Aurora García-Dorado

THE RATE AND EFFECTS DISTRIBUTION OF VIABILITY MUTATION IN DROSOPHILA : MINIMUM DISTANCE ESTIMATION

The empirical distribution of the mean viability of mutation accumulation lines, obtained from three published experiments, was analyzed using minimum‐distance estimation. In two cases (Mukai et al. 1972; Ohnishi 1977), mutations were allowed to accumulate in copies of chromosome II protected from natural selection and recombination. In the other one (Fernández and López‐Fanjul 1996), they accumulated in inbred lines derived from an isogenic stock. In contrast with currently accepted hypotheses, we consistently estimated low (about 0.01) genomic viability mutation rates, λ, and a small kurtosis of the distribution of mutational effects on viability (a) in the three datasets. Minimum‐distance estimates of the per‐generation mean viability change due to mutation (λE[a]) were also obtained. These were very similar for both chromosomal datasets, their absolute values being about five times smaller than estimates obtained from the observed change in mean viability during the mutation process. It must be noted that, in both experiments, viability was measured relative to the Cy chromosome of a Cy/Pm stock. Thus, an unnoticed viability increase in this Cy chromosome may have resulted in overestimation of the mean viability reduction in the lines. In parallel, minimum‐distance estimation of λE(a) from inbred lines data (where the selective pressure during the accumulation process was larger) was even somewhat smaller, in absolute value, and very close to the estimate obtained by comparing the mean viability of the lines with that of the control isogenic line. The evolutionary importance of these results, as well as their relevance to the solution of the mutational load paradox, is discussed.

THE GENETICS OF VIABILITY IN DROSOPHILA MELANOGASTER: EFFECTS OF INBREEDING AND ARTIFICIAL SELECTION

Inbreeding and artificial selection experiments were conducted to investigate the genetic properties of egg‐to‐pupa viability in a population of Drosophila melanogaster. The effect of different levels of inbreeding (F = 0, 0.25, 0.50, and 0.73) was studied. Up to F = 0.50, a linear depression of the mean viability was observed, accompanied by a significant increase of both within‐line additive variance and between‐line variance. At F = 0.73, no further changes were detected. This can be attributed to natural selection opposing high levels of homozygosity. In parallel, artificial selection to increase viability was performed for 27 generations in (1) a single undivided population (U) and (2) two populations with cycles of subdivision and between‐line selection, followed by reconstitution of selected lines (SO and SI). During the first cycle (generations 0–4), most of the final total response was achieved under all selection regimes. An advantage of the SO and SI strategies was observed after the completion of the first cycle. However, the same limit was reached in all cases because of a delayed response experienced by line U. Reverse selection for viability resulted in positive correlated responses for fecundity and mating success. Both inbreeding and selection results are compatible with the genetic variance of viability in the base population being generated by segregation at a few loci with substantial additive effects and several deleterious recessives at low initial frequencies. Possible reasons for the maintenance of that variance in natural populations are discussed.

THE EFFECT OF NICHE PREFERENCE ON POLYMORPHISM PROTECTION IN A HETEROGENEOUS ENVIRONMENT

A model is proposed in which niche choice precedes natural selection in an individuals life span; the panmictic population occupies an environment divided into niches, each contributing a constant proportion of parents to the next generation. Niche choice and fitness within the niche are controlled either by the same locus or by two linked loci. The conditions for a protected polymorphism are derived.

Understanding Inbreeding Depression, Purging, and Genetic Rescue

Inbreeding depression, the reduction of fitness caused by inbreeding, is a nearly universal phenomenon that depends on past mutation, selection, and genetic drift. Recent estimates suggest that its impact on individual fitness is even greater than previously thought. Genomic information is contributing to its detection and can enlighten important aspects of its genetic architecture. In natural populations, purging and genetic rescue mitigate fitness decline during inbreeding periods, and might be critical to population survival, thus, both mechanisms should be considered when assessing extinction risks. However, deliberate purging and genetic rescue involve considerable risk in the short and medium term, so that neither appears to be a panacea against high inbreeding depression.

Allelic Diversity and Its Implications for the Rate of Adaptation

Genetic variation is usually estimated empirically from statistics based on population gene frequencies, but alternative statistics based on allelic diversity (number of allelic types) can provide complementary information. There is a lack of knowledge, however, on the evolutionary implications attached to allelic-diversity measures, particularly in structured populations. In this article we simulated multiple scenarios of single and structured populations in which a quantitative trait subject to stabilizing selection is adapted to different fitness optima. By forcing a global change in the optima we evaluated which diversity variables are more strongly correlated with both short- and long-term adaptation to the new optima. We found that quantitative genetic variance components for the trait and gene-frequency-diversity measures are generally more strongly correlated with short-term response to selection, whereas allelic-diversity measures are more correlated with long-term and total response to selection. Thus, allelic-diversity variables are better predictors of long-term adaptation than gene-frequency variables. This observation is also extended to unlinked neutral markers as a result of the information they convey on the demographic population history. Diffusion approximations for the allelic-diversity measures in a finite island model under the infinite-allele neutral mutation model are also provided.

