It is often assumed that the mutation rate is an evolutionarily optimized property of a taxon. The relevant mutation rate is for mutations that affect fitness, U, but the strength of selection on the mutation rate depends on the average effect of a mutation. Determination of U is complicated by the possibility that mutational effects depend on the particular environmental context in which the organism exists. It has been suggested that the effects of deleterious mutations are typically magnified in stressful environments, but most studies confound genotype with environment, so it is unclear to what extent environmental specificity of mutations is specific to a particular starting genotype. We report a study designed to separate effects of species, genotype, and environment on the degradation of fitness resulting from new mutations. Mutations accumulated for .200 generations at 20°in two strains of two species of nematodes that differ in thermal sensitivity. Caenorhabditis briggsae and C. elegans have similar demography at 20°, but C. elegans suffers markedly reduced fitness at 25°. We find little evidence that mutational properties differ depending on environmental conditions and mutational correlations between environments are close to those expected if effects were identical in both environments.T HE importance of deleterious mutations to the evolutionary process is well appreciated (Morgan 1903;Haldane 1927;Fisher 1930;Sturtevant 1937;Kondrashov 1988), and much effort has been expended in understanding the processes by which new mutations arise and their effects on the phenotype and on fitness (reviewed by Simmons and Crow 1977;Drake et al. 1998;Keightley and Eyre-Walker 1999;Lynch et al. 1999;Houle and Kondrashov 2006). Drake, especially, has emphasized the remarkable consistency of the per-genome mutation rate across very broad taxonomic categories, but has also noted that there is considerable variation within those broad taxa (e.g., Drake et al. 1998). The idea that certain mutational properties vary between related species and even within species is venerable (Sturtevant 1937 and references therein), but there is as yet nothing approaching a comprehensive understanding of the variation in mutational properties-rate, distribution of effects, environmental sensitivity, molecular spectrum-of any species or group of closely related species, with the possible exception of the bacterium Escherichia coli (Matic et al. 1997;Sniegowski et al. 1997;Bjedov et al. 2003).An intriguing but almost completely unsubstantiated possibility (but see Nöthel 1987;Bjedov et al. 2003) is that mutation rates are themselves an evolutionarily optimized property (Fisher 1930;Sturtevant 1937;Kimura 1960Kimura , 1967Leigh 1970Leigh , 1973 Kondrashov 1995a,b;Dawson 1998). Because the vast majority of mutations with observable effects are deleterious, it is generally accepted that direct selection (almost) always favors a reduction in the mutation rate, with an optimal mutation rate of zero, at least in sexual taxa (e.g., Leig...

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Highly aggressive Africanized honeybees (AHB) invaded Puerto Rico (PR) in 1994, displacing gentle European honeybees (EHB) in many locations. Gentle AHB (gAHB), unknown anywhere else in the world, subsequently evolved on the island within a few generations. Here we sequence whole genomes from gAHB and EHB populations, as well as a North American AHB population, a likely source of the founder AHB on PR. We show that gAHB retains high levels of genetic diversity after evolution of gentle behaviour, despite selection on standing variation. We observe multiple genomic loci with significant signatures of selection. Rapid evolution during colonization of novel habitats can generate major changes to characteristics such as morphological or colouration traits, usually controlled by one or more major genetic loci. Here we describe a soft selective sweep, acting at multiple loci across the genome, that occurred during, and may have mediated, the rapid evolution of a behavioural trait.

Mutational bias is a potentially important agent of evolution, but it is difficult to disentangle the effects of mutation from those of natural selection. Mutation-accumulation experiments, in which mutations are allowed to accumulate at very small population size, thus minimizing the efficiency of natural selection, are the best way to separate the effects of mutation from those of selection. Body size varies greatly among species of nematode in the family rhabditidae; mutational biases are both a potential cause and a consequence of that variation. We report data on the cumulative effects of mutations that affect body size in three species of rhabditid nematode that vary fivefold in adult size. Results are very consistent with previous studies of mutations underlying fitness in the same strains: two strains of Caenorhabditis briggsae decline in body size about twice as fast as two strains of C. elegans, with a concomitant higher point estimate of the genomic mutation rate; the confamilial Oscheius myriophila is intermediate. There is an overall mutational bias, such that mutations reduce size on average, but the bias appears consistent between species. The genetic correlation between mutations that affect size and those underlying fitness is large and positive, on average.T HE importance of mutation to the evolutionary process is universally appreciated by biologists, both in terms of the deleterious effects on fitness (Morgan 1903;Fisher 1930;Haldane 1937;Sturtevant 1937) and as the ultimate source of potentially adaptive genetic variation. It has been recognized for a long time that there is substantial variation in the mutational process at a variety of taxonomic levels, even among genotypes within species (Sturtevant 1937 and references therein;Woodruff et al. 1984;Fry 2004b;Baer et al. 2005;Á vila et al. 2006;Haag-Liautard et al. 2007). The factors responsible for that variation are poorly understood, but there are two classes of potential explanations. First, the mutation rate may be primarily a by-product of intrinsic or extrinsic environmental factors, e.g., temperature, metabolic rate, UV exposure, etc. (Martin and Palumbi 1993;Hebert et al. 2002;Gillooly et al. 2005). Alternatively, the mutation rate may be an evolutionarily optimized property, with either the optimum or the deviation from the optimum varying among taxa (Kimura 1967;Leigh 1973;Kondrashov 1995;Dawson 1998). Elucidating the taxonomic distribution of variation in mutational properties may provide important insights into several disparate areas of evolutionary biology, among them the causes of adaptive radiation (Bjedov et al. 2003;Sikorski and Nevo 2005) and cladogenesis (Shpak 2005), the rate of molecular evolution (Martin and Palumbi 1993;Gillooly et al. 2005), the nature of selection on modifier loci (Kondrashov 1995), the evolution of genetic architecture underlying the phenotype ( Jones et al. 2003), and the evolution of mating system and sexual reproduction (Kondrashov 1988(Kondrashov , 1995Keightley and Otto 2006).Of part...

