Novel sex differences in linkage values and meiotic chromosome behaviour in a marsupial

There have been few reported family studies of gene segregation in marsupials and no report on genetic linkage in a marsupial species. The paucity of such genetic data probably stems from the lack of marsupial species sufficiently well adapted to a laboratory environment to give the abundant progeny from controlled crosses which are required in such work. A colony of the fat-tailed insectivore Sminthopsis crassicaudata (Marsupialia: Dasyuridae), which was established in the R. A. Fisher Laboratories, University of Adelaide, more than 20 years ago, has now provided unusual linkage data. These data indicate that the linkage situation in this marsupial species differs markedly from that in eutherian mammals. Extremely large differences exist between the sexes in the values of the recombination frequencies, with much closer linkage occurring in females. Cytological examination of meiosis in males and females has revealed a major difference between the sexes in chromosome behaviour.     

Rsx is a metatherian RNA with Xist-like properties in X-chromosome inactivation

In female (XX) mammals, one of the two X chromosomes is inactivated to ensure an equal dose of X-linked genes with males (XY). X-chromosome inactivation in eutherian mammals is mediated by the non-coding RNA Xist. Xist is not found in metatherians (marsupials), and how X-chromosome inactivation is initiated in these mammals has been the subject of speculation for decades. Using the marsupial Monodelphis domestica, here we identify Rsx (RNA-on-the-silent X), an RNA that has properties consistent with a role in X-chromosome inactivation. Rsx is a large, repeat-rich RNA that is expressed only in females and is transcribed from, and coats, the inactive X chromosome. In female germ cells, in which both X chromosomes are active, Rsx is silenced, linking Rsx expression to X-chromosome inactivation and reactivation. Integration of an Rsx transgene on an autosome in mouse embryonic stem cells leads to gene silencing in cis. Our findings permit comparative studies of X-chromosome inactivation in mammals and pose questions about the mechanisms by which X-chromosome inactivation is achieved in eutherians.     

限食程度对KM小鼠体重和能量代谢的影响

为阐明体重和能量代谢的生理调节对动物适应食物短缺的作用和意义,将成年KM小鼠按自由取食量的75%和50%随机限食四周,再重喂食四周.分别测定了摄食量、基础代谢率(BMR)和脂肪含量.限食75%和50%使BMR和消化道重量显著增加,体重、胴体重、脂肪含量显著降低;重喂食后,BMR和体重恢复到对照组水平,脂肪含量显著高于对照组.结果表明,动物通过能量代谢和体重调节以适应不同程度的食物资源短缺的应激环境,通过增加身体能量贮存以增强应对食物短缺的适应能力.     

Sequences homologous to ZFY, a candidate human sex-determining gene, are autosomal in marsupials

Sexual differentiation in placental mammals results from the action of a testis-determining gene encoded by the Y chromosome. This gene causes the indifferent gonad to develop as a testis, thereby initiating a hormonal cascade which produces a male phenotype. Recently, a candidate for the testis-determining gene (ZFY, Y-borne zinc-finger protein) has been cloned. The ZFY probe detects a male-specific (Y-linked) sequence in DNA from a range of eutherian mammals, as well as an X-linked sequence (ZFX) which maps to the human X chromosome. In marsupials it is also the Y chromosome that seems to determine the fate of the gonad, but not all sexual dimorphisms. Using the ZFY probe we find, surprisingly, that the ZFY homologous sequences are not on either the X or the Y chromosome in marsupials, but map to the autosomes. This implies ZFY is not the primary sex-determining gene in marsupials. Either the genetic pathways of sex determination in marsupials and eutherians differ, or they are identical and ZFY is not the primary signal in human sex determination.     

Evolution of sex determination and the Y chromosome: SRY-related sequences in marsupials.

In mammals, testis determination is under the control of the testis-determining factor borne by the Y chromosome. SRY, a gene cloned from the sex-determining region of the human Y chromosome, has been equated with the testis-determining factor in man and mouse. We have used a human SRY probe to identify and clone related genes from the Y chromosome of two marsupial species. Comparisons of eutherian and metatherian Y-located SRY sequences suggest rapid evolution of these genes, especially outside the region encoding the DNA-binding HMG box. The SRY homologues, together with the mouse Ube1y homologues, are the first genes to be identified on the marsupial Y chromosome.     

