Restriction fragment length variants in the marsupialSminthopsis crassicaudata

First-generation linkage map of the gray, short-tailed opossum, Monodelphis domestica, reveals genome-wide reduction in female recombination rates

The gray, short-tailed opossum, Monodelphis domestica, is the most extensively used, laboratory-bred marsupial resource for basic biologic and biomedical research worldwide. To enhance the research utility of this species, we are building a linkage map, using both anonymous markers and functional gene loci, that will enable the localization of quantitative trait loci (QTL) and provide comparative information regarding the evolution of mammalian and other vertebrate genomes. The current map is composed of 83 loci distributed among eight autosomal linkage groups and the X chromosome. The autosomal linkage groups appear to encompass a very large portion of the genome, yet span a sex-average distance of only 633.0 cM, making this the most compact linkage map known among vertebrates. Most surprising, the male map is much larger than the female map (884.6 cM vs. 443.1 cM), a pattern contrary to that in eutherian mammals and other vertebrates. The finding of genome-wide reduction in female recombination in M. domestica, coupled with recombination data from two other, distantly related marsupial species, suggests that reduced female recombination might be a widespread metatherian attribute. We discuss possible explanations for reduced female recombination in marsupials as a consequence of the metatherian characteristic of determinate paternal X chromosome inactivation.

Evolution and molecular characterization of a beta-globin gene from the Australian Echidna Tachyglossus aculeatus (Monotremata)

Coinciding with a period in evolution when monotremes, marsupials, and eutherians diverged from a common ancestor, a proto-beta-globin gene duplicated, producing the progenitors of mammalian embryonic and adult beta-like globin genes. To determine whether monotremes contain orthologues of these genes and to further investigate the evolutionary relationships of monotremes, marsupials, and eutherians, we have determined the complete DNA sequence of an echidna (Tachyglossus aculeatus) beta-like globin gene. Conceptual translation of the gene and sequence comparisons with eutherian and marsupial beta-like globin genes and echidna adult beta-globin indicate that the gene is adult expressed. Phylogenetic analyses do not clearly resolve the branching pattern of mammalian beta-like globin gene lineages and it is therefore uncertain whether monotremes have orthologues of the embryonic beta-like globin genes of marsupials and eutherians. Four models are proposed that provide a framework for interpreting further studies on the evolution of beta-like globin genes in the context of the evolution of monotremes, marsupials, and eutherians.

Evolution and expression of a beta-like globin gene of the Australian marsupial Sminthopsis crassicaudata

A beta-like globin gene was isolated from the Australian dasyurid marsupial Sminthopsis crassicaudata. Nucleotide-sequence analysis of promoter and coding regions of the gene revealed that it was orthologous to eutherian early-expressed (epsilon, gamma, eta) beta-like globin genes. Comparison of the conceptually translated sequence of the gene with a partial amino acid sequence of the adult beta-globin chain from S. crassicaudata provided evidence that the gene was not expressed in adult tissues. In addition, Northern analysis of RNA isolated from an embryo, pouch young, and adult bone marrow indicated that the gene was expressed predominantly in embryonic tissues and that there was a significant reduction in the expression of the gene within a day of birth. These results provide strong support for the hypothesis of Koop and Goodman [Koop, B. F. & Goodman, M. (1988) Proc. Natl. Acad. Sci. USA 85, 3893-3897] that an embryonic beta-like globin gene existed prior to the divergence of the eutherian and marsupial lineages and that this gene was already differentiated with respect to its promoter regions and developmental expression. The observation that epsilon-globin mRNA was present at least until day 4 postpartum suggests that the epsilon-globin chain may play some role in influencing the physiological properties of hemoglobin in S. crassicaudata neonates.

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.