The compositional distribution of coding sequences and DNA molecules in humans and murids

Summary The compositional distributions of coding sequences and DNA molecules (in the 50-100-kb range) are remarkably narrower in murids (rat and mouse) compared to humans (as well as to all other mammals explored so far). In murids, both distributions begin at higher and end at lower GC values. A comparison of homologous coding sequences from murids and humans revealed that their different compositional distributions are due to differences in GC levels in all three codon positions, particularly of genes located at both ends of the distribution. In turn, these differences are responsible for differences in both codon usage and amino acids. When GC levels at first+second codon positions and third codon positions, respectively, of murid genes are plotted against corresponding GC levels of homologous human genes, linear relationships (with very high correlation coefficients and slopes of about 0.78 and 0.60, respectively) are found. This indicates a conservation of the order of GC levels in homologous genes from humans and murids. (The same comparison for mouse and rat genes indicates a conservation of GC levels of homologous genes.) A similar linear relationship was observed when plotting GC levels of corresponding DNA fractions (as obtained by density gradient centrifugation in the presence of a sequence-specific ligand) from mouse and human. These findings indicate that orderly compositional changes affecting not only coding sequences but also noncoding sequences took place since the divergence of murids. Such directional fixations of mutations point to the existence of selective pressures affecting the genome as a whole.     

Spacing behaviour of juvenile corn mice, Calomys musculinus, at the beginning of the breeding period, in absence of adult males

This research was carried out to examine the hypothesis that the absence of fathers promotes a different spacing behaviour in juveniles Calomys musculinus at the beginning of the breeding period. The study was carried out in four 0.25-ha enclosures (two control and two experimental), in a natural pasture, between November 2003 and February 2004. In this study the fathers were removed from the experimental enclosures after juveniles were born. Home-range size depended on sex of juveniles and treatment (father removal). In control and experimental enclosures, female home-range sizes were always smaller than male home-ranges. Male home-ranges were always larger in experimental enclosures than in control enclosures. Treatment and overlap type (intra- and inter-sexual) were not independent. The overlap proportions of male home-ranges were greatest in experimental enclosures than in control enclosures, in both the overlap types (male/male, males/females). The intra- (females/females) and inter-sexual (females/males) overlap proportions of female home-ranges were independent of treatment. In C. musculinus, at the beginning of the breeding period and in absence of adult males, juvenile males increase their home-range size and therefore the degree of inter- and intra-sexual home-range overlap as a mechanism for enlarging the number of receptive females that they encounter.     

Improved visualization of the immunoreactive hypothalamo-neurohypophysial system by use of immuno-gold techniques

Summary Ultrastructural post-embedding immuno-gold techniques were applied to the supraoptic nucleus and the neurohypophysis of mice and rats. The primary antibodies were three different monoclonal antineurophysins, used in protein A-gold and immunoglobulin-gold procedures. Conventional plastic embedding as well as hydrophilic media (L.R. White) were used; non-osmicated and osmicated tissues were immunolabeled; sodium metaperiodate oxidation was used, but was not essential for immunolabeling.Vasopressinergic and oxytocinergic NSGs were identified by the specific immunoreactivity of their respective neurophysins on adjacent thin sections, and by sequential double labeling on the same thin section using two different antibodies associated with gold probes of different diameters. The immunoidentification indicates that vasopressin NSGs can additionally be differentiated as larger, with more electron-dense matrix, and susceptible to damage by sodium metaperiodate.The only organelles consistently labeled were neurosecretory granules (NSGs), either intact or within lysosomal configurations. Some lysosomal dense bodies were immunoreactive even when discrete NSGs were no longer morphologically recognisable within them. Labeled NSGs were located within neuronal cell bodies, along axonal shafts and within axonal swellings and endings; occasionally immunoreactive NSGs were observed within synaptic boutons. Labeling intensity was semi-quantitatively gauged by counting gold particles in relation to numbers of NSGs per axonal varicosity.The precise localisation achieved with particulate immunogold labeling surpasses that previously obtained with diffuse electron-dense immunoreaction products.     