Fitness costs of gestation and lactation in wild mammals

Like a number of plants, some mammals commonly produce more progeny than they can afford to rear, terminating investment in some or even all of their offspring once the resources available for breeding are known. Adaptive interpretations of juvenile wastage rely on the argument that the costs of gestation are small compared to those of feeding offspring. Though energetic evidence supports this conclusion, it is unsafe to assume that the relative costs of gestation and lactation to the mother's survival and future reproductive success follow the same pattern because lactation commonly coincides with the period of maximum food availability. Controlling for individual variation, we show that in wild red deer (Cervus elaphus L) any costs of gestation to the mother's subsequent survival and reproductive success are slight compared to those of lactation.     

Tribosphenic mammal from the North American Early Cretaceous

The main groups of living mammals, marsupials and eutherians, are presumed to have diverged in the Early Cretaceous, but their early history and biogeography are poorly understood. Dental remains have suggested that the eutherians may have originated in Asia, spreading to North America in the Late Cretaceous, where an endemic radiation of marsupials was already well underway. Here I describe a new tribosphenic mammal (a mammal with lower molar heels that are three-cusped and basined) from the Early Cretaceous of North America, based on an unusually complete specimen. The new taxon bears characteristics (molarized last premolar, reduction to three molars) otherwise known only for Eutheria among the tribosphenic mammals. Morphometric analysis and character comparisons show, however, that its molar structure is primitive (and thus phylogenetically uninformative), emphasizing the need for caution in interpretation of isolated teeth. The new mammal is approximately contemporaneous with the oldest known Eutheria from Asia. If it is a eutherian, as is indicated by the available evidence, then this group was far more widely distributed in the Early Cretaceous than previously appreciated. An early presence of Eutheria in North America offers a potential source for the continent's Late Cretaceous radiations, which have, in part, proven difficult to relate to contemporary taxa in Asia.     

热驯化对高山姬鼠体重调节的影响

为了研究高温对高山姬鼠体重调节的影响,测定了30℃条件下高山姬鼠的体重、基础代谢率、非颤抖性产热、摄入能和血清瘦素含量。结果表明:在热驯化过程中下,高山姬鼠体重变化差异不显著;但摄入能、基础代谢率和非颤抖性产热显著降低。血清瘦素含量和摄入量呈负相关。以上结果表明高山姬鼠在高温条件下会降低产热来适应这一环境条件,瘦素可能参与了高山姬鼠的体重调节。     

IRS-2 pathways integrate female reproduction and energy homeostasis

Severe dietary restriction, catabolic states and even short-term caloric deprivation impair fertility in mammals. Likewise, obesity is associated with infertile conditions such as polycystic ovary syndrome. The reproductive status of lower organisms such as Caenorhabditis elegans is also modulated by availability of nutrients. Thus, fertility requires the integration of reproductive and metabolic signals. Here we show that deletion of insulin receptor substrate-2 (IRS-2), a component of the insulin/insulin-like growth factor-1 signalling cascade, causes female infertility. Mice lacking IRS-2 have small, anovulatory ovaries with reduced numbers of follicles. Plasma concentrations of luteinizing hormone, prolactin and sex steroids are low in these animals. Pituitaries are decreased in size and contain reduced numbers of gonadotrophs. Females lacking IRS-2 have increased food intake and obesity, despite elevated levels of leptin. Our findings indicate that insulin, together with leptin and other neuropeptides, may modulate hypothalamic control of appetite and reproductive endocrinology. Coupled with findings on the role of insulin-signalling pathways in the regulation of fertility, metabolism and longevity in C. elegans and Drosophila, we have identified an evolutionarily conserved mechanism in mammals that regulates both reproduction and energy homeostasis.     

Dinosaurian growth patterns and rapid avian growth rates

Did dinosaurs grow in a manner similar to extant reptiles, mammals or birds, or were they unique? Are rapid avian growth rates an innovation unique to birds, or were they inherited from dinosaurian precursors? We quantified growth rates for a group of dinosaurs spanning the phylogenetic and size diversity for the clade and used regression analysis to characterize the results. Here we show that dinosaurs exhibited sigmoidal growth curves similar to those of other vertebrates, but had unique growth rates with respect to body mass. All dinosaurs grew at accelerated rates relative to the primitive condition seen in extant reptiles. Small dinosaurs grew at moderately rapid rates, similar to those of marsupials, but large species attained rates comparable to those of eutherian mammals and precocial birds. Growth in giant sauropods was similar to that of whales of comparable size. Non-avian dinosaurs did not attain rates like those of altricial birds. Avian growth rates were attained in a stepwise fashion after birds diverged from theropod ancestors in the Jurassic period.     

