SPONTANEOUS MUTATIONAL GENOTYPE-ENVIRONMENT INTERACTION FOR FITNESS-RELATED TRAITS IN DROSOPHILA MELANOGASTER

Life history consequences of temperature transients in Drosophila melanogaster

The physiological and life history consequences of chronic temperatures are well studied in ectotherms. However, little is known about the consequences of short-term exposure to unusually high or low temperatures, as would occur during a weather front. What are the immediate life-history effects of such thermal transients? Can ectotherms recover quickly or do they suffer carry-over effects that persist after weather returns to normal? We measured the impact of thermal transients on egg and progeny production of Drosophila melanogaster Meigen from Washington State. We reared flies at 25°C and then transferred 3- to 5-day old adults to one of three transient treatments (1 or 3 days at 18°C, 1 day at 29°C) before returning them to 25°C. We monitored daily egg production and egg-to-adult viability before (as a control), during, and after the transient as well as fecundity and viability of flies held at constant 18°, 25° and 29°C. This population appears particularly heat tolerant as neither constant nor transient exposure to 29°C (usually a stressful temperature for this species) affected female fecundity or the viability of her progeny. However, a 1- or 3-day exposure to 18°C reduced female fecundity by 75–90% relative to controls, and eggs laid during the 3-day exposure had greatly reduced viability. When returned to 25°C after transient exposure to 18°C, females immediately matched the fecundity and viability of females maintained constantly at 25°C. Therefore, these flies do not suffer negative carry-over effects from these moderate thermal transients. Surprisingly, fitness (intrinsic rate of population growth) was not depressed by transient temperature exposure. However, the severity and especially the timing of the transient will probably determine the likelihood of carry-over effects as well as its effect on fitness.

Cumulative effects of spontaneous mutations for fitness in Caenorhabditis: role of genotype, environment and stress

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 degrees in two strains of two species of nematodes that differ in thermal sensitivity. Caenorhabditis briggsae and C. elegans have similar demography at 20 degrees, but C. elegans suffers markedly reduced fitness at 25 degrees. 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.

Comparative evolutionary genetics of spontaneous mutations affecting fitness in rhabditid nematodes

Deleterious mutations are of fundamental importance to all aspects of organismal biology. Evolutionary geneticists have expended tremendous effort to estimate the genome-wide rate of mutation and the effects of new mutations on fitness, but the degree to which genomic mutational properties vary within and between taxa is largely unknown, particularly in multicellular organisms. Beginning with two highly inbred strains from each of three species in the nematode family Rhabditidae (Caenorhabditis briggsae, Caenorhabditis elegans, and Oscheius myriophila), we allowed mutations to accumulate in the relative absence of natural selection for 200 generations. We document significant variation in the rate of decay of fitness because of new mutations between strains and between species. Estimates of the per-generation mutational decay of fitness were very consistent within strains between assays 100 generations apart. Rate of mutational decay in fitness was positively associated with genomic mutation rate and negatively associated with average mutational effect. These results provide unambiguous experimental evidence for substantial variation in genome-wide properties of mutation both within and between species and reinforce conclusions from previous experiments that the cumulative effects on fitness of new mutations can differ markedly among related taxa.

Environmental stresses can alleviate the average deleterious effect of mutations

BACKGROUND

Fundamental questions in evolutionary genetics, including the possible advantage of sexual reproduction, depend critically on the effects of deleterious mutations on fitness. Limited existing experimental evidence suggests that, on average, such effects tend to be aggravated under environmental stresses, consistent with the perception that stress diminishes the organism's ability to tolerate deleterious mutations. Here, we ask whether there are also stresses with the opposite influence, under which the organism becomes more tolerant to mutations.

RESULTS

We developed a technique, based on bioluminescence, which allows accurate automated measurements of bacterial growth rates at very low cell densities. Using this system, we measured growth rates of Escherichia coli mutants under a diverse set of environmental stresses. In contrast to the perception that stress always reduces the organism's ability to tolerate mutations, our measurements identified stresses that do the opposite - that is, despite decreasing wild-type growth, they alleviate, on average, the effect of deleterious mutations.

CONCLUSIONS

Our results show a qualitative difference between various environmental stresses ranging from alleviation to aggravation of the average effect of mutations. We further show how the existence of stresses that are biased towards alleviation of the effects of mutations may imply the existence of average epistatic interactions between mutations. The results thus offer a connection between the two main factors controlling the effects of deleterious mutations: environmental conditions and epistatic interactions.

