New components of Drosophila leg development identified through genome wide association studies.

The adult Drosophila melanogaster body develops from imaginal discs, groups of cells set-aside during embryogenesis and expanded in number during larval stages. Specification and development of Drosophila imaginal discs have been studied for many years as models of morphogenesis. These studies are often based on mutations with large developmental effects, mutations that are often lethal in embryos when homozygous. Such forward genetic screens can be limited by factors such as early lethality and genetic redundancy. To identify additional genes and genetic pathways involved in leg imaginal disc development, we employed a Genome Wide Association Study utilizing the natural genetic variation in leg proportionality found in the Drosophila Genetic Reference Panel fly lines. In addition to identifying genes already known to be involved in leg development, we identified several genes involved in pathways that had not previously been linked with leg development. Several of the genes appear to be involved in signaling activities, while others have no known roles at this time. Many of these uncharacterized genes are conserved in mammals, so we can now begin to place these genes into developmental contexts. Interestingly, we identified five genes which, when their function is reduced by RNAi, cause an antenna-to-leg transformation. Our results demonstrate the utility of this approach, integrating the tools of quantitative and molecular genetics to study developmental processes, and provide new insights into the pathways and networks involved in Drosophila leg development.

Effect of Crop Rotations on Rotylenchulus reniformis Population Structure.

Rotylenchulus reniformis is a highly variable nematode species and an economically important pest in many cotton fields across the south-eastern United States. Rotation with resistant or poor host crops is a method for management of reniform nematode. We studied the effect of six planting schemes covering four 120-day planting cycles on the predominant genotype of R. reniformis. Rotations used were: (i) cotton to corn; (ii) susceptible soybean to corn; (iii) resistant soybean to cotton; (iv) corn to cotton; (v) continuous susceptible soybean; (vi) continuous cotton. After each 120-day cycle, amplified fragment length polymorphisms (AFLPs) produced from four primer pairs were used to determine the effect of crop rotation on the predominant genotype of reniform nematode. A total of 279 polymorphic bands were scored using four primer combinations. Distinct changes in genotype composition were observed following rotations with resistant soybean or corn. Rotations involving soybean (susceptible and resistant) had the greatest effect on population structure. The characterization of field population variability of reniform nematode and of population responses to host plants used in rotations can help extend the durability of resistant varieties and can help identify effective rotation schemes.

Genetic Variability of Rotylenchulus reniformis.

Rotylenchulus reniformis, reniform nematode, is a polyphagous pest commonly found parasitizing cotton in the southeastern United States. We developed and optimized 10 polymorphic microsatellite loci found in reniform nematode and tested them on 160 individual reniform nematodes to determine informative genetic variation of isolates from the southeastern United States, Colombia, Japan, and from the species Rotylenchulus parvus. No significant gametic disequilibrium was detected between any pair of loci, and most loci were not in expected Hardy-Weinberg proportions. A positive FIS coefficient was observed at all 10 loci, suggesting a high level of inbreeding at these loci. Most isolate locations exhibited significant genotypic differentiation and moderate to very high genetic differentiation based on FST analysis. The most consistently differentiated isolates were found reproducing parthenogenetically in Japan. These isolates were also found to represent the most basal locality in this study based on unweighted pair group method with arithmetic mean (UPGMA) clustering analysis and were distinct from other localities based on STRUCTURE V 2.3 analysis. These results support previous reports suggesting that the parthenogenetically reproducing isolates from Japan are another species. Taken together, our results can serve as the foundation for more extensive characterization of population structure and genetic variation among isolates of R. reniformis variants to help discern the impact of alternative processes on genetic connectivity and differentiation in the genetically undercharacterized reniform nematode.

Effect of Temperature on the Embryogenesis of Geographic Populations of Rotylenchulus reniformis.

The effect of temperature on the embryonic development of three populations of reniform nematode (Rotylenchulus reniformis) from the southeastern United States was studied. The development of eggs from single-cell stage to eclosion of second-stage juvenile was monitored at 20, 25, 30, and 35°C. All populations completed embryogenesis in 7 days at 25°C. The greatest differences among populations in time to completion of embryogenesis were observed at 20 and 35°C. Results at the intermediate temperatures (25 and 30°C) were similar for the three populations. The optimal temperature for embryogenesis was calculated to be 31.4°C for the population from Alabama, 28.4°C for the one from Mississippi, and 37.5°C for the one from South Carolina.