Acarologic risk of exposure to Borrelia burgdorferi spirochaetes: long-term evaluations in north-western California, with implications for Lyme borreliosis risk-assessment models

Over a 5-year period (1997-2001) the population densities of Ixodes pacificus Cooley & Kohls (Acari: Ixodidae) nymphs infected with spirochaetes of Borrelia burgdorferi sensu lato (s.l.) were evaluated in areas of 2000 ha at two localities (CHR, nine sites; HREC, seven sites) 25 km apart in Mendocino County, north-western California. The 5-year median density of infected nymphs was significantly higher at CHR than at HREC (0.51 vs. 0.09 per 100 m(2) and site-specific yearly densities exceeding one infected nymph per 100 m2 were 10-fold more likely to occur at CHR than at HREC. The importance of long-term data in acarologic risk assessment was demonstrated by significantly higher median yearly densities of infected nymphs at CHR from 1997 to 1999, whereas both areas had similar densities during 2000-2001. Overall, the causative agent of Lyme borreliosis in North America, B. burgdorferi Johnson et al. sensu stricto (s.s.) accounted for 76% of 46 genetically characterized B. burgdorferi s.l. infections from I. pacificus nymphs. Tremendous variability in acarologic risk was recorded within both areas: yearly densities of infected nymphs varied 11-97-fold between sites at CHR and 8-30-fold at HREC. Part of this variation could be explained by environmental traits, most notably deer usage. However, correlations between environmental factors and density of infected nymphs (for CHR and HREC combined) did not necessarily apply when these areas were considered separately. Thus, a Lyme borreliosis ecology model developed in one of these areas needs testing in the other area.     

Phylogenomics of the reproductive parasite Wolbachia pipientis wMel: a streamlined genome overrun by mobile genetic elements

The complete sequence of the 1,267,782 bp genome of Wolbachia pipientis wMel, an obligate intracellular bacteria of Drosophila melanogaster, has been determined. Wolbachia, which are found in a variety of invertebrate species, are of great interest due to their diverse interactions with different hosts, which range from many forms of reproductive parasitism to mutualistic symbioses. Analysis of the wMel genome, in particular phylogenomic comparisons with other intracellular bacteria, has revealed many insights into the biology and evolution of wMel and Wolbachia in general. For example, the wMel genome is unique among sequenced obligate intracellular species in both being highly streamlined and containing very high levels of repetitive DNA and mobile DNA elements. This observation, coupled with multiple evolutionary reconstructions, suggests that natural selection is somewhat inefficient in wMel, most likely owing to the occurrence of repeated population bottlenecks. Genome analysis predicts many metabolic differences with the closely related Rickettsia species, including the presence of intact glycolysis and purine synthesis, which may compensate for an inability to obtain ATP directly from its host, as Rickettsia can. Other discoveries include the apparent inability of wMel to synthesize lipopolysaccharide and the presence of the most genes encoding proteins with ankyrin repeat domains of any prokaryotic genome yet sequenced. Despite the ability of wMel to infect the germline of its host, we find no evidence for either recent lateral gene transfer between wMel and D. melanogaster or older transfers between Wolbachia and any host. Evolutionary analysis further supports the hypothesis that mitochondria share a common ancestor with the α-Proteobacteria, but shows little support for the grouping of mitochondria with species in the order Rickettsiales. With the availability of the complete genomes of both species and excellent genetic tools for the host, the wMel–D. melanogaster symbiosis is now an ideal system for studying the biology and evolution of Wolbachia infections.     

Population genetic variation in gene expression is associated with phenotypic variation in <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis>

Background

The relationship between genetic variation in gene expression and phenotypic variation observable in nature is not well understood. Identifying how many phenotypes are associated with differences in gene expression and how many gene-expression differences are associated with a phenotype is important to understanding the molecular basis and evolution of complex traits.

