To breed or not to breed: an analysis of the social and density-dependent constraints on the fecundity of female badgers (Meles meles).

Data from post-mortem examinations, population density estimates and long term capture-mark-recapture studies have been combined to look at the pattern of reproductive behaviour and the social factors leading to reproductive failure in badgers in Britain. The results are used to evaluate whether the hypothesis that the defence of oestrous females (as opposed to defence of food resources) best explains territorial behaviour and the social organization of badgers. Badgers in Britain have two peaks of reproductive activity, one immediately post partum and one in the summer/autumn. These coincide with two peaks of ovulation, and in the late winter/spring there is a steep rise in the number of sows carrying blastocysts, to reach an asymptote in June for yearling sows and April in older sows. Measured by their contribution to overall productivity, winter/spring matings were much more important than summer/autumn matings, contributing 65% of total autumn blastocysts in yearling sows and 71% of autumn blastocysts in older sows. The relative importance of the two mating periods is reflected in the seasonal pattern of bite wounding in adult male badgers; minor bite wounding in January-March was 2.3 times as frequent as in August-October, and moderate-extensive bite wounding was 3.1 times more frequent. In the populations studied, pre- and post-natal losses were high, with reproductive failure occurring at all stages of the breeding cycle, so that less than 30% of potential productivity was achieved. Indeed 22% of sows failed to develop blastocysts; these had a lower body mass, less body fat, larger adrenal glands, poorer health and higher bite wound scores than sows with blastocysts. Only 44% of adult sows implanted their blastocysts and proceeded to the end of pregnancy. However, it was less easy to identify features characteristic of sows that did or did not go on to implant their blastocysts. Finally, 35% of sows that produced cubs ceased lactation early, and this loss of entire litters was thought to be due to infanticide by dominant sows. The presence of annexe setts correlates with increased productivity in younger sows, and this is thought to be because annexe setts enable younger sows and their cubs to avoid the aggression of older, more dominant sows. Living in large social groups has no net reproductive gain for adult males or females, and there was a decline in productivity (per adult) with increasing group size.(ABSTRACT TRUNCATED AT 400 WORDS)     

The effect of oxygen on nitrate and nitrite assimilation in leaves of Zea mays L. under dark conditions

The assimilation of nitrate under dark-N₂ and dark-O₂ conditions in Zea mays leaf tissue was investigated using colourimetric and ¹⁵N techniques for the determination of organic and inorganic nitrogen. Studies using ¹⁵N indicated that nitrate was assimilated under dark conditions. However, the rate of nitrate assimilation in the dark was only 28% of the rate under non-saturating light conditions. No nitrite accumulated under dark aerobiosis, even though nitrate reduction occurred under these conditions. The pattern of nitrite accumulation in leaf tissue in response to dark-N₂ conditions consisted of three phases: an initial lag phase, followed by a period of rapid nitrite accumulation and finally a phase during which the rate of nitrite accumulation declined. After a 1-h period of dark-anaerobiosis, both nitrate reduction and nitrite accumulation declined considerably. However, when O₂ was supplied, nitrate reduction was stimulated and the accumulated nitrite was rapidly reduced. Anaerobic conditions stimulated nitrate reduction in leaf tissue after a period of dark-aerobic pretreatment.     

Invariant chain trimers are sequestered in the rough endoplasmic reticulum in the absence of association with HLA class II antigens

HLA class II antigens are heterodimeric cell surface glycoproteins that interact with antigenic peptides to form complexes recognizable by CD4-positive T cells. During their biosynthesis, class II antigens are retained in a post-Golgi compartment in association with the invariant chain, which dissociates before class II cell surface expression. To address whether the invariant chain mediates this post-Golgi retention, its transport and assembly were examined in cells that do not express HLA class II antigens. Pulse-chase analysis and endoglycosidase digestions showed that very little invariant chain proceeded as far as the trans-Golgi in class II-negative cell lines. Immunofluorescence studies suggested that in these cells the invariant chain is sequestered in the RER. Gel filtration and cross-linking data showed that RER-localized invariant chain is present as trimers or aggregated trimers. Multimerization is mediated by lumenal interactions; a proteolytic fragment of the invariant chain corresponding to the lumenal domain remained trimeric as determined by cross-linking analysis. Similar transport and structural characteristics were observed for a pool of excess invariant chain in class II-positive cells, suggesting that an excess of invariant chain in the ER may be important for class II antigen function. These results have important implications for the transport of cellular proteins in general and for the role of the invariant chain in class II antigen biosynthesis.     

The transport of class I major histocompatibility complex antigens is determined by sequences in the α1 and α2 protein domains

The transport of human-mouse hybrid class I histocompatibility antigens has been studied in a mutant human cell line, 174 × CEM.T2 (T2). T2, a somatic cell hybrid of human B- and T-lymphoblastoid cell lines (B-LCL and T-LCL, respectively), synthesizes HLA-A2 and HLA-B5 glycoproteins, but expresses only low levels of A2 and undetectable levels of B5 at the cell surface. We have previously shown that the products of human class I genes introduced into T2 by transfection behave like the endogenous HLA-B5 glycoproteins, while the products of mouse class I alleles similarly introduced are transported normally to the cell surface. We have now determined that the surface expression of class I glycoproteins in T2 depends on the origin of the ₁ and ₂ domains. Human (HLA-B7) and mouse (H-2D ^p ) hybrid class I genes, encoding the leader, ₁, and ₂ sequences of one species fused to the ₃, transmembrane, and cytoplasmic domains of the other, were transfected into T2. Normal surface expression of the hybrid class I molecule was observed in T2 only when the leader, ₁, and ₂-encoding exons were derived from the mouse gene. The reciprocal construct, encoding human leader, ₁, and ₂ domains fused to the mouse ₃, transmembrane, and cytoplasmic regions, resulted in biosynthesis of a hybrid glycoprotein which was not transported to the cell surface. The products of both constructs were expressed normally in control cells. The effects of glycosylation on class I antigen transport were also studied using mutant class I constructs with altered glycosylation sites. Two mutant B7 genes encoding either an extra glycosylation site at position 176 or no glycosylation sites were transfected into T2. These mutant products were expressed at the cell surface in control cells, but were synthesized and not surface-expressed in T2. These data demonstrate that the HLA/H-2 transport dichotomy in T2 is a function of the origin of the ₁ and/or ₂ domains of the class I glycoprotein, and is not a reflection of glycosylation differences between the human and mouse molecules. Offprint requests to: P. Cresswell.     