Compositional properties of coding sequences and mammalian phylogeny

The compositional distributions of large DNA fragments reflect those of the isochores that make up vertebrate genomes and can provide novel phylogenetic insights in the case of mammalian genomes (see Sabeur et al. 1993). This approach has been complemented here by an analysis of the compositional patterns of coding sequences and their codon positions (which also reflect the isochore pattern) and by a comparison of the base compositions of codon positions from homologous genes in a number of pairs of species. The results obtained using these two approaches support the existence of a general compositional pattern for mammalian genomes and of a distinct pattern for Myomorpha. The other two “special” patterns identified in a megachiropteran and in pangolin could not be tested here. Presented at the NATO Advanced Research Workshop onGenome Organization and Evolution, Spetsai, Greece, 16–22 September 1992     

Territorial behaviour in corn mice, <Emphasis Type="Italic">Calomys musculinus</Emphasis> (Muridae: Sigmodontinae), with regard to mating system

We studied territorial behaviour of Calomys musculinus, a promiscuous-polygynous species, in order to test the predictions that during the breeding period, home-range-owner females are more aggressive than home-range intruders and that males do not display territorial behaviour. At the core of the breeding season, we conducted 136 intrasexual and 66 intersexual encounters. The study was carried out in four 0.25-ha enclosures, each containing one independent population. We studied territorial behaviour utilising focal animal sampling to record behaviour. Encounters were conducted in a circular opaque arena (COA) near the home-range’s activity mathematical centre (AMC). Amicable behaviour was not observed in any of the 78 trials among females. The rates of aggressive and submissive behaviour per female–female encounters varied in relation to whether they were home-range owners or intruders, the former being aggressive, the latter submissive. So we could link these drives to the territorial behaviour of C. musculinus. This result fits well with the already known territoriality of C. musculinus during the breeding season. In contrast, the 58 intrasexual male encounters did not show direct aggressive interactions between them. The rates of the different behaviours between males did not vary in relation to whether they were home-range owners or intruders. In our study, which took place during the breeding period, C. musculinus males were nonterritorial, and they never exhibited aggressiveness towards females.     

Ultrastructural immunohistochemical localization of vasopressin in the hypothalamic-neurohypophysial system of three murids

Vasopressin was immunohistochemically localized at the electron microscopic (EM) level in the hypothalamic-neurohypophysial system (HNS) of three murids. Antiserum to vasopressin was produced in rabbits injected with lysine vasopressin (LVP) conjugated to egg albumin (EA), anti-EA being precipitated prior to staining. Sternberger's unlabeled antibody peroxidase technique was employed, immunoreactivity being designated by peroxidase anti-peroxidase (PAP) molecules and electron opacity. Immunoreactive neurosecretory granules (NSG) were found in the perikarya of the supraoptic nucleus (SON) in all three murids investigated, although far more profusely in the two wild strains. Immunoreactive axonal NSG were observed in the inner and outer zones of the median eminence (ME), and within most of the axons and terminals in the neurohypophysis. The concentration of primary serum effective for staining the SON (110–150) was far higher than that required for the ME and the neurohypophysis (1:500–1:1,200). AntiLVP also induced electron opacity of granules in cells of the pars intermedia (PI). Discussion centers on the significance of immunoreactive NSG in the neurosecretory (NS) perikarya, on the possibility of an extragranular pool of hormone, and on speculation about the electron opacity of the PI granules.     

Juvenile dispersal in Calomys venustus (Muridae: Sigmodontinae)

Both spacing behaviour and dispersal movement are viewed as hierarchical processes in which the effects may be expressed at spatial scale. This research was carried out to examine the hypothesis that the presence of parents promotes the dispersal of juveniles from their natal nest and their father or mother home-range, in Calomys venustus.The study was carried out in four 0.25 ha fences (two controls and two experimentals), in a natural pasture. This study had two periods: Father Removal (FR) (August and December 1997; year one) and Mother Removal (MR) (August 1998 and January 1999; year two). For the FR treatment fathers were removed after juveniles were born, but in the MR treatment mothers were removed after the juveniles were weaned. The effect of parents on the dispersal distance of juveniles was analysed with respect to their natal nest and their mother and father home-range. Dispersal distance from the nest of C. venustus was independent of either male or female parent. Juveniles were more dispersing in relation to the centre of activity of their mothers than to that of their fathers, and females were more dispersing than males. Female juveniles overlap their home-range with their parents less than male juveniles do. The differences observed between female and male juveniles would be related to their different sexual maturation times, as well as to the female territoriality.