Cytogenetic studies of the Australian rodent, Uromys caudimaculatus, a species showing extensive heterochromatin variation

The Australian rodent, Uromys caudimaculatus, consists of two chromosome races. a) The Southern Race is characterized by the possession of two to twelve B-chromosomes. These vary considerably in size, morphology, and C- and G-banding characteristics, they behave as univalent at meiosis and are inherited in a random manner. b) The Northern Race lacks B-chromosomes, but possesses large blocks of distal C-positive heterochromatin on between 18 and 28 of the 46 chromosomes. The C-blocks may be entirely G-positive entirely G-negative, or G-banded, suggesting heterogeneity with the C-blocks. There is extensive variation both between and within populations of the northern race in the number and size of the distal heterochromatic blocks. There is no apparent difference between the races in chiasma frequency. The northern race does have much higher proportion of interstitial versus distal chiasmata, although it is probable that this is merely a reflection of lack of crossing over in the heterochromatic blocks rather than an actual shift of chiasma localisation within the euchromatin. Despite the extensive differences between the races in the amount and organization of constitutive heterochromatin, hybrids show no abnormalities at meiosis and they are fully fertile.

Water balance of small lactating rodents--III. Estimates of milk production and water recycling in lactating Mus musculus under various water regimes

1. The exchanges of water between lactating female and young Mus musculus were modelled on the computer. 2. The model was used to estimate rates of milk production and water recycling in various litter sizes under various water regimes by following the time course of injected tritiated water. 3. The high correlation between estimated rates of milk production and actual growth rates of young was taken to indicate that the method gave if not the actual rates of milk production a very constant proportion of it. 4. Approximately 50% of the water secreted in milk is returned to the mother by recycling.

Genetical Relationships among Australian Rodents (Muridae)

A total of 168 specimens of 38 species of Australian native murid rodents and three species of nonnative rodents were screened electrophoretically at 20 loci. Genetic and phylogenetic relationships among species were assessed by means of Average Linkage Cluster and Wagner Analysis respectively. The major conclusions of the analysis are: the genera Melomys, Leggadina, Zyzomys, Notomys and Rattus ire each monophyletic, at least for the species we had; the genus Pseudomys is largely monophyletic; although it may be paraphyletic with respect to Mastacomys and Leporillus; the genus Pseudomys is nevertheless a genetically diverse group consisting of at least four groups, none of which correspond to the earlier genera Gyomys and Thetomys; the 'Old Endemics', the water rats, and the Uromys- Melomys groups are genetically more similar to each other than any is to Rattus. Within Rattus, the following groupings emerge: R. sordidus, R. colletti and R. villosissimus: R. t. tunneyi and R. t. culmorum; R. l. leucopus and R. l. cooktownensis; R. fuscipes coracius, R. f: assimilis and R. f. greyii.

Taxonomic Separation of Eptesicus (Chiroptera:Vespertilionidae) in South Eastern Australia by Discriminant Analysis and Electrophoresis

Eptesicus from south-eastern New South Wales and the Australian Capital Territory were used in a combination of multivariate (discriminant) analysis of skeletal dimensions and isozyme electrophoresis of liver, plasma and red cell proteins. Both techniques separated the bats unequivocally into three groups. The groups identified by discriminant analysis were the same as those derived by the electrophoretic method and almost certainly belong to distinct biological species, for even when they occur sympatrically there are many fixed gene differences between them. The three groups are shown to correspond to the species E. vulturnus, E. regulus and E. sagittula. Structural differences between these species and methods of identifying them are discussed.

Rat kappa-chain allotypes. III. Serological and genetic analysis of RI-1b cross-reactivity in Australian Rattus

The presence of rat kappa-chain allotype specificities (RI-1a and b) has been studied in 13 subspecies of the seven native Australian species of Rattus. RI-1a reactivity was not detected among these rats. On the other hand, extensive cross-reactivity was seen with RI-1b, some sera cross-reacting totally (R. leucopus cooktownensis), some not at all (R. colletti), and the remainder showing at least two distinct levels of partial cross-reactivity, confirming the existence of multiple specificities for RI-1b. Three subspecies show polymorphism with respect to RI-1b cross-reactivity (R. sordidus, R. colletti, and R. l. leucopus) and in one case (leucopus) breeding studies have indicated that there is allelic inheritance of this trait. The segregation of RI-1b reactivity has been studied in crosses and backcrosses made between species differing in their RI-1b reactivity, and the results are consistent with the existence of codominant alleles at a single locus. The fact that these species differ extensively in their karyotype opens the door to possible chromosomal localization of this and other genetic traits.