STABILIZING SELECTION DETECTED FOR BRISTLE NUMBER IN DROSOPHILA MELANOGASTER

Stabilizing selection, which favors intermediate phenotypes, is frequently invoked as the selective force maintaining a populations status quo. Two main alternative reasons for stabilizing selection on a quantitative trait are possible: (1) intermediate trait values can be favored through the causal effect of the trait on fitness (direct stabilizing selection); or (2) through a pleiotropic, deleterious side effect on fitness of mutants affecting the trait (apparent stabilizing selection). Up to now, these alternatives have never been experimentally disentangled. Here we measure fitness as a function of the number of abdominal bristles within four Drosophila melanogaster lines, one with high, one with low, and two with intermediate average bristle number. The four were inbred nonsegregating lines, so that apparent selection due to pleiotropy is not possible. Individual fitness significantly increased (decreased) with bristles number in the low (high) line. No significant fitness‐trait association was detected within each intermediate line. These results reveal substantial direct stabilizing selection on the trait.

Understanding and Predicting the Fitness Decline of Shrunk Populations: Inbreeding, Purging, Mutation, and Standard Selection

The joint consequences of inbreeding, natural selection, and deleterious mutation on mean fitness after population shrinkage are of great importance in evolution and can be critical to the conservation of endangered populations. I present simple analytical equations that predict these consequences, improving and extending a previous heuristic treatment. Purge is defined as the “extra” selection induced by inbreeding, due to the “extra” fitness disadvantage (2d) of homozygotes for (partially) recessive deleterious alleles. Its effect is accounted for by using, instead of the classical inbreeding coefficient f, a purged inbreeding coefficient g that is weighed by the reduction of the frequency of deleterious alleles caused by purging. When the effective size of a large population is reduced to a smaller stable value N (with Nd ≥ 1), the purged inbreeding coefficient after t generations can be predicted as gt ≈ [(1 – 1/2N) gt-1 + 1/2N](1 – 2d ft-1), showing how purging acts upon previously accumulated inbreeding and how its efficiency increases with N. This implies an early fitness decay, followed by some recovery. During this process, the inbreeding depression rate shifts from its ancestral value (δ) to that of the mutation–selection–drift balance corresponding to N (δ*), and standard selection cancels out the inbreeding depression ascribed to δ*. Therefore, purge and inbreeding operate only upon the remaining δ − δ*. The method is applied to the conservation strategy in which family contributions to the breeding pool are equal and is extended to make use of genealogical information. All these predictions are checked using computer simulation.

Increase of the Spontaneous Mutation Rate in a Long-Term Experiment With Drosophila melanogaster

In a previous experiment, the effect of 255 generations of mutation accumulation (MA) on the second chromosome viability of Drosophila melanogaster was studied using 200 full-sib MA1 lines and a large C1 control, both derived from a genetically homogeneous base population. At generation 265, one of those MA1 lines was expanded to start 150 new full-sib MA2 lines and a new C2 large control. After 46 generations, the rate of decline in mean viability in MA2 was ∼2.5 times that estimated in MA1, while the average degree of dominance of mutations was small and nonsignificant by generation 40 and moderate by generation 80. In parallel, the inbreeding depression rate for viability and the amount of additive variance for two bristle traits in C2 were 2–3 times larger than those in C1. The results are consistent with a mutation rate in the line from which MA2 and C2 were derived about 2.5 times larger than that in MA1. The mean viability of C2 remained roughly similar to that of C1, but the rate of MA2 line extinction increased progressively, leading to mutational collapse, which can be ascribed to accelerated mutation and/or synergy after important deleterious accumulation.

Population genetics: Surviving under mutation pressure

Concern has been voiced about the survival of endangered species, and even the long-term prospects for humans, in the face of accumulating deleterious mutations. Two experiments have investigated the mutation accumulation process in laboratory Drosophila populations, with apparently conflicting results.

On the persistence and pervasiveness of a new mutation

Abstract It has frequently been assumed that the persistence of a deleterious mutation (the average number of generations before its loss) and its pervasiveness (the average number of individuals carrying the gene before its loss) are equal. This is true for a particular simple, widely used infinite model, but this agreement is not general. If hs > 1/(4Ne), where hs is the selective disadvantage of mutant heterozygotes and Ne is the effective population number, the contribution of homozygous mutants can be neglected and the simple approximate formula 1/hs gives the mean pervasiveness. But the expected persistence is usually much smaller, 2(loge(1/2hs) + 1—y) where y = 0.5772. For neutral mutations, the total number of heterozygotes until fixation or loss is often the quantity of interest, and its expected value is 2Ne, with remarkable generality for various population structures. In contrast, the number of generations until fixation or loss, 2(Ne/N)(1 + loge2N), is much smaller than the total number of heterozygotes. In general the number of generations is less than the number of individuals.