Results

We compared levels of gene expression among nine natural isolates of Saccharomyces cerevisiae grown either in the presence or absence of copper sulfate. Of the nine strains, two show a reduced growth rate and two others are rust colored in the presence of copper sulfate. We identified 633 genes that show significant differences in expression among strains. Of these genes, 20 were correlated with resistance to copper sulfate and 24 were correlated with rust coloration. The function of these genes in combination with their expression pattern suggests the presence of both correlative and causative expression differences. But the majority of differentially expressed genes were not correlated with either phenotype and showed the same expression pattern both in the presence and absence of copper sulfate. To determine whether these expression differences may contribute to phenotypic variation under other environmental conditions, we examined one phenotype, freeze tolerance, predicted by the differential expression of the aquaporin gene AQY2. We found freeze tolerance is associated with the expression of AQY2.

Conclusions

Gene expression differences provide substantial insight into the molecular basis of naturally occurring traits and can be used to predict environment dependent phenotypic variation.

     

Single-cell analysis of lambda immunity regulation

We have examined expression of the lambdacI operon in single cells via a rex Colon, two colons gfp substitution. Although average fluorescence agreed with expectations for expression of lambda-repressor, fluorescence fluctuated greatly from cell-to-cell. Fluctuations in repressor concentration are not predicted by previous models and are tolerated in part by a regulatory response to DNA damage.     

Zebrafish smoothened functions in ventral neural tube specification and axon tract formation

Sonic hedgehog (Shh) signaling patterns many vertebrate tissues. shh mutations dramatically affect mouse ventral forebrain and floor plate but produce minor defects in zebrafish. Zebrafish have two mammalian Shh orthologs, sonic hedgehog and tiggy-winkle hedgehog, and another gene, echidna hedgehog, that could have overlapping functions. To examine the role of Hedgehog signaling in zebrafish, we have characterized slow muscle omitted (smu) mutants. We show that smu encodes a zebrafish ortholog of Smoothened that transduces Hedgehog signals. Zebrafish smoothened is expressed maternally and zygotically and supports specification of motoneurons, pituitary cells and ventral forebrain. We propose that smoothened is required for induction of lateral floor plate and a subpopulation of hypothalamic cells and for maintenance of medial floor plate and hypothalamic cells.     

Gametocyte sex ratio of a malaria parasite: experimental test of heritability

The gametocyte sex ratio of Plasmodium mexicanum, a malaria parasite of western fence lizards, was studied in a modified garden experiment. Each of 6 naturally infected lizards was used to initiate 20 replicate-infections in naive western fence lizards. A significant donor effect was observed for the sex ratios of recipient infections at their maximal parasitemia, and this effect was associated with the sex ratio of the donor infection. In 20 infections in which sex ratio was followed during the course of the infection, 9 revealed constant sex ratios and 11 showed an increase in proportion of males over time. Recipient sex ratio was correlated with another life-history trait, a composite of rate of asexual replication and peak parasitemia, such that higher Rate-Peak scores were associated with infections with less female-biased sex ratios. These results are placed into the context of sex ratio theory that concludes that the degree of selfing of parasite genotypes (number of parasite clones) within the vector will influence the evolution of gametocyte sex ratio. The theory predicts that the sex ratio should be under some genetic control and thus be heritable as observed in the experiment. Clonal diversity should also influence the life-history trait, Rate-Peak, which was found to be correlated with sex ratio.     

Effect of ascorbic acid and acute heat exposure on heat shock protein 70 expression by young white Leghorn chickens

Dietary ascorbic acid (AA) and heat stress (HS) affect heat shock protein 70 (hsp70) and body temperature (BT) of strain cross white Leghorn chickens. At five weeks of age, chicks fed either no supplemental AA (N-AA) or 200 mg/kg AA (AA) were subjected to either HS (42 degrees C) or maintained in control (CN, 23 degrees C) ambient temperature (T(a)) for 1 h. Body temperature (BT) was recorded for each bird before collection of heart and liver for hsp70 assay. In the CN AA-fed groups, neither the lower constitutive hsc70 nor the decreased hsp70 response to HS in the heart and liver were sex-dependent. The BT was increased by HS, but neither AA nor sex of the bird affected BT response. A diet X T(a) interaction revealed that BT of CN AA-fed chickens was lower than in N-AA-fed chickens, but BT of HS AA-fed chicks was greater than BT in HS N-AA-fed chickens. The BT and hsp70 responses were positively correlated. A lower expression of hsp70 indicated less of a stress response in the AA-fed chickens.