Effects of Single P-Element Insertions on Bristle Number and Viability in Drosophila Melanogaster

Single P-element mutagenesis was used to construct 1094 lines with P[lArB] inserts on all three major chromosomes in an isogenic background previously free of P elements. The effects of insertions on bristle number and on viability were assessed by comparison to 392 control lines. The variance and effects of P-element inserts on bristle number and viability were larger than those inferred from spontaneous mutations. The distributions of effects on bristle number were symmetrical and highly leptokurtic, such that a few inserts with large effects caused most of the increase in variance. The distribution of effects on viability were negatively skewed and platykurtic. On average, the effects of P-element insertions on bristle number were partly recessive and on viability were completely recessive. P-element inserts with large effects on bristle number tended to have reduced viability, but the correlation between the absolute value of the effects on bristle number and on viability was not strong. Fifty P-element inserts tagging quantitative trait loci (QTLs) with large effects on bristle number were mapped cytogenetically. Two P-element-induced scabrous alleles and five extramacrochaetae alleles were generated. Single P-element mutagenesis is a powerful method for identifying QTLs at the level of genetic locus.     

Structure,molecular evolution and maintenance of copy number of extended repeated structures in the X-heterochromatin of Drosophila melanogaster

The 60 kb repeats located in the distal heterochromatin of the X chromosome of Drosophila melanogaster were cloned in overlapping cosmids. These regions, designated as SCLRs, comprised the following types of repeated elements Stellate genes, which are known to be involved in spermatogenesis; copia-like retrotransposons; LINE elements, including amplified Type rDNA insertions; and rDNA fragments. The following steps in SCLR formation were hypothesized: insertion of mobile elements into the rDNA and Stellate gene clusters: internal tandem duplication events; recombination between the rDNA cluster and Stellate tandem repeat; and amplification of the whole SCLR structure. There are about nine SCLR copies per haploid genome, but there is approximately a twofold variation in copy number between fly stocks. The SCLR copy number differences between closely related stocks are suggested to be the result of unequal sister chromatid exchange (USCE). The restricted variation in SCLR copy number between unrelated stocks and the absence of chromosomes free of SCLRs suggests that natural selection is active in copy number maintenance.     

Polygenic Mutation in Drosophila Melanogaster: Estimates from Response to Selection of Inbred Strains

Replicated divergent artificial selection for abdominal and sternopleural bristle number from a highly inbred strain of Drosophila melanogaster resulted in an average divergence after 125 generations of selection of 12.0 abdominal and 8.2 sternopleural bristles from the accumulation of new mutations affecting bristle number. Responses to selection were highly asymmetrical, with greater responses for low abdominal and high sternopleural bristle numbers. Estimates of V(M), the mutational variance arising per generation, based on the infinitesimal model and averaged over the responses to the first 25 generations of selection, were 4.32 X 10(-3) V(E) for abdominal bristle number and 3.66 X 10(-3) V(E) for sternopleural bristle number, where V(E) is the environmental variance. Based on 10 generations of divergent selection within lines from generation 93, V(M) for abdominal bristle number was 6.75 X 10(-3) V(E) and for sternopleural bristle number was 5.31 X 10(-3) V(E). However, estimates of V(M) using the entire 125 generations of response to selection were lower and generally did not fit the infinitesimal model largely because the observed decelerating responses were not compatible with the predicted increasing genetic variance over time. These decelerating responses, periods of response in the opposite direction to artificial selection, and rapid responses to reverse selection all suggest new mutations affecting bristle number on average have deleterious effects on fitness. Commonly observed periods of accelerated responses followed by long periods of stasis suggest a leptokurtic distribution of mutational effects for bristles.     

The Search for Agroislands in the Chickpea Genome

Biophysics - The concentration of genes that control domestication traits in so-called “agroislands” complicates breeding. Therefore, information about where such islands are located in...     

An Artificial Neural Network Model to Predict the Phenology of Early-Maturing Soybean Varieties from Climatic Factors

Biophysics - Abstract—Soybean phenology is strongly influenced by temperature and day length, and phenological records clearly reflect the changes in climatic conditions. A model including...     

Natural genetic variation in cuticular hydrocarbon expression in male and female Drosophila melanogaster

Cuticular hydrocarbons (CHCs) act as contact pheromones in Drosophila melanogaster and are an important component of several ecological traits. Segregating genetic variation in the expression of CHCs at the population level in D. melanogaster is likely to be important for mate choice and climatic adaptation; however, this variation has never been characterized. Using a panel of recombinant inbred lines (RILs) derived from a natural population, we found significant between-line variation for nearly all CHCs in both sexes. We identified 25 QTL in females and 15 QTL in males that pleiotropically influence CHC expression. There was no evidence of colocalization of QTL for homologous traits across the sexes, indicating that sexual dimorphism and low intersex genetic correlations between homologous CHCs are a consequence of largely independent genetic control. This is consistent with a pattern of divergent sexual and natural selection between the sexes.     

Quantitative trait loci for lifespan of mated Drosophila melanogaster affect both sexes

The properties of alleles at quantitative trait loci (QTLs) contributing to variation in lifespan should be described to determine the mechanisms of evolution of life length and to predict its future changes. Previously, we and others conducted genome-wide screens for QTLs that segregate among one panel of recombinant inbred lines (RILs) using a dense molecular marker map. In non-stressful conditions, QTLs effecting the lifespans of virgin females and males were frequently sex specific. In an unrelated panel of RILs, the effects of QTLs in flies maintained in cages with mixed sexes were similar in both sexes. Here, we re-measured the lifespans of the former panel of RILs in cages with mixed sex cohorts. Lifespan declined owing to mating. The amount of decline correlated between sexes within lines. QTLs mapping to the intervals 15A-19C, 50B-57C, 63A-65A, and 96F-99B had similar effects on the lifespans of both males and females. These QTLs have previously been detected in virgin flies surveys and had sex- and/or environment-specific effects.     

Fitness costs of Doc expression are insufficient to stabilize its copy number in Drosophila melanogaster

The stable coexistence of transposable elements (TEs) with their host genome over long periods of time suggests TEs have to impose some deleterious effect upon their host fitness. Three mechanisms have been proposed to account for the deleterious effect caused by TEs: host gene interruptions by TE insertions, chromosomal rearrangements by TE-induced ectopic recombination, and costly TE expression. However, the relative importance of these mechanisms remains controversial. Here, we test specifically if TE expression accounts for the host fitness cost imposed by TE insertions. In the retrotransposon Doc, expression requires binding of the host RNA polymerase to the internal promoter. If expression of Doc elements is deleterious to their host, Doc copies with promoters would be more strongly selected against and would persist in the population for shorter periods of time compared with Docs lacking promoters. We tested this prediction using sequence-specific amplified polymorphism (SSAP) analyses. We compared the populations of these two types of Doc elements in two sets of lines of Drosophila melanogaster: selection-free isogenic lines accumulating new Doc insertions and isogenized isofemale lines sampled from a natural population. We found that (1) there is no difference in the proportion of promoter-bearing and promoter-lacking copies between sets of lines, and (2) the site occupancy distribution of promoter-bearing copies does not skew toward lower frequency compared with that of promoter-lacking copies. Thus, selection against promoter-bearing copies does not appear to be stronger than that of promoter-lacking copies. Our results show that expression is not playing a major role in stabilizing Doc copy numbers.