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PubMed1985

Effects of water restriction on nitrogen metabolism and urea recycling in the macropodid marsupials Macropus eugenii (tammar wallaby) and Thylogale thetis (red-necked pademelon).

The effects of water restriction on nitrogen metabolism were compared in the semi-arid adapted tammar wallaby (Macropus eugenii) and a wallaby from a moist forest environment, the red-necked pademelon (Thylogale thetis). On a medium (9 to 13%) protein diet water restriction depressed dry matter and nitrogen intakes in both species. Nitrogen balance remained positive, but fell further in the pademelons. Urea excretion decreased in the tammars, but increased in the pademelons. Urea recycling as a proportion of urea entry rate tended to increase in the tammars but not in the pademelons. These findings suggest that T. thetis is not as well adapted as M. eugenii to coping with water shortages. In a second experiment water restriction depressed nitrogen balance in tammars on both high (15%) and low (5%) protein diets, but only on the latter diet did nitrogen balance become negative. Urea recycling was greater on the low than on the high protein diet, but was unaffected by water restriction. Although better able to withstand water stress, M. eugenii was unable to cope with both water stress and a low protein diet together. The reported ability of M. eugenii to maintain water intake by drinking sea water during the dry season when fresh water is unavailable and vegetation is of low protein content is thus of great ecological significance.

Animals↗
PubMed1985

Postnatal development of the telencephalon of the tammar wallaby (Macropus eugenii). An accessible model of neocortical differentiation.

The sequence of development of cell layers in the neocortex of the tammar has been followed from 24 days gestation to 213 days postnatal. The tammar is born at 27 days gestation and the major period of its development occurs during the subsequent 250 days, most of this time being spent within the pouch. Although the pattern of differentiation of the cell layers appears to resemble that described for many Eutherian mammals, the neocortex is at an embryonic 2 layered stage at birth and a cortical plate is not present throughout the telencephalon until 10-15 days postnatal. A transient subplate zone, presenting a characteristic appearance with widely spaced rows of cells aligned parallel to the cortical surface, develops between 20 and 70 days postnatal, but no secondary proliferative region is seen in the subventricular zone of the dorso-lateral wall. Preliminary experiments with (3H)-thymidine injections indicate that the cortical plate follows the "inside-out" pattern of development described in many Eutherian mammals and that the oldest neurons are found in the parallel cell rows of the subplate zone. The importance of the late differentiation of the neocortex in relation to the time of birth and the resulting usefulness of the tammar as an experimental model of cortical development is discussed.

Aging↗
PubMed1984

Divergent neuropeptide evolutionary drifts between American and Australian marsupials.

Present-day marsupials, which are supposed to have arisen from a single stem diverging from the placental stem some 130 million years ago, exist only in the American and Australian continents. Comparison of the homologous genes and their protein products, which evolved under different environmental conditions, may provide arguments for either selective or neutral evolution. In contrast to Australian Macropodidae, which have peculiar neurohypophysial peptides, namely mesotocin and two pressor peptides, lysine vasopressin and phenypressin, the South American oppossum, Didelphis marsupialis, has oxytocin, lysine vasopressin, and arginine vasopressin. Because placental mammals have oxytocin and usually arginine vasopressin, and nonmammalian tetrapods have mesotocin and arginine vasotocin, it is assumed that (1) selective change of arginine vasotocin into arginine vasopressin occurred in mammalian ancestors and a subsequent gene duplication in the marsupial line gave rise to two pressor peptides with divergent neutral drifts in American and Australian groups, and (2) mesotocin of nonmammalian tetrapods has been preserved in Australian marsupials and reclaimed for milk-ejecting function whereas it has been converted into oxytocin in South American oppossums. The change of mesotocin into oxytocin seems neutral rather than selective.

Amino Acid Sequence↗
PubMed1980

Segmental amplification in a satellite DNA: restriction enzyme analysis of the major satellite of Macropus rufogriseus.

The 1.708 g/cc satellite DNA of the red necked wallaby is shown to consist of a number of related families of sequences arranged in tandem arrays. Particular families are subpopulations of other families; their distribution supports a model of successive amplification events during the generation of the satellite. In each amplification episode one 2,500 bp unit is multiplied into a tandemly repeated array of that unit (segmental amplification). The 1.708 g/cc sequences can be detected in related kangaroo species in much reduced amount, and with changes to the long order periodicity of the repeat units.

Animals↗
PubMed1993

Different rates of axonal degeneration in the crossed and uncrossed retinofugal pathways of Monodelphis domestica.