哺乳动物母体繁殖对策的研究

哺乳动物以雌体哺乳和雄性有限的 贡献共同哺育。雌体是整个繁殖过程的中心事件,也是最耗能量的过程。哺乳动物哺乳的行为表现可影响母体民仔未来的繁殖。哺乳期越长对后代越有利,但另一方面,母体的繁殖代价可影响其存活和繁殖力;后代质量与胎仔数间存在着权衡,作为胎仔数的函数,后代质量以种群新生个体补充概率潜在地影响亲本的适合度;与出生时不明确的格局相比较,育幼结束时的权衡通常非常明显;后代体重与生长速率的差异可影响新生个体的补充数;如果幼仔体重差异保持到成体,这些差异将超越简单的后代补充数而持续影响母体的广义适合度。雌性成体体重与胎仔数之间存在明显的相关性。     

地源热泵与传统热泵工程应用的对比分析

简述了地源热泵的形式,并基于热泵系统的技术性能、初投资和运行经济性的对比分析了地源热泵与传统热泵的工程应用,得出了地源热泵与传统热泵相比具有初投资少、运行费用低、运行安全可靠、无污染和可再生能源合理利用的特点。     

Body weight, diet and home range area in primates.

Primates show a strong positive relationship between body weight and home range area. Dietary habits also influence home range area. Folivorous primates occupy smaller home range areas for their body weight than do frugivores and omnivores. Primates generally require smaller home range area per individual than solitary terrestrial mammals, but primates living in social groups have much larger total home range than individual solitary mammals. This trend may necessitate higher expenditures of energy in food-gathering or modifications in movement patterns.     

Dual origin of tribosphenic mammals

Marsupials, placentals and their close therian relatives possess complex (tribosphenic) molars that are capable of versatile occlusal functions. This functional complex is widely thought to be a key to the early diversification and evolutionary success of extant therians and their close relatives (tribosphenidans). Long thought to have arisen on northern continents, tribosphenic mammals have recently been reported from southern landmasses. The great age and advanced morphology of these new mammals has led to the alternative suggestion of a Gondwanan origin for the group. Implicit in both biogeographic hypotheses is the assumption that tribosphenic molars evolved only once in mammalian evolutionary history. Phylogenetic and morphometric analyses including these newly discovered taxa suggest a different interpretation: that mammals with tribosphenic molars are not monophyletic. Tribosphenic molars evolved independently in two ancient (holotherian) mammalian groups with different geographic distributions during the Jurassic/Early Cretaceous: an australosphenidan clade endemic to Gondwanan landmasses, survived by extant monotremes; and a boreosphenidan clade of Laurasian continents, including extant marsupials, placentals and their relatives.     

Convergent dental adaptations in pseudo-tribosphenic and tribosphenic mammals

Tribosphenic molars of basal marsupials and placentals are a major adaptation, with the protocone (pestle) of the upper molar crushing and grinding in the talonid basin (mortar) on the lower molar. The extinct pseudo-tribosphenic mammals have a reversed tribosphenic molar in which a pseudo-talonid is anterior to the trigonid, to receive the pseudo-protocone of the upper molar. The pseudo-protocone is analogous to the protocone, but the anteriorly placed pseudo-talonid is opposite to the posterior talonid basin of true tribosphenic mammals. Here we describe a mammal of the Middle Jurassic period with highly derived pseudo-tribosphenic molars but predominantly primitive mandibular and skeletal features, and place it in a basal position in mammal phylogeny. Its shoulder girdle and limbs show fossorial features similar to those of mammaliaforms and monotremes, but different compared with those of the earliest-known Laurasian tribosphenic (boreosphenid) mammals. The find reveals a much greater range of dental evolution in Mesozoic mammals than in their extant descendants, and strengthens the hypothesis of homoplasy of 'tribosphenic-like' molars among mammals.     

