The genetic basis of variation in immune defense against Lysinibacillus fusiformis infection in Drosophila melanogaster

The genetic causes of phenotypic variation often differ depending on the population examined, particularly if the populations were founded by relatively small numbers of genotypes. Similarly, the genetic causes of phenotypic variation among similar traits (resistance to different xenobiotic compounds or pathogens) may also be completely different or only partially overlapping. Differences in genetic causes for variation in the same trait among populations suggests context dependence for how selection acts on those traits. Similarities in the genetic causes of variation for different traits, on the other hand, suggests pleiotropy which would also influence how natural selection shapes variation in a trait. We characterized immune defense against a natural Drosophila pathogen, the Gram-positive bacterium Lysinibacillus fusiformis, in three different populations and found almost no overlap in the genetic architecture of variation in survival post infection. However, when comparing our results to a similar experiment with the fungal pathogen, B. bassiana, we found a convincing shared QTL peak for both pathogens. This peak contains the Bomanin cluster of Drosophila immune effectors. Loss of function mutants and RNAi knockdown experiments confirms a role of some of these genes in immune defense against both pathogens. This suggests that natural selection may act on the entire cluster of Bomanin genes (and the linked region under the QTL) or specific peptides for specific pathogens.

Hoisted with his own petard: How sex-ratio meiotic drive in Drosophila affinis creates resistance alleles that limit its spread

Meiotic drivers are selfish genetic elements that tinker with gametogenesis to bias their own transmission into the next generation of offspring. Such tinkering can have significant consequences on gametogenesis and end up hampering the spread of the driver. In Drosophila affinis, sex-ratio meiotic drive is caused by an X-linked complex that, when in males with a susceptible Y chromosome, results in broods that are typically more than 95% female. Interestingly, D. affinis males lacking a Y chromosome (XO) are fertile and males with the meiotic drive X and no Y produce only sons-effectively reversing the sex-ratio effect. Here, we show that meiotic drive dramatically increases the rate of nondisjunction of the Y chromosome (at least 750X), meaning that the driver is creating resistant alleles through the process of driving. We then model how the O might influence the spread, dynamics and equilibrium of the sex-ratio X chromosome. We find that the O can prevent the spread or reduce the equilibrium frequency of the sex-ratio X chromosome, and it can even lead to oscillations in frequency. Finally, with reasonable parameters, the O is unlikely to lead to the loss of the Y chromosome, but we discuss how it might lead to sex-chromosome turnover indirectly.

Dynamic Acquisition and Loss of Dual-Obligate Symbionts in the Plant-Sap-Feeding Adelgidae (Hemiptera: Sternorrhyncha: Aphidoidea)

Sap-sucking insects typically engage in obligate relationships with symbiotic bacteria that play nutritional roles in synthesizing nutrients unavailable or in scarce supply from the plant-sap diets of their hosts. Adelgids are sap-sucking insects with complex life cycles that involve alternation between conifer tree species. While all adelgid species feed on spruce during the sexual phase of their life cycle, each adelgid species belongs to a major lineage that feeds on a distinct genus of conifers as their alternate host. Previous work on adelgid symbionts had discovered pairs of symbionts within each host species, and unusual diversity across the insect family, but left several open questions regarding the status of bacterial associates. Here, we explored the consistency of symbionts within and across adelgid lineages, and sought evidence for facultative vs. obligate symbiont status. Representative species were surveyed for symbionts using 16S ribosomal DNA gene sequencing, confirming that different symbiont pairs were consistently present within each major adelgid lineage. Several approaches were used to establish whether symbionts exhibited characteristics of long-term, obligate mutualists. Patterns of symbiont presence across adelgid species and diversification with host insects suggested obligate relationships. Fluorescent in situ hybridization and electron microscopy localized symbionts to bacteriocyte cells within the bacteriome of each species (with one previously known exception), and detection of symbionts in eggs indicated their vertical transmission. Common characteristics of long-term obligate symbionts, such as nucleotide compositional bias and pleomorphic symbiont cell shape were also observed. Superimposing microbial symbionts on the adelgid phylogeny revealed a dynamic pattern of symbiont gains and losses over a relatively short period of time compared to other symbionts associated with sap-sucking insects, with each adelgid species possessing an older, “senior” symbiont and a younger “junior” symbiont. A hypothesis relating adelgid life cycles to relaxed constraints on symbionts is proposed, with the degradation of senior symbionts and repeated acquisition of more junior symbionts creating opportunities for repeated colonization of new alternate-conifer hosts by adelgids.

