal and coamplitude charts were made for the largest diurnal constituents, K(1) and O(1) and a provisional cotidal map was made for the semidiurnal constituent M(2). The amplitudes of the diurnal tide constituents are larger in the Ross Sea than in the adjacent southern Pacific Ocean, indicating the existence of a diurnal resonance related to the shape and depth of the sea. Waves related to ocean swell propagate into the ice-covered region from the northern Ross Sea. These waves have amplitudes near 1 centimeter, and periods in the range 1 to 15 minutes. The speed at which these waves travel is successfully predicted by flexural wave theory.

Acute stress and the brain norepinephrine uptake mechanism in the rat

The kinetic constants for norepinephrine uptake in cerebral cortical homogenates were determined in vitro immediately following an acute stress consisting of either forced immobilization, cold-wet exposure, combined cold-plus-restraint, swim stress, or electric footshock in the rat. The kinetic constants, apparent Km and Vmax, for uptake of 3H-l-norepinephrine were significantly increased only following 10 min swim at 22 degrees or following 5 min electric footshock. When severe hypothermia accompanied the stress, the findings suggested that a profound reduction in body temperature was associated with depressed responsiveness of brain noradrenergic mechanisms to stress including decreased uptake kinetic constants. In a series in which the duration of electric footshock was varied from 2 to 30 min, it was noted that the NE uptake kinetic constants were increased at 5 min, but were similar to paired controls at 2, 10 and 30 min following the onset of footshock. It was concluded that various acute stresses did not elicit a generalized response of the cortical NE uptake mechanism to stress in the rat. Furthermore, when uptake kinetic constants did change with stress, the values were often within the range of normal values seen in the rat.

Karyotypic variation within the genus Leiopelma (Amphibia: Anura)

The chromosome complements of four specimens of the anatomically primitive frog Leiopelma hochstetteri are described from cultured cells and squashes. The basic karyotype in all cases consists of 22 chromosomes, 12 of which are acrocentric. Supernumerary chromosomes are either absent or variable in number, but appear to be constant in the somatic cells of any one individual. The limited evidence available suggests that the supernumerary chromosomes do not pair during male meiosis.The karotype of L. archeyi is described for the first time. Only the smallest pair of the total complement of 18 chromosomes is acrocentric. Supernumeraries are absent.The distribution and probable relationships of the species of Leiopelma are discussed. The karyotypes of Leiopelma and the North American ascaphid frog Ascaphus truei are compared, with particular reference to the relationship of the supernumerary chromosomes of Leiopelma and the microchromosomes of Ascaphus.