Interactions between Pesticide Genes: Model and Experiment

In response to years of intense selection pressure by organophosphate insecticides, several different insecticide resistance mechanisms have evolved in natural populations of the mosquito Culex pipiens. We examined interactions between two of the most important mechanisms using a four-compartment model of insecticide pharmacokinetics. The joint effect of different mechanisms of resistance can be expressed in terms of epistasis at the physiological level in this model. The type of epistasis predicted by the model depends on the particular physiological mechanisms of resistance involved. Resistance due to a reduced penetration of the insecticide combines multiplicatively with other resistance factors, but resistance due to detoxicative processes and to insensitivity of the target site combines additively. How the pattern of epistasis at the physiological level is translated into fitness epistasis in natural populations of this mosquito depends on the intensity and pattern of insecticide selection in the field.     

Effect of Moniliformis moniliformis density on distribution within the definitive host population (Rattus norvegicus)

The population dynamics of Moniliformis moniliformis was studied in ‘free-ranging’ laboratory rats, Rattus norvegicus, presented with different relative density levels of M. moniliformis in cockroaches, Periplaneta americana. Changes in selected population parameters of the negative binomial distribution were evaluated as indicators of changes in aggregation. A significant increase in the degree of aggregation of parasites occurred as a result of the increase in relative density of infective stages available to the rats. This increase in aggregation was due to the increase in over-dispersion that occurred in female rats only. The degree of aggregation in females was found to be significantly higher than that in males at both treatment levels. The best indicators of the degree of aggregation were found to be the ratio of the variance to the relative density and the ratio of the log-variance to log-relative density. Changes in k were not correlated with changes in over-dispersion or the relative density.     

Evidence for a stem cell-specific repressor of Moloney murine leukemia virus expression in embryonal carcinoma cells.

A negative regulatory element (NRE) spanning the tRNA primer-binding site (PBS) of Moloney murine leukemia virus (M-MuLV) mediates repression of M-MuLV expression specifically in embryonal carcinoma (EC) cells. We precisely defined the element by base-pair mutagenesis to an 18-base-pair segment of the tRNA PBS and showed that the element also restricted expression when moved upstream of the long terminal repeat. A DNA-binding activity specific for the M-MuLV NRE was detected in vitro by using crude EC nuclear extracts in exonuclease III protection assays. Binding was strongly correlated with repression in EC cells. Mutations within the NRE that relieved repression disrupted binding activity. Also, nuclear extracts prepared from permissive, differentiated EC cell cultures showed reduced binding activity for the NRE. These results indicate the presence of a stem cell-specific repressor that extinguishes M-MuLV expression via the NRE at the tRNA PBS.     

Ktaadn: Thoreau the anthropologist

Timothy Troy is a Librarian at the Center for Creative Photography, University of Arizona.     

Discrimination of jittered sonar echoes by the echolocating bat,Eptesicus fuscus: The shape of target images in echolocation

1. Behavioral experiments with jittering echoes examined acoustic images of sonar targets in the echolocating bat, Eptesicus fuscus, along the echo delay or target range axis. Echo phase, amplitude, bandwidth, and signal-to-noise ratio were manipulated to assess the underlying auditory processes for image formation. 2. Fine delay acuity is about 10 ns. Calibration and control procedures indicate that this represents temporal acuity rather than spectral discrimination. Jitter discrimination curves change in phase when the phase of one jittering echo is shifted by 180 degrees relative to the other, showing that echo phase is involved in delay estimation. At an echo detectability index of about 36 dB, fine acuity is 40 ns, which is approximately as predicted for the delay accuracy of an ideal receiver. 3. Compound performance curves for 0 degrees and 180 degrees phase conditions match the crosscorrelation function of the echoes. The locations of both 0 degrees and 180 degrees phase peaks in the performance curves shift along the time axis by an amount that matches neural amplitude-latency trading in Eptesicus, confirming a temporal basis for jitter discrimination.     

Sequence diversity of pistil S-proteins associated with gametophytic self-incompatibility in Nicotiana alata

Summary In order to study the extent and nature of differences among various S-allele-associated proteins in N. alata, we carried out comparative studies of seven such proteins. We first isolated and sequenced cDNA clones for the S_z-, S_F11-, S₁-, and S_a-alleles, and then we compared the deduced amino acid sequences both of these four S-proteins and of three previously published S₂-, S₃-, and S₆-proteins. This comparison revealed (1) an average homology of 53.8% among the seven proteins and (2) two homology classes, with S_z and S_F11 in one class and S₁, S₂, S₃, and S₆ in the other class. There are 60 conserved residues, including 9 cysteines. Of the 144 variable residues, 50 were identified as hypervariable based on a calculation of their Similarity Indices. Although conserved, variable, and hypervariable residues are dispersed throughout the protein, some are clustered to form five conserved, five hypervariable, and a number of variable regions. Those variable sites which contain residues conserved within one class of S-proteins but different between classes might provide a clue to the evolutionary relationship of these two classes of S-proteins. The hypervariable residues, which account for sequence variability, may contribute to allelic specificity.     

Expression and function of the homoeotic genes Antennapedia and Sex combs reduced in the embryonic midgut of Drosophila

Drosophila homoeotic genes control the formation of external morphological features of the embryo and adult, and in addition affect differentiation of the nervous system. Here we describe the morphogenetic events in the midgut that are controlled by the homoeotic genes Sex combs reduced (Scr) and Antennapedia (Antp). The midgut is composed of two cell layers, an inner endoderm and an outer visceral mesoderm that surround the yolk. Scr and Antp are expressed in the visceral mesoderm but not in the endoderm. The two genes are required for different aspects of the midgut morphogenesis. In Scr null mutant embryos the gastric caeca fail to form. Scr is expressed in the visceral mesoderm cells posterior to the primordia of the gastric caeca and appears to be indirectly required for the formation of the caeca. Antp is expressed in visceral mesoderm cells that overlie a part of the midgut where a constriction will form, and Antp null mutant embryos fail to form this constriction. An ultrastructural analysis of the midgut reveals that the visceral mesoderm imposes the constriction on the endoderm and the yolk. The mesodermal tissue contracts within the constriction and thereby penetrates the layer of the midgut endoderm. Microtubules participate in the morphological changes of the visceral mesoderm cells. The analysis of the expression of Scr in Antp mutant embryos revealed a case of tissue-specific regulation of Scr expression by Antp. In the epidermis, Antp has been shown to negatively regulate Scr, but it positively regulates Scr in the visceral mesoderm.     

Habitat usage by red (Cervus elaphus) and roe (Capreolus capreolus) deer in a Scottish Sitka spruce plantation

Habitat usage in a 1825 ha block of mixed-age forest was assessed by counts of accumulated pellet groups on permanent plots. This monitoring took place from 1978 to 1984 with some 300 plots in 13 types of habitat. Tests on the method are described; losses of pellet groups were insufficient to bias the conclusions.
Usage was least in the forest stages lacking ground vegetation, i.e. thicket, pole and high-canopy forest; stages with ground vegetation had moderately heavy usage. The most preferred habitats were of small extent, treeless, and situated near or within closed forest, i.e. rides, openings and the margin between forest and unplanted hill ground. Red and roe deer showed broadly similar patterns of preference but differed significantly in the use of some habitats. Thickets received more use from red than roe deer, whereas pre-thicket and vegetated high-canopy forest had most use from roe deer, being richer in forbs on which this species feeds heavily.