Predation pressure on an imperfect Batesian micicry complex in the presence of alternative prey

Summary Differential predation pressure and the probability of predation on a Batesian mimicry complex and on alternative prey were estimatedin a field experiment. The mimicry complex was composed of a noxious model (Eleodes obscura (Say)) and a palatable mimic (Stenomorpha marginata (LeConte)). House crickets (Acheta domesticus) (Linn.) were used as alternative prey. The experiment was conducted for 23 nights in August and September to approximate the peak seasonal activity time period during which both models and mimics normally are exposed to predation while foraging and depositing eggs. Each night thirty prey in ratios of 16 models: 7 mimics: 7 crickets were exposed for 2.5 h to a suite of predators consisting of pallid bats (Antrozous pallidus), striped skunks (Mephitis mephitis) and ringtails (Bassariscus astutus) that had free access to the prey. The model-mimic ratio was similar to that found in nature. Predators obtained prey on 11 of the 23 nights and preferred the alternative prey (crickets) in proportions higher than was expected from a predation rate that was equal on all species of prey. Mimics were taken by predators at a rate proportional to their abundance, while models were taken at a rate considerably lower than their relative abundance. This suggests that at least some of the predators could distinguish between models and mimics and were willing to eat the mimics at higher frequencies than they were willing to eat the models. However, although the mimicry is not perfect with respect to the entire predator suite, the mimics still gain an advantage by resembling the models, compared to the predation levels on the alternate prey.     

Ecophysiological Significance of CO(2)-Recycling via Crassulacean Acid Metabolism in Talinum calycinum Engelm. (Portulacaceae)

High levels of variability in gas exchange characteristics and degree of CAM-cycling were found in the same and different individuals of Talinum calycinum Engelm. collected from rock outcrops in Missouri. Differences in CO₂ assimilation were mostly correlated with differences in shoot conductance to CO₂ not shoot internal CO₂ concentration. As found previously, CAM acid fluctuations were evident in well-watered plants exhibiting C₃ gas exchange patterns (CAM-cycling) and also in drought-stressed plants with stomata closed, or nearly so, day and night (CAM-idling). Drought stress also resulted in rapid stomatal closure, conserving water during droughts. Maximal CO₂ uptake rates occurred below 35°C; higher temperatures induced decreases in CO₂ assimilation and conductance while shoot internal CO₂ concentrations remained similar. Plant water-use-efficiency was severely curtailed at temperatures above 30°C. Tissue acid fluctuations were the result of changes in malic acid concentrations. Calculations of the amount of water potentially conserved by CAM-cycling yielded values of approximately 5 to 44% of daytime water loss. Thus, CAM-cycling may be an important adaptation minimizing water loss by perennial succulents growing in shallow soil on rock outcrops.     

Sequence specificity of 125I-labelled Hoechst 33258 in intact human cells

Using polyacrylamide/urea DNA sequencing gels, the DNA sequence selectivity of 125I-labelled Hoechst 33258 damage has been determined in intact human cells to the exact base-pair. This was accomplished using a novel procedure with human alpha RI-DNA as the target DNA sequence. In this procedure, after size fractionation, the alpha RI-DNA is selectively purified by hybridization to a single-stranded M13 clone containing an alpha RI-DNA insert. The sequence specificity of [125I]Hoechst 33258 was indistinguishable in intact cells from purified high molecular weight DNA; and this is surprising considering the more complex environment of DNA in the nucleus where DNA is bound to nucleosomes and other DNA binding proteins. The ligand preferentially binds to DNA sequences which have four or more consecutive A.T base-pairs. The extent of damage was measured with a densitometer and, relative to the damage hotspot at base-pair 94, the extent of damage was similar in both purified high molecular weight DNA and intact cells. [125I]Hoechst 33258 causes only double-strand breaks, since single-strand breaks or base damage were not detected. These experiments represent the first occasion that the sequence specificity of a DNA damaging agent, which causes only double-strand breaks, has been determined to the exact base-pair in intact cells.     

Differences in CO2 threshold of respiratory muscles in preterm infants

Because neonatal apnea is frequently associated with airway obstruction, we compared relative changes in activity between various upper airway muscles and the diaphragm during hypercapnic stimulation. The technique of hyperoxic CO2 rebreathing was employed in 17 healthy, sleeping preterm infants studied at a postnatal age of 32 +/- 12 days. Surface diaphragm (DIA) electromyograms (EMGs) were recorded in all infants, and noninvasive measurements of posterior cricoarytenoid (PCA), genioglossus (GG), and alae nasi (AN) EMGs were analyzed in 11, 9, and 8 infants, respectively. During the control period, consistent phasic EMGs were recorded from the DIA in all infants and from the PCA in 8 infants, but from the GG and AN each in only one infant. During CO2 rebreathing, minute ventilation and end-tidal CO2 increased linearly as CO2 rose from 31 +/- 5 to 51 +/- 5 Torr. DIA and PCA EMGs also had proportional and comparable increases throughout rebreathing. In contrast, both GG and AN responses differed from the DIA and PCA (P less than 0.001) and exhibited minimal or absent responses at low levels of hypercapnia. Consistent GG and AN EMGs appeared at comparable levels of end-tidal CO2 (47 +/- 5 and 45 +/- 5 Torr, respectively) and subsequently increased linearly in most infants. We conclude that during CO2 rebreathing the initially delayed and subsequently linear responses of the GG and AN EMGs indicate a high CO2 threshold for these muscles.     