Environmental stress and the effects of mutation

Mutations are the ultimate fuel for evolution, but most mutations have a negative effect on fitness. It has been widely accepted that these deleterious fitness effects are, on average, magnified in stressful environments. Recent results suggest that the effects of deleterious mutations can, instead, sometimes be ameliorated in stressful environments.

Environment dependence of mutational parameters for viability in Drosophila melanogaster

The genomic rate of mildly deleterious mutations (U) figures prominently in much evolutionary and ecological theory. In Drosophila melanogaster, estimates of U have varied widely, from <0.1 to nearly 1 per zygote. The source of this variation is unknown, but could include differences in the conditions used for assaying fitness traits. We examined how assay conditions affect estimates of the rates and effects of viability-depressing mutations in two sets of lines with accumulated spontaneous mutations on the second chromosome. In each set, the among-line variance in egg-to-adult viability was significantly greater when viability was assayed using a high parental density than when it was assayed using a low density. In contrast, the proportional decline in viability due to new mutations did not differ between densities. Two other manipulations, lowering the temperature and adding ethanol to the medium, had no significant effects on either the mean decline or among-line variance. Cross-environment genetic correlations in viability were generally close to one, implying that most mutations reduced viability in all environments. Using data from the low-density, lower-bound estimates of U approached the classic, high values of Mukai and Ohnishi; at the high density, U estimates were similar to recently reported low values. The difference in estimated mutation rates, taken at face value, would imply that many mutations affected fitness at low density but not at high density, but this is shown to be incompatible with the observed high cross-environment correlations. Possible reasons for this discrepancy are discussed. Regardless of the interpretation, the results show that assay conditions can have a large effect on estimates of mutational parameters for fitness traits.

The rate of mutation and the homozygous and heterozygous mutational effects for competitive viability: a long-term experiment with Drosophila melanogaster

The effect of 250 generations of mutation accumulation (MA) on the second chromosome competitive viability of Drosophila melanogaster was analyzed both in homozygous and heterozygous conditions. We used full-sib MA lines, where selection hampers the accumulation of severely deleterious mutations but is ineffective against mildly deleterious ones. A large control population was simultaneously evaluated. Competitive viability scores, unaffected by the expression of mutations in heterozygosis, were obtained relative to a Cy/L(2) genotype. The rate of decline in mean DeltaM approximately 0.1% was small. However, that of increase in variance DeltaV approximately 0.08 x 10(-3) was similar to the values obtained in previous experiments when severely deleterious mutations were excluded. The corresponding estimates of the mutation rate lambda > or = 0.01 and the average effect of mutations E(s) < or = 0.08 are in good agreement with Bateman-Mukai and minimum distance estimates for noncompetitive viability obtained from the same MA lines after 105 generations. Thus, competitive and noncompetitive viability show similar mutational properties. The regression estimate of the degree of dominance for mild-to-moderate deleterious mutations was approximately 0.3, suggesting that the pertinent value for new unselected mutations should be somewhat smaller.

Spontaneous changes in Drosophila melanogaster transposable elements and their effects on fitness

Twenty-eight spontaneous alterations modifying the hybridization banding pattern of six families of transposable elements (297, Foldback, copia, jockey, P and hobo) have been fixed in a set of mutation-accumulation lines of Drosophila melanogaster. Their effect on fitness has been studied by competition with the original pattern. Most alterations affecting transposable elements were shown to be rearrangements with no detectable effect on fitness, showing that spontaneous transposable element mutations mainly generate minor fitness mutations.

The rate of spontaneous mutation for life-history traits in Caenorhabditis elegans

Spontaneous mutations were accumulated in 100 replicate lines of Caenorhabditis elegans over a period of approximately 50 generations. Periodic assays of these lines and comparison to a frozen control suggest that the deleterious mutation rate for typical life-history characters in this species is at least 0.05 per diploid genome per generation, with the average mutational effect on the order of 14% or less in the homozygous state and the average mutational heritability approximately 0.0034. While the average mutation rate per character and the average mutational heritability for this species are somewhat lower than previous estimates for Drosophila, these differences can be reconciled to a large extent when the biological differences between these species are taken into consideration.