The abundance and distribution of transposable elements (TEs) in a representative part of the euchromatic genome of Drosophila melanogaster were studied by analyzing the sizes and locations of TEs of all known families in the genomic sequences of chromosomes 2R, X, and 4. TEs contribute to up to 2% of the sequenced DNA, which corresponds roughly to the euchromatin of these chromosomes. This estimate is lower than that previously available from in situ data and suggests that TEs accumulate in the heterochromatin more intensively than was previously thought. We have also found that TEs are not distributed at random in the chromosomes and that their abundance is more strongly associated with local recombination rates, rather than with gene density. The results are compatible with the ectopic exchange model, which proposes that selection against deleterious effects of chromosomal rearrangements is a major force opposing element spread in the genome of this species. Selection against insertional mutations also influences the observed patterns, such as an absence of insertions in coding regions. The results of the analyses are discussed in the light of recent findings on the distribution of TEs in other species.

A genome-wide comparison of transposable elements reveals evidence for unexpectedly high rates of horizontal transfer between three species of Drosophila

The worldwide beekeeping sector has been facing a grave threat, with losses up to 100-1000 times greater than those previously reported. Despite the scale of this honey bee mortality, the causes underlying this phenomenon remain unclear, yet they are thought to be multifactorial processes. Nosema ceranae, a microsporidium recently detected in the European bee all over the world, has been implicated in the global phenomenon of colony loss, although its role remains controversial. A review of the current knowledge about this pathogen is presented focussing on discussion related with divergent results, trying to analyse the differences specially based on different methodologies applied and divisive aspects on pathology while considering a biological or veterinarian point of view. For authors, the disease produced by N. ceranae infection cannot be considered a regional problem but rather a global one, as indicated by the wide prevalence of this parasite in multiple hosts. Not only does this type of nosemosis causes a clear pathology on honeybees at both the individual and colony levels, but it also has significant effects on the production of honeybee products.

The distribution of mutational effects on fitness is of fundamental importance for many aspects of evolution. We develop two methods for characterizing the fitness effects of deleterious, nonsynonymous mutations, using polymorphism data from two related species. These methods also provide estimates of the proportion of amino acid substitutions that are selectively favorable, when combined with data on between-species sequence divergence. The methods are applicable to species with different effective population sizes, but that share the same distribution of mutational effects. The first, simpler, method assumes that diversity for all nonneutral mutations is given by the value under mutation-selection balance, while the second method allows for stronger effects of genetic drift and yields estimates of the parameters of the probability distribution of mutational effects. We apply these methods to data on populations of Drosophila miranda and D. pseudoobscura and find evidence for the presence of deleterious nonsynonymous mutations, mostly with small heterozygous selection coefficients (a mean of the order of 10 ÿ5 for segregating variants). A leptokurtic gamma distribution of mutational effects with a shape parameter between 0.1 and 1 can explain observed diversities, in the absence of a separate class of completely neutral nonsynonymous mutations. We also describe a simple approximate method for estimating the harmonic mean selection coefficient from diversity data on a single species. S URVEYS of DNA sequence polymorphisms in many species have revealed substantial variation in the aminoacid sequences of proteins, although the nonsynonymous nucleotide site diversity is usually much less than that for silent variants (Li 1997). This is consistent with the action of purifying selection on protein sequences, removing deleterious amino acid mutations from the population while neutral or nearly neutral silent variants can persist (Kimura 1983;Li 1997). It is clearly important to characterize the distribution of fitness effects of new nonsynonymous mutations. This distribution is relevant to a broad range of problems in evolutionary genetics, and a variety of methods have been used to characterize it, including direct estimates from mutation-accumulation experiments and indirect estimates from comparisons of DNA sequences among related species (Keightley and Eyre-Walker 1999). The extent, nature, and magnitude of selection on amino acid variants are also relevant to understanding the relation between human disease and genetic variation (Sunyaev et al. 2001;Wright et al. 2003).Several methods have been used to detect purifying selection from data on variability in natural populations and to estimate the parameters describing such selection. An important method was introduced by Sawyer and Hartl (1992), based on the McDonald-Kreitman test (McDonald and Kreitman 1991). It compares the ratio of the number of within-species nonsynonymous polymorphisms to the number of synonymous polymorphisms and the corresponding ra...