Nucleotide sequence of Acinetobacter baumannii aphA-6 gene: evolutionary and functional implications of sequence homologies with nucleotide-binding proteins, kinases and other aminoglycoside-modifying enzymes

A new kanamycin-resistance gene, detected in Acinetobacter baumannii and designated aphA-6, was sequenced. It specifies a 30319 Dalton 3'-aminoglycoside phosphotransferase (APH(3'] that mediates resistance to kanamycin and structurally related aminoglycosides, including amikacin. Pairwise comparisons of the six types of APH(3') so far detected in human pathogens (types I, II, III and VI) and in amino-glycoside-producing microorganisms (types IV and V), confirm that APH(3') enzymes have diverged from a common ancestor. Three highly retained motifs (1: V--HGD----N; 2: G--D-GR/K-G and 3: D--K/R--Y/F---LDE) located in the C-terminal part of the enzymes were defined. Screening of protein sequence data bases fore each of these motifs revealed that motifs 1 and 2 are both found in nucleotide-binding phosphotransferases associated with a variety of biological processes, namely adenylate kinase, viral oncogenic protein kinases, elongation factors, Na+/K+-transporting ATPase, myosin and antibiotic-modifying enzymes. Motif 2 probably corresponds to the MgATP binding site, while motifs 3 and 1 could be involved in the splitting of the phosphodiester bond and in the phosphate transfer, respectively. Moreover, an additional motif, almost invariably centrally located, was found in all aminoglycoside-modifying enzymes. The occurrence of this motif, possibly a recombination site which would have allowed the association of units of separate functions, is compatible with a modular concept for the structure of aminoglycoside-modifying enzymes.     

Relatedness of penicillin-binding protein 1a genes from different clones of penicillin-resistant Streptococcus pneumoniae isolated in South Africa and Spain.

Penicillin-resistant strains of Streptococcus pneumoniae have been common in South Africa and Spain for several years. Multilocus enzyme electrophoresis identified one clone of capsular type 6B which was prevalent in Spain and another clone of type 23F that was present in both countries. Genes for penicillin-binding proteins (PBPs) in penicillin-resistant strains are often mosaics where parts of the pneumococcal genes are replaced by homologous genes from other species. We have compared the mosaic structures of the PBP 1a genes from the two clones as well as from genetically distinct South African isolates. Four classes of mosaic PBP 1a genes were found that contained blocks of sequences divergent by 6-22% from those of sensitive genes; two classes contained sequences coming from more than one external source. Data are presented showing that the PBP 1a genes from the 23F and the 6B clone are related, and that the two PBP 1a genes from the South African isolates are also related. We suggest that the type 23F clone originated in Spain prior to distribution into other continents.     

Comparative efficiencies of bispecific F(ab′γ)2 and chimeric mouse/human IgG antibodies in recruiting cellular effectors for cytotoxicity via Fcγ receptors

Summary The three forms of Fc receptor carried by monocytes (FcRI, II) and natural killer (NK) cells (FcRIII) are all capable of mediating cell lysis. Here we compare the use of F(ab)₂ bispecific antibodies, specifically targetting individual FcR, and chimeric IgG mouse/human antibodies which are capable of targetting all FcR, for their ability to mediate target cell destruction. The derivatives are prepared by linking hinge sulphydryl residues via tandem thioether bonds, using a bismaleimide crosslinker: Fab from an anti-FcR mAb linked to Fab from a common anti-target mAb (BsAb), or Fab from the common anti-target mouse antibody linked to human Fc (FabFc or bisFabFc). All the derivatives targetting chick red blood cells gave efficient lysis, although different effector cell donors yielded differences in both the lytic levels achieved and the comparative efficiencies of derivatives. In contrast, significant lysis of the guinea pig lymphoblastic leukaemia, L₂C, regularly resulted only via the anti-FcRIII BsAb and the chimeric derivatives. These results suggest that the chimeric, Fc-containing derivatives mediate tumour cell lysis principally through FcRIII on NK cells. This is in contrast to the situation with the chick red blood cells where the chimeric derivatives appear capable of lysing erythrocytes by utilizing either monocytes or NK cells, because significant (50%) lysis occurred with effector cell populations magnetically depleted through either FcRII or FcRIII. A major difference between these two types of antibody derivative was their ability to function in the presence of high concentrations of normal human Fc. The lysis mediated by BsAb reactive with FcRI or II was unaffected by the presence of human Fc at 2.5 mg/ml (a concentration comparable with that yielded by IgG in plasma) whereas the BsAb recognizing FcRIII and all the Fc-containing derivatives were completely inhibited.This work has been supported by Tenovus, the Cancer Research Campaign, the Leukaemia Research Fund, Italfarmaco, Milano, Italy and the Imperial Cancer Research Fund