Temporal variability of top-down forces and their role in host choice evolution of phytophagous arthropods

The role of top-down forces in host choice evolution of phytophagous arthropods is the subject of a vividly animated debate. Empirical evidence for the evolutionary role of top-down forces comes from studies showing that phytophagous arthropods prefer hosts that entail enemy-free space. The aim of this paper is to draw the attention of plant–arthropod researchers to the potentially, temporally variable nature of third trophic level effects. We show that this aspect is largely neglected in studies on enemy-free space, despite the fact that relative enemy impact varies seasonally among plants in at least some studies. We conclude that rigorous testing of the enemy-free space hypothesis can only be performed when within and between season variation in higher trophic level effects is taken into account.     

Glycolipid anchors are attached to Thy-1 glycoprotein rapidly after translation.

The attachment of glycolipid anchors to the Thy-1 glycoprotein during biosynthesis was followed by the change of detergent-binding properties of biosynthetically labelled Thy-1 precursors upon phospholipase C treatment in the murine thymoma lines BW5147 and S1A. In S1A, 80% of the Thy-1 molecules were phospholipase-C-sensitive after a 2 min pulse with [35S]methionine, indicating that these molecules were already anchored via a glycolipid tail. In BW5147, 47% of the Thy-1 molecules had phospholipase-C-sensitive anchors attached after a 1.5 min labelling and, with longer pulses, this percentage rose to 76%. Tunicamycin did not block the addition of glycolipid anchors, and glycolipid attachment also occurred at 21 degrees C. The findings suggest that the attachment of glycolipid anchors occurs in the rough endoplasmic reticulum.     

Expression of UDP-N-acetylgalactosamine: beta-galactose beta 1,4-N-acetylgalactosaminyltransferase in functionally defined T-cell clones.

To measure UDP-N-acetylgalactosamine: beta-galactose beta 1,4-N-acetylgalactosaminyltransferase (beta 1,4-GalNActransferase) in crude cell and tissue extracts we designed an assay containing UDP-[3H]N-acetylgalactosamine as donor and biotinylated human glycophorin A as an acceptor. After incubation the labelled acceptor was separated by the use of avidin-agarose from extract-derived endogenous acceptors. This assay permitted one to measure specifically the beta 1,4-GalNActransferase in crude extracts. This glycosyltransferase has previously been shown to be involved in the biosynthesis of Vicia villosa (hairy winter vetch)-lectin (VV)-binding sites of the murine cytotoxic T-cell line B6.1. Since VV-binding sites are a distinct marker for the cytotoxic subclass of murine T-lymphocytes, we used this assay to determine enzyme levels in a panel of functionally defined murine T-cell clones. Non-cytolytic T-cell lines generally have low activity, whereas most cytotoxic lines have high levels of activity. However, one cytotoxic T-cell line does not express the enzyme, although it has large numbers of VV-binding sites. This suggests the existence of another type of VV-binding sites which is independent of the beta 1,4-GalNActransferase in some cytotoxic-T-lymphocyte lines. The enzyme was also assayed in a variety of other tissues and found to have a very high activity in the intestine but a low activity in most other tissues. This was in considerable contrast with the ubiquitously high expression of UDP-GalNAc:peptide alpha 1-GalNActransferase. Therefore, the beta 1,4-GalNActransferase seems to be regulated during differentiation.     

Cloning and comparison of Aα mating-type alleles of the basidiomycete Schizophyllum commune

Summary An A mating-type allele (A4) was isolated by walking the chromosome from the closely linked PAB1 gene. A cosmid clone containing the A1 allele isolated from the walk was used as a probe to recover the A1 allele from another cosmid library. Cosmids encoding mating-type activity were identified by transforming Schizophyllum cells and screening for activation of A-regulated development. Putative mating-type transformants were confirmed in mating tests and genetic analyses of progeny. The identity of the specific alleles isolated was demonstrated by showing that their effectiveness in transforming for mating type is limited to recipient strains possessing an A allele different from the one encoded by the cloned sequences. Transforming DNA is active in trans, suggesting that A encodes a diffusible product. Restriction mapping shows that A1 and A4 are coded in the same physical region of the genome, but within a subregion that contains extensive sequence divergence. In addition, Southern analyses show that there is only one copy of A1 or A4 per haploid genome, and that they do not cross-hybridize to one another or to any of the other A alleles. A1 and A4 were subcloned as 2.8 and 1.2 kb fragments, respectively, retaining in transformation all the mating-type activity demonstrated of the original cosmids.     

Ontogeny of hormone-secreting cells of the rat pituitary gland: An immunocytochemical study on dissociated cells

Summary An immunocytochemical study was undertaken in foetal, prepubertal and mature rats to determine the time of differentiation of various types of adenohypophyseal cells during development. Freshly dissociated pituitary cells from foetal (18–21 days postconception), neonatal (from birth up to 30 days) and adult rats (more than 8 weeks) were characterized using immunocytochemical methods. All types of hormone-producing cells were present at day 18 postconception, although only 20% of the cells were immunolabelled. Adrenocorticotropin (ACTH)-secreting cells accounted for the highest number of hormone-positive cells. Growth hormone-secreting cells increased remarkably from day 18 postconception onwards. Prolactin-secreting cells were not seen in the foetal adenohypophysis and did not start to increase until 10 days after birth, whereas by that time the number of ACTH, thyrotropin, follicle-stimulating and luteinizing hormone-secreting cells had stopped increasing. By day 30 after birth, 80–95% of the cells were immunoreactive.     

Development of a beta-galactosidase alpha-complementation system for molecular cloning in Bacillus subtilis

A versatile beta-galactosidase alpha-complementation system for Bacillus subtilis was developed, which can be used for molecular cloning in this Gram+ organism. The cloning system, which is based on the highly efficient host-vector system 6GM-pHP13, offers several advantages over previously described systems: (1) convenient direct selection of recombinants; (2) the cloning of large heterologous DNA fragments with high efficiency; and (3) the availability of six unique target sites: SphI, NdeI, NheI, BamHI, SmaI and EcoRI.     

Accumulation of mansonones E and F in seedlings of Ulmus americana in response to inoculation with Ophiostoma ulmi

Summary The accumulation of mansonones E and F was investigated in Ulmus americana L. seedlings 5 weeks after inoculation with three aggressive and three non-aggressive isolates of Ophiostoma ulmi (Buism.) Nannf. The three non-aggressive isolates stimulated significantly more mansonone E and F accumulation than the three aggressive isolates of O. ulmi. Mansonone induction also varied within both the aggressive and the non-aggressive groups. Aggressive and non-aggressive isolates were recovered in equal frequencies from the inoculation wounds, whereas the aggressive isolates were recovered more frequently than the non-aggressive isolates 15 cm and 25 cm up the seedlings' stem. Vascular browning in the outer xylem of the seedlings correlated with mansonone E and F accumulation. Mansonone accumulation in U. americana seedlings is therefore associated with vascular browning and a reduction in fungal spread.