Restriction endonuclease and molecular analyses of three rat genomes with special reference to chromosome rearrangement and speciation problems

When differences are found between related species of organisms, it is often assumed that the differences themselves are causal factors either in speciation itself or in processes related to speciation. Two recent proposals on the functions of satellite DNA (Hatch et al., 1976 and Fry and Salser 1977) are that (a) large amounts of satellite DNA are important in facilitating chromosome rearrangements and hence cytogenetic evolution, and (b) satellite DNA differences between homologous chromosomes lead to pairing difficulties and are important in generating infertility barriers and hence speciation. If these proposals were to have some generality, one could expect organisms with very low amounts of highly repeated DNA to exhibit few chromosome rearrangements and to be evolutionarily conservative in a cytogenetic sense.--We have chosen two very closely related species of rat which are phenotypically almost indistinguishable and which have undergone massive genome reorganization. They differ by 11 major centric rearrangements (2n = 32, 2n = 50). We have characterised their genomes by restriction endonuclease digestions, thermal denaturations, analytical ultracentrifugations and reassociation techniques, and have found that they have virtually no highly repeated DNA. Thus the 11 major chromosomal rearrangements have been fixed in present day genomes with hardly any highly repeated DNA, centric or otherwise.--It appears therefore, that a large amount of highly repeated DNA is not obligatory for the formation and fixation of chromosome rearrangements. In addition, the existing literature reveals that one can find almost any situation at all, from species groups with high amounts of satellite DNA and no gross chromosomal rearrangements, to ones such as those described here, with tiny amounts of highly repeated DNA and massive chromosomal reorganisation. Since direct experimental data indicates that satellite DNA differences per se between homologous chromosomes do not cause infertility, speculations concerning modes of speciation based on satellite DNA differences between otherwise homologous chromosomes would appear to be ill founded.

The probable basis of the relationship between growth rate and winter mortality in the lizard Amphibolurus ornatus (Agamidae)

At Bakers Hill, differential winter mortality of juvenile slowgrowing Amphibolurus ornatus is correlated with the incidence of frosts (Bradshaw, 1971). The present study shows that the probability an individual will spend the night in a refuge that is safe from the lethal effect of a frost is directly related to the individual's size. Thus should frosts occur during any particular winter, juvenile slow-growers, will suffer a higher mortality than juvenile fast-growers. At Tuttanning, animals do not segregate according to size, and consequently juvenile slow-growers do not suffer increased mortality during frosty winters.

Genetic similarities and differences between sublines of certain inbred strains of mice held within Australia

Inbred mice of the strains C57B1/6, CBA, C3H, Balb/c and DBA/2 from a total of 17 different sources within Australia were compared, utilizing 12 different blood proteins and a morphometric analysis of a series of lower jaws. Evidence for genetic differentiation between sublines was found in only two cases. Evidence for the recent contamination of an inbred strain with another stock was found in one case.

Polymorphism of the X-chromosome, Y-chromosome and autosomes in the Australian hopping mice, Notomys alexis, N. cervinus and N. fuscus (rodentia, muridae)

All three species of Notomys so far studied possess a diploid number of 48. Many elements in the karyotype of N. alexis are polymorphic due to variation in heterochromatin, but the variation is most marked in autosomal pair 1, which occurs in at least four forms, the X-chromosome, which occurs in three forms, and the Y-chromosome which occurs in many forms. N. cervinus is unique in the genus in possessing an entirely biarmed karyotype due mainly to the addition of heterochromatic short arms. The X-chromosome of N. cervinus occurs in three forms and the Y-chromosome in two forms. The karyotype of N. fuscus is mainly telocentric although two autosomal pairs are polymorphic due to pericentric inversions. The X- and Y-chromosomes both occur in two forms in N. fuscus. Chromosome measurements and C-banding show that most of the variation in the size and morphology of the sex-pair both within and between species is due to variation in constitutive heterochromatin.