Mammal teeth from the Cretaceous of Africa

We report here the discovery of two mammal teeth from the early Cretaceous of Cameroon. These, and some jaw fragments, all from Cameroon, are the only fossil evidence of mammalian evolution from Africa between late Jurassic and Paleocene, a span of at least 85 million years. A triangular upper tooth lacks the principal internal cusp of marsupials and placentals and is therefore of a similar evolutionary grade to most Jurassic and early Cretaceous therian mammals, but more primitive than the metatherian-eutherian grade. Early Cretaceous, or older, therian mammals are now known from all southern continents except Antarctica. The new find from Cameroon is consistent with the hypothesis that marsupials, the dominant living mammals of South America and Australia, were not present on any Gondwana continents until after the early Cretaceous separation of Africa by the opening of the South Atlantic.     

Late Cretaceous relatives of rabbits, rodents, and other extant eutherian mammals.

Extant eutherian mammals and their most recent common ancestor constitute the crown group Placentalia. This taxon, plus all extinct taxa that share a more recent common ancestor with placentals than they do with Metatheria (including marsupials), constitute Eutheria. The oldest well documented eutherian-dominated fauna in the world is Dzharakuduk, Uzbekistan. Among eutherians that it yields is Kulbeckia, an 85-90-Myr-old member of Zalambdalestidae (a family of Late Cretaceous Asian eutherians). This extends Zalambdalestidae back by some 10 million years from sites in the Gobi Desert, Mongolia. A phylogenetic analysis of well described Late Cretaceous eutherians strongly supports Zalambdalestidae, less strongly supports 'Zhelestidae' (a Late Cretaceous clade related to Tertiary ungulates), but does not support Asioryctitheria (a group of Late Cretaceous Asian eutherians). A second analysis incorporating placentals from clades that include rodents (Tribosphenomys), lagomorphs (Mimotona) and archaic ungulates (Protungulatum and Oxyprimus) strongly supports Zalambdalestidae in a clade with Glires (rabbits, rodents and extinct relatives) and less strongly 'Zhelestidae' within a clade that includes archaic ungulates ('condylarths'). This argues that some Late Cretaceous eutherians belong within the crown group Placentalia. The ages of these taxa are in line with molecularly based estimates of 64-104 Myr ago (median 84 Myr ago) for the superordinal diversification of some placentals, but provide no support for a Late Cretaceous diversification of extant placental orders.     

Birth of parthenogenetic mice that can develop to adulthood

Only mammals have relinquished parthenogenesis, a means of producing descendants solely from maternal germ cells. Mouse parthenogenetic embryos die by day 10 of gestation. Bi-parental reproduction is necessary because of parent-specific epigenetic modification of the genome during gametogenesis. This leads to unequal expression of imprinted genes from the maternal and paternal alleles. However, there is no direct evidence that genomic imprinting is the only barrier to parthenogenetic development. Here we show the development of a viable parthenogenetic mouse individual from a reconstructed oocyte containing two haploid sets of maternal genome, derived from non-growing and fully grown oocytes. This development was made possible by the appropriate expression of the Igf2 and H19 genes with other imprinted genes, using mutant mice with a 13-kilobase deletion in the H19 gene as non-growing oocytes donors. This full-term development is associated with a marked reduction in aberrantly expressed genes. The parthenote developed to adulthood with the ability to reproduce offspring. These results suggest that paternal imprinting prevents parthenogenesis, ensuring that the paternal contribution is obligatory for the descendant.     

Is the guinea-pig a rodent?

The guinea-pig (Cavia porcellus), traditionally classified as a New World hystricomorph rodent, often shows anomalous morphological and molecular features in comparison with other eutherian mammals. For example, its insulin differs from that of other mammals in anabolic and growth-promoting activities and in its capability to form hexamers. Indeed, the literature about the molecular evolution of guinea-pigs abounds in references to 'convergent evolution', 'extremely rapid rates of substitution', and 'unique evolutionary mechanisms'. These claims are based on the assumption that the guinea-pig is a rodent. Our phylogenetic analyses of amino-acid sequence data, however, imply that the guinea-pig diverged before the separation of the primates and the artiodactyls from the myomorph rodents (rats and mice). If true, then the myomorphs and the caviomorphs do not constitute a natural clade, and the Caviomorpha (or the Histricomorpha) should be elevated in taxonomical rank and regarded as a separate mammalian order distinct from the Rodentia. If, as suggested by recent data, the myomorphs branched off before the divergence among the carnivores, lagomorphs, artiodactyls and primates, then the new order would represent an early divergence in eutherian radiation.