For social animals, the genotypes of group members affect the social environment, and thus individual behavior, often indirectly. We used genome-wide association studies (GWAS) to determine the influence of individual vs. group genotypes on aggression in honey bees. Aggression in honey bees arises from the coordinated actions of colony members, primarily nonreproductive “soldier” bees, and thus, experiences evolutionary selection at the colony level. Here, we show that individual behavior is influenced by colony environment, which in turn, is shaped by allele frequency within colonies. Using a population with a range of aggression, we sequenced individual whole genomes and looked for genotype–behavior associations within colonies in a common environment. There were no significant correlations between individual aggression and specific alleles. By contrast, we found strong correlations between colony aggression and the frequencies of specific alleles within colonies, despite a small number of colonies. Associations at the colony level were highly significant and were very similar among both soldiers and foragers, but they covaried with one another. One strongly significant association peak, containing an ortholog of the Drosophila sensory gene dpr4 on linkage group (chromosome) 7, showed strong signals of both selection and admixture during the evolution of gentleness in a honey bee population. We thus found links between colony genetics and group behavior and also, molecular evidence for group-level selection, acting at the colony level. We conclude that group genetics dominates individual genetics in determining the fatal decision of honey bees to sting.

Background The population genetics of U.S. honey bee stocks remain poorly characterized despite the agricultural importance of Apis mellifera as the major crop pollinator. Commercial and research-based breeding programs have made significant improvements of favorable genetic traits (e.g. production and disease resistance). The variety of bees produced by artificial selection provides an opportunity to characterize the genetic diversity and regions of the genome undergoing selection in commonly managed stocks. Results Pooled sequencing of eight honey bee stocks found strong genetic similarity among six of the stocks. Two stocks, Pol-line and Hilo, showed significant differentiation likely due to their intense and largely closed breeding for resistance to the parasitic Varroa mite. Few variants were identified as being specific to any one stock, indicating potential admixture among the sequenced stocks. Juxtaposing the underlying genetic variation of stocks selected for disease- and parasite-resistance behavior, we identified genes and candidate regions putatively associated with resistance regulated by hygienic behavior. Conclusion This study provides important insights into the distinct genetic characteristics and population diversity of honey bee stocks used in the United States, and provides further evidence of high levels of admixture in commercially managed honey bee stocks. Furthermore, breeding efforts to enhance parasite resistance in honey bees may have created unique genetic profiles. Genomic regions of interest have been highlighted for potential future work related to developing genetic markers for selection of disease and parasite resistance traits. Due to the vast genomic similarities found among stocks in general, our findings suggest that additional data regarding gene expression, epigenetic and regulatory information are needed to more fully determine how stock phenotypic diversity is regulated.

Developmental plasticity generates phenotypic variation, but how it contributes to evolutionary change is unclear. Phenotypes of individuals in caste-based (eusocial) societies are particularly sensitive to developmental processes, and the evolutionary origins of eusociality may be rooted in developmental plasticity of ancestral forms. We used an integrative genomics approach to evaluate the relationships among developmental plasticity, molecular evolution, and social behavior in a bee species (Megalopta genalis) that expresses flexible sociality, and thus provides a window into the factors that may have been important at the evolutionary origins of eusociality. We find that differences in social behavior are derived from genes that also regulate sex differentiation and metamorphosis. Positive selection on social traits is influenced by the function of these genes in development. We further identify evidence that social polyphenisms may become encoded in the genome via genetic changes in regulatory regions, specifically in transcription factor binding sites. Taken together, our results provide evidence that developmental plasticity provides the substrate for evolutionary novelty and shapes the selective landscape for molecular evolution in a major evolutionary innovation: Eusociality.

Circadian rhythms in social insects are highly plastic and are modulated by multiple factors. In addition, complex behaviors such as sun-compass orientation and time learning are clearly regulated by the circadian system in these organisms. Despite these unique features of social insect clocks, the mechanisms as well as the functional and evolutionary relevance of these traits remain largely unknown. Here we show a modification of the Drosophila activity monitoring (DAM) system that allowed us to measure locomotor rhythms of the honey bee, Apis mellifera (three variants; gAHB, carnica and caucasica), and two paper wasps (Polistes crinitus and Mischocyttarus phthisicus). A side-by-side comparison of the endogenous period under constant darkness (free-running period) led us to the realization that these social insects exhibit significant deviations from the Earth's 24 h rotational period as well as a large degree of inter-individual variation compared with Drosophila. Experiments at different temperatures, using honey bees as a model, revealed that testing the endogenous rhythm at 35°C, which is the hive's core temperature, results in average periods closer to 24 h compared with 25°C (23.8 h at 35°C versus 22.7 h at 25°C). This finding suggests that the degree of tuning of circadian temperature compensation varies among different organisms. We expect that the commercial availability, cost-effectiveness and integrated nature of this monitoring system will facilitate the growth of the circadian field in these social insects and catalyze our understanding of the mechanisms as well as the functional and evolutionary relevance of circadian rhythms.