Constitutive and inducible aspects of nitrate-nitrogen uptake by Chlamydomonas reinhardtii

Similarly to higher plant root systems, Chlamydomonas reinhardtii Dangeard (UTEX 90) cells exhibited biphasic NO₃^? uptake kinetics. The uptake pattern was similar in cells cultured in 10 mM NO₃^? (NO₃^?-grown), 0.25 mM NO₃^? (N-limited) or 10 mM NO₃^? followed by an 18-h period of N-deprivation (N-starved). In all cell types there was an apparent phase transition in uptake at 1.1 mM NO₃^?, although there were variations in the uptake V_max of both isotherms. The rate of uptake via isotherm 0 ([NO₃^?]<1.1 mM) in N-limited cells was higher than that of either NO₃^?-grown or N-starved cells. In contrast, NO₃^?-grown and N-limited cells exhibited comparable V_max values when supplied with 1.1 to 1.8 mM NO₃^? (isotherm 1). When supplied with 1.6 mM NO₃^?, both N-limited and N-starved cells exhibited enhanced linear uptake after 60 min of incubation. We ascribed this to an induction phenomenon. This trend was not observed when NO₃^?-grown cells were supplied with 1.6 mM NO₃^?, or when N-limited and N-starved cells were supplied with 0.6 mM NO₃^?. The ‘inducible’ aspect of uptake by N-limited cells was blocked by cycloheximide (10 mg l^?1), but not by actinomycin D (5 mg l^?1), thus indicating the involvement of a translational or post-translational event. To investigate this phenomenon further, we analysed the cell proteins of N-limited cells supplied with either 0.6 or 1.6 mM NO₃^? for 90 min, using two-dimensional gel electrophoresis. Comparison of protein profiles enabled the identification of a single cell membrane-associated polypeptide (21 kDa, pI ca 5.5) and ten soluble fraction polypeptides (17–73 kDa, pI ca 5.0 to 7.1) unique to the high NO₃^? treatment. We propose that the ‘inducible’ portion of NO₃^? uptake may provide the means by which C. reinhardtii cells regulate uptake in accordance with assimilatory capacity.     

An Ustilago maydis Mutant Partially Blocked in P45014DM Activity Is Hypersensitive to Azole Fungicides

An Ustilago maydis ergosterol biosynthesis mutant (A14) which is partially blocked in sterol 14alpha-demethylase (P45014DM) activity is described. This mutant accumulated the abnormal 14alpha-methyl sterols, eburicol, 14alpha-methylfecosterol, and obtusifoliol, along with significant amounts of ergosterol. Although the A14 mutant grew nearly as well as the wild type, it was impaired in cell extension growth, which indicated a dysfunction in apical cell wall synthesis. The mutant was also found to be hypersensitive to the azole fungicides penconazole and tebuconazole.     

Role of endogenous peptides in murine allogenic cytotoxic T cell responses assessed using transfectants of the antigen-processing mutant 174xCEM.T2.

One model to explain the high frequency of alloreactive T cells proposes that allogeneic MHC molecules are recognized together with host cell-derived peptides. A model system was developed to investigate the relevance of this mechanism by expression of H-2Dd or H-2Ld in 174xCEM.T2 (T2) cells. This human cell line contains a mutation in its Ag-processing pathway that should restrict the association of endogenous peptides with cell surface class I molecules. CTL generated by stimulating C57BL/6 (H-2b) responder cells with H-2Dd or H-2Ld transfectants of the human B cell line C1R or the murine T cell lymphoma EL4 were assayed for their ability to recognize alloantigenic determinants on these transfectants. The major fraction of the H-2Dd-specific allogeneic CTL response, generated in a MLC or under clonal limiting dilution conditions, was composed of T cells that recognized H-2Dd expressed on C1R or EL4 cells, but failed to recognize this molecule on T2 cells. Clonal analysis indicated that approximately one-third of these CTL recognized determinants that were unique to H-2Dd expressed on C1R stimulator cells whereas the remainder recognized determinants that were also found on EL4 transfectants. Less than 10% of H-2Dd-reactive CTL recognized the T2 transfectant, and these clones also killed C1R-Dd and EL4-Dd. This result suggests that the great majority of H-2Dd-specific alloreactive CTL recognize determinants that are formed by a complex of H-2Dd with endogenous peptides that are absent or significantly reduced in T2 cells. Based on recognition of human or murine transfectants, these CTL exhibit some level of specificity for the structure or composition of the bound peptides. Examination of allogeneic CTL specific for H-2Ld revealed populations similar to those described for H-2Dd. In addition, a major new population was present that recognized determinants shared between C1R-Ld and T2-Ld but not present on EL4-Ld. These results are consistent with the idea that the alloreactive response to H-2Ld is also largely dependent on the presence of bound peptide. However, they also may indicate that the H-2Ld molecule expressed on T2 cells is occupied by one or more peptides that are shared with other human, but not murine, cells. The significance of these results to current models of alloreactivity is discussed.