The uncrossed retinofugal fibres in the marsupial Monodelphis domestica form a separate bundle as they pass through the optic chiasm. The uncrossed fibres segregate from the crossed fibres a short distance before they reach the chiasm, gathering as an essentially exclusive bundle in the ventral part of the optic nerve. This bundle then passes laterally through the optic chiasm and into the optic tract. The distinctive position of the uncrossed fibres has allowed us to recognise that, surprisingly, the uncrossed fibres degenerate more rapidly than the rest. Seven days after a monocular enucleation approximately 60-80% of the fibres of the crossed component in the main part of the optic nerve near the chiasm have a normal cross sectional appearance in electron micrographs whereas less than 20% of the fibres in the uncrossed bundle look normal. The rapid degeneration of the uncrossed fibres cannot be related to any morphological parameter of the axons. Their fibre diameters are mainly medium to thick, lying within the range of axon diameters found in the rest of the nerve. The axon-myelin ratios of the uncrossed fibres are also no different from those of the crossed optic fibres. There are no structural peculiarities identifiable with light or electron microscopical methods in either the axons or in the glia of the uncrossed bundle that might account for the more rapid degeneration. There is evidence that the degenerative change in the main part of the optic nerve progresses from the lesion towards the chiasm, and that for the crossed fibres it may progress slightly faster for the thicker than for the thinner fibres. The degeneration in the uncrossed bundle does not fit any of the rules that have been proposed for relating rate of degeneration to fibre diameter. We conclude that the rate of Wallerian degeneration is determined by factors that yet remain to be defined.

Animals↗
PubMed1979

Preferential occurrence of sister chromatid exchanges at heterochromatin-euchromatin junctions in the wallaby and hamster chromosomes.

Chromosomes of two mammalian species, the white-throated wallaby and the rat-like hamster, possessed large amounts of constitutive heterochromatin which is detectable as C bands. By making use of this character the frequency of sister chromatid exchanges (SCEs) was determined for the C band and the euchromatic regions of the chromosome. In both species, the distribution of SCEs in the euchromatin of chromosomes was found to be proportional to its metaphase length, while the number of SCEs localized in the C band regions was clearly fewer than expected on the basis of the relative length of those regions at metaphase. Many SCE's were, however, detected at the junctions between the euchromatin and the C band heterochromatin. All of these findings were consistent with previous observations on the Indian muntjac and the kangaroo rat chromosomes.

Animals↗
PubMed1982

The structure of the lung of the newborn marsupial bandicoot, Isoodon macrourus.

In bandicoots, implantation takes place 9.5 days after mating and birth occurs 3 days later. It is therefore of interest to examine the lungs of newborn marsupials to ascertain the stage of lung development with respect to other organs and other mammals in view of the short period of time allowed for maturation of lung tissue. The newborn bandicoot lung is composed of a primitive system of branching airways which terminate in large blind sacs; alveolar ducts or alveoli are not present. The terminating sacs, which are delineated by thick connective-tissue septae, have a highly vascularised internal lining. Thin squamous cells and numerous type-II "surfactant"-secreting cells constitute the air interface of the sac. Corticosteroids influence the development of the eutherian fetal lung by accelerating pulmonary maturation and increasing the synthesis of surfactant. Since recent evidence indicates that the adrenal of the bandicoot is active at, or before birth, it is possible that this organ also plays an important role in preparing the lungs of the marsupial for extra-uterine life in the pouch.

Animals↗
PubMed1982

A geographic approach to senile cataracts: possible links with milk consumption, lactase activity, and galactose metabolism.

Examined in this article is presently available evidence for the hypothesis that some types of senile cataracts may be brought on by decades-long consumption of milk and milk products. The author approaches the question from a background of research in the geography and history of dairying as these relate to present-day differences among the world's peoples in prevalence of primary adult lactose malabsorption, which is based on a deficiency of the enzyme lactase in adulthood. Among peoples who have consumed milk in lactose-rich forms over a long historical period, there seems to have been a mutation for persistence of high lactase activity throughout life (PHLA), which distinguishes them from human populations of nonmilking tradition and from most land mammals. PHLA permits greater intestinal hydrolysis of lactose and absorption of galactose by adults. The mutation for PHLA, however, was not accompanied by a second one raising galactokinase activity to high levels through life. The result may be that adults who consume large quantities of milk, who have high lactase activity, lactose hydrolysis, and galactose absorption, suffer repeated small galactose challenges, accumulation of galactitol in the lens, and a greater likelihood of developing senile cataracts.

Adult↗
PubMed1987

Restriction enzyme maps of the macropodid herpesvirus 2 genome.