Lack of genetic differentiation between domestic dogs and dingoes at a further 16 loci

Clarke, Ryan and Czuppon (1975) failed to detect electrophoretic differences between domestic dogs and dingoes at 14 loci. We have found that domestic dogs and dingoes are monomorphic for the same electrophoretic alleles at a further 15 loci, and polymorphic for the same alleles at a 30th locus.

Electrophoretic Comparisons between Allopatric Populations of Five Australian Pseudomyine Rodents (Muridae)

Allopatric populations of the pseudomyine rodents Pseudomys albocinereus, P. delicatulus, the P. nanus-P. gracilicaudatus complex, Zyzomys argurus and Mesembriomys gouldi were surveyed for electrophoretic variability of 14-17 red cell and plasma proteins. Few or no electrophoretic differences were found to parallel the chromosomal differences between populations of P. delicatulus, Z. argurus, and M. gouldi. Populations of the P. nanus-P. gracilicaudatus complex, however, fell into two groups defined both chromosomally and electrophoretically. The western form (P. nanus) extends from Western Australia into the Northern Territory whilst the eastern form (P. gracilicaudatus) occurs only along the east coast of Queensland and New South Wales. The biochemical differentiation between South Australian, Western Australian mainland and Bernier Island P. albocinereus parallels the chromosomal, morphological and breeding data, all of which indicate that the Western Australian mainland and Bernier Island forms belong to one species (P. albocinereus), whilst the South Australian form represents a distinct biological species (P. apodemoides).

Heterochromatin variation in the Australian rodent Uromys caudimaculatus

Ten individuals of Uromys caudimaculatus sampled from Queensland gave evidence for the occurrence of two distinct chromosome races characterised by marked differences in their pattern of C-banding. In all four individuals from the north, thirteen of the twenty three chromosome which make up the standard haploid set had substantial distal C-blocks in addition to the smaller centric blocks which characterise all chromosomes other than the Y. Additionally two pairs had an interstitial block. By contrast none of the six southern individuals had fixed distal blocks though all of them except the Y carry pro-centric C-blocks and again one pair showed an interstitial block. The southern karyotype was, however, characterised by the presence of from six to nine mitotically stable supernumerary chromosomes all of which were totally C-positive despite the fact that at least five morphologically distinguishable types have been defined. While the relationship of these two types of constitutive heterochromatin remains to be clarified the large amount present in both northern and southern animals suggests that heterochromatin plays an important role in the basic biology of this species.

Water Balance and Kidney Function in Four Species of Rattus From Ecologically Diverse Environments

While Rattus fuscipes survived only 4 days of water deprivation at 21�C, R. norvegicus, R. villosissimus and R. lutreolus survived 13-16 days. There was considerable inter-individual variation in the response of water-deprived R. villosissimus. Analysis of osmotic pressure, urea, sodium, potassium and chloride of both plasma and urine of rats with and without drinking water revealed that: (1) the abilities of R. norvegicus and R. villosissimus to tolerate water deprivation were due in large part to their abilities to produce highly concentrated urine; (2) R. lutreolus tolerated long periods of water deprivation not by urine-concentrating ability but by partly abandoning homeostasis and tolerating elevated levels of plasma solutes; (3) water-deprived R. fuscipes excreted large volumes of concentrated urine, possibly because their relatively high metabolic rate necessitated the excretion of excess metabolites. In all of the rats, urea constituted an unusually low proportion of the total osmotic pressure. The water-balance response of water-deprived rats is at variance with both their macrogeographical distribution and microhabitat preferences.

Karyotypic data and the specific status and taxonomic affinites of Rattus villosissimus and R. sordidus

Although R. villosissimus and R. sordidus have been considered to be conspecific, they differ karyotypically in at least 11 chromosomal rearrangements. It seems likely that they diverged early in the evolution of Rattus, and they should be regarded as separate species.