Malaria severity and human nitric oxide synthase type 2 (NOS2) promoter haplotypes

Nitric oxide (NO) mediates host resistance to severe malaria and other infectious diseases. NO production and mononuclear cell expression of the NO producing enzyme-inducible nitric oxide synthase (NOS2) have been associated with protection from severe falciparum malaria. The purpose of this study was to identify single nucleotide polymorphisms (SNPs) and haplotypes in the NOS2 promoter, to identify associations of these haplotypes with malaria severity and to test the effects of these polymorphisms on promoter activity. We identified 34 SNPs in the proximal 7.3 kb region of the NOS2 promoter and inferred NOS2 promoter haplotypes based on genotyping 24 of these SNPs in a population of Tanzanian children with and without cerebral malaria. We identified 71 haplotypes; 24 of these haplotypes comprised 82% of the alleles. We determined whether NOS2 promoter haplotypes were associated with malaria severity in two groups of subjects from Dar es Salaam (N = 185 and N = 250) and in an inception cohort of children from Muheza-Tanga, Tanzania (N = 883). We did not find consistent associations of NOS2 promoter haplotypes with malaria severity or malarial anemia, although interpretation of these results was potentially limited by the sample size of each group. Furthermore, cytokine-induced NOS2 promoter activity determined using luciferase reporter constructs containing the proximal 7.3 kb region of the NOS2 promoter and the G-954C or C-1173T SNPs did not differ from NOS2 promoter constructs that lacked these polymorphisms. Taken together, these studies suggest that the relationship between NOS2 promoter polymorphisms and malaria severity is more complex than previously described.

Normative values for exhaled breath condensate pH and its relationship to exhaled nitric oxide in healthy African Americans

BACKGROUND

Exhaled breath condensate (EBC) pH and exhaled nitric oxide (FeNO) have been proposed as markers of asthma severity. EBC pH values below 6.5 have been associated with asthma exacerbations. Protonation of airway nitrite occurs at low pH and may contribute to FeNO.

OBJECTIVE

To establish normative EBC pH values and to determine the contribution of EBC pH to FeNO in healthy African Americans.

METHODS

Two hundred seventy healthy African American subjects without asthma between 18 and 40 years old were evaluated. Subjects had simultaneous measurement of EBC pH, EBC nitrite, nitrate, and FeNO.

RESULTS

The median EBC pH was 8.14 (interquartile range, 7.83-8.28). Of subjects, 11.9% had an EBC pH < or = 6.5. In subjects with EBC pH values below 6.5, there was an inverse correlation between EBC pH and FeNO (r(2) = 0.158; P = .0245; n = 32). In the entire cohort, there was a direct correlation between EBC pH and EBC nitrite (r(2) = 0.163; P < .0001), but there was no correlation between EBC nitrite and FeNO. In multivariate analyses, EBC pH and nitrite did not contribute significantly to FeNO variation while controlling for other confounders of FeNO.

CONCLUSION

There was an increased prevalence (11.9%) of low EBC pH (less than 6.5) in healthy African American subjects compared with white subjects (<5%). EBC pH and nitrite were directly correlated, but there was no correlation between EBC nitrite and FeNO. FeNO correlated with EBC pH at pH values less than 6.5 in univariate but not multivariate analyses. This suggests that EBC pH and nitrite are not significant determinants of FeNO in healthy subjects.

Determinants of exhaled nitric oxide levels in healthy, nonsmoking African American adults

BACKGROUND

Asthma is a significant cause of morbidity and mortality for African Americans. Fraction of exhaled nitric oxide (FeNO) levels are increased in patients with asthma, and airway levels of nitric oxide metabolites regulate airway inflammation and airway diameter. More needs to be known about the factors that regulate FeNO. There is a need for FeNO reference values for African Americans.

OBJECTIVE

We sought to establish reference values and identify factors associated with FeNO levels in healthy African American adults.

METHODS

FeNO levels were measured in 895 healthy, nonsmoking African Americans between the ages of 18 and 40 years. FeNO measurements were repeated in 84 subjects. Factors potentially associated with FeNO were measured, including blood pressure, height, weight, and serum total IgE, eosinophil cationic protein, C-reactive protein, and nitrate levels. Data on respiratory symptoms, including upper respiratory tract infection (URI) symptoms, were collected. Univariate and multivariate analyses of the relationship between these variables and FeNO levels were performed.

RESULTS

In healthy, nonsmoking African Americans FeNO levels were stable during repeated measurements (intraclass correlation coefficient, 0.81). Sex (P < .0001), serum total IgE levels (P < .0001), and current URI symptoms (P = .0002) contributed significantly to FeNO variability but together accounted for less than 50% of the variation in FeNO levels.

CONCLUSION

The high correlation between repeated measurements of FeNO and the low correlation coefficients of known factors associated with FeNO suggest that other factors might contribute substantially to variability of FeNO levels in African Americans.