The DNA of macropodid herpesvirus 2 (dorcopsis wallaby herpesvirus) was analysed using restriction enzymes and molecular cloning techniques. Sets of EcoRI and SalI clones were prepared which represented approximately 85 per cent of the genome, and these clones were used to map the DNA by double-digestion and hybridization experiments. Data on uncloned regions were obtained by analysing fragments excised from agarose gels, and terminal fragments were identified by exonuclease III digestion. The genome was shown to be approximately 135 kilobases (kb) long. It has a long unique sequence (95 kb) bounded by repeat sequences (5.5 kb) and joined at one end to a short unique sequence (15 kb) also bounded by repeat sequences (7.0 kb) in a manner similar to that of herpes simplex viruses 1 and 2.

Animals↗
PubMed1993

Selected features of marsupial genetics.

As a consequence of the ancient separation of the marsupial and eutherian lineages, comparative genetical studies of these two mammalian taxa can be particularly informative. The potential for marsupial genetical research has been enhanced by the development of laboratory colonies of three 'model' species--Macropus eugenii, Monodelphis domestica and Sminthopsis crassicaudata. In this paper two selected aspects of marsupial genetics are reviewed, one involving cytogenetics and the other linkage. Marsupials provide a spectacular example of karyotypic conservation. The so-called 'basic karyotype' (2n = 14) is probably ancestral in all extant marsupials. Karyotypes that do not conform to this basic arrangement are thought to have been derived from it. A notable feature of the basic karyotype is that it has been retained, possibly for as long as 150 million years, in morphologically, behaviourally and ecologically diverse species from at least five Australian and two American families; this suggests that selective forces, presently unknown, have acted to conserve the basic chromosome form and number in these species. With respect to genetic linkage, family studies in S. crassicaudata and more recently M. domestica have indicated extreme differences between the sexes with the recombination frequencies for linked loci being very much greater in males than in females, a situation that is strikingly different from that in eutherian mammals. These differences in linkage values are paralleled by differences in the number and distribution of chiasmata during male and female meiosis. Prospects for further research in marsupials, particularly research that builds upon the observations of karyotypic conservation and genetic linkage, are noted.

Animals↗
PubMed1979

Marsupial--mouse cell hybrids containing fragments of the marsupial X chromosome.

Hybrids were obtained from fusions of HPRT-deficient mouse fibroblasts and marsupial lymphocytes. These hybrids retained no identifiable marsupial chromosomes, but all expressed the marsupial form of HPRT. Half the clones also expressed marsupial PGK-A, and half of these also marsupial G6PD; no other marsupial allozyme markers were detected. Since G6PD is known to be sex linked in these species, we conclude that Hpt and Pgk-A are also located on the X chromosome and the markers lie in the order Hpt-Pgk-A-Gpd.

Animals↗
PubMed1990

Calcium and strontium activation characteristics of skeletal muscle fibres from the small marsupial Sminthopsis macroura.

Mechanically skinned skeletal muscle fibres from the soleus and tibialis anterior muscles of the small marsupial Sminthopsis macroura were activated by Ca2+ and Sr2+ so that their isometric force properties could be determined. The properties characterized were the shape, slope and positions of the curves generated by plotting isometric force vs. pCa (-log10[Ca2+]) and pSr (-log10[Sr2+]), the maximum Ca2(+)-activated and Sr2(+)-activated tension (Ncm-2) and the frequency of force oscillations of myofibrillar origin during submaximal activations. The effect of caffeine on force activation was also studied. Apart from the fibres which exhibited physiological characteristics similar to those observed previously in mammalian fibres, a large proportion of fibres exhibited characteristics or combinations of characteristics which have not previously been described from healthy adult mammals. The results from 32 soleus fibres showed that only 23 could be categorized as either typical fast-twitch or slow-twitch fibres. The rest possessed unusual physiological characteristics which suggested the co-existence in the same fibre of Ca2(+)-regulatory and contractile properties from different categories of fast-twitch and slow-twitch fibres. We could distinguish two major fast twitch populations of tibialis anterior fibres which occurred in similar proportions. There were significant differences in the maximum tension produced by some of these groups of fibres. The tibialis anterior population fibres produced the highest maximum tension (ToCa 44.6 +/- 4.6 Ncm-2, n = 7) while the soleus combined type fibres produced the lowest maximum tension (ToCa 18.1 +/- 2.1 Ncm-2, n = 8). Our physiological observations of the Ca2(+)-activation and Sr2(+)-activation properties of soleus fibres in this study provide new evidence that there can be combinations of characteristics in single fibres and a continuum of properties between fibre types in normal mammalian skeletal muscles. These animals can therefore be used as a source of fibres with a wide range of properties.

Animals↗