FITNESS SENSITIVITY AND THE CANALIZATION OF LIFE-HISTORY TRAITS

Canalization is an abstract term that describes unknown developmental mechanisms that reduce phenotypic variation. A trait can be canalized against environmental perturbations (e.g., changes in temperature or nutrient quality), or genetic perturbations (e.g., mutations or recombination); this paper is about genetic canalization. Stabilizing selection should improve the canalization of traits, and the degree of canalization should be positively correlated with the traits' impact on fitness. Experiments testing this idea should measure the canalization of a series of traits whose impact on fitness is known or can be inferred, exclude differences among traits in the number of loci and alleles segregating as an explanation for the pattern of variability found, and distinguish between canalization against genetic and environmental variation. These conditions were met by three experiments within which the variation of fitness components among Drosophila melanogaster lines was measured and among which the genetic contribution to the variation among lines was clearly different. The canalization of the traits increased with their impact on fitness and did not depend on the degree of genetic differences among lines. That the flies used had been transformed by a P-element insert suggests that canalization was also effective against novel genetic variation. The results reported here cannot be explained by the classical hypothesis of reduction in the number of loci segregating for traits with greater impact on fitness and confirm that traits with greater impact on fitness are more strongly canalized. This pattern of canalization reveals an underappreciated role for development in microevolution. There is differential genetic canalization of fitness components in D. melanogaster.     

Perturbation of gene frequencies in a natural population of Drosophila melanogaster: evidence for selection at the Adh locus

The cellar population of Drosophila melanogaster at the Chateau Tahbilk Winery (Victoria, Australia) was perturbed for alcohol dehydrogenase (Adh) gene frequencies. Phenol oxidase (Phox) frequencies were also perturbed and monitored as a control. Subsequent gene frequency changes, together with information on population structure, indicated that selection acted on the chromosome regions of both loci. Adh gene frequencies returned to preperturbation levels in a predictable manner. A model in which the relative fitness of Adh phenotypes was determined by temperature-dependent specific activities of enzymes of Adh genotypes adequately accounts for the rate of gene frequency change at this locus. Thus temperature behaves as a selective agent in modulating Adh gene frequencies in this cellar environment.     

Estimating selection by comparing synonymous and substitutional changes

A higher ratio of substitutional to synonymous changes in between-species than in within-species comparisons has been taken as evidence for positive selection changing amino acids. A model is presented in which a difference of this kind arises as a result of purely neutral mutations, provided that the species compared are sufficiently different to approach a steady state between forward and backward mutation. In Neissseria, substitutions are twice as frequent, relative to synonymous changes, in between-species comparisons: it is shown that the data are consistent with the neutral model. The argument does not invalidate evidence for positive selection, for example in Drosophila, when the species compared are fairly similar.     

Keepers of maize in Chiapas,Mexico

This study concerns the knowledge and selection of biological diversity of maize (Zea mays) within its center of domestication in Mesoamerica. Maize farmers in central Chiapas of Southern México keep local varieties (landraces) belonging to six races and four race mixtures. Fifteen local varieties are recognized. In spite of widespread adoption of a modern, high yielding variety, maize farmers continue to select local varieties for specific soils and because of agronomic and use criteria. Farmers maintain maize varieties primarily through seed selection. Spatial and temporal separation do not seem sufficient to maintain varieties. The management of improved varieties can lead from a uniform population to a heterogeneous one as hybridization with local maize populations occurs.     

Chlorsulfuron resistance in Daucus carota cell lines and plants:Involvement of gene amplification

Daucus carota L. cell lines stably resistant to the herbicide chlorsulfuron (CS) have been isolated according to a stepwise selection. Studies carried out during different selection steps show that the specific activity of the target enzyme acetohydroxyacid synthase (AHAS) increases along with CS resistance. Southern hybridization analysis performed with aBrassica napus AHAS probe in a CS highly-resistant cell line reveals the presence of a greatly amplifiedEcoRI fragment of genomic DNA. This indicates that AHAS overproduction induced by stepwise selection is due to gene amplification. Regenerants from some resistant cell lines maintained the CS-resistant trait at the whole plant level.     

Sporophytic response to pollen selection for Alachlor tolerance in maize

In order to assess the efficiency of male gametophytic selection (MGS) for crop improvement, pollen selection for tolerance to herbicide was applied in maize. The experiment was designed to test the parallel reactivity to Alachlor of pollen and plants grown in controlled conditions or in the field, the response to pollen selection in the sporophytic progeny, the response to a second cycle of MGS, and the transmission of the selected trait to the following generations. The results demonstrated that pollen assay can be used to predict Alachlor tolerance under field conditions and to monitor the response to selection. A positive response to selection applied to pollen in the sporophytic progeny was obtained in diverse genetic backgrounds, indicating that the technique can be generally included in standard breeding programs; the analysis of the data produced in a second selection cycle indicated that the selected trait is maintained in the next generation.     

Linkage disequilibrium in crosses between Illinois maize strains divergently selected for protein percentage

The objectives of this study were two fold: (1) to determine whether divergent selection for kernel protein concentration, which produced the Illinois high protein (IHP), Illinois low protein (ILP), reverse low protein (RLP), and reverse high protein (RHP) maize (Zea mays L.) strains, had generated coupling-phase linkages among genes controlling protein concentration or other traits and (2) to measure the effectiveness of random mating in reducing linkage disequilibrium in segregating generations from crosses between the strains. To achieve these objectives, design III progenies from the F₂ and F₆ (produced by random mating the F₂) from the crosses of IHP × ILP, IHP × RHP, ILP × RLP, and RHP × RLP were evaluated. Estimates of additive variance for percent protein in the crosses of IHP × ILP and ILP × RLP were significantly less in the F₆ than in the F₂ indicating the presence of coupling-phase linkages in the parents and their breakup by random mating. In addition, a significant reduction in dominance variance for grain yield from the F₂ to the F₆ in IHP × ILP suggested the presence of repulsion-phase linkages. No other evidence of coupling or repulsion-phase linkages was found for any of the traits measured. These results demonstrate the effectiveness of long-term divergent selection in the development of coupling-phase linkages and of random mating to dissipate linkage disequilibrium.Research supported by the Illinois Agricultural Experiment Station     

Diet ofColobus polykomos on Tiwai Island: Selection of food in relation to its seasonal abundance and nutritional quality

A group of Colobus polykomosat Tiwai, Sierra Leone, demonstrated seasonal flexibility in its diet, with seeds, young leaves, and mature leaves each dominating the diet at different times. Comparison of food consumption with phenological data indicates that seeds are eaten whenever available and are preferred to other foods, while young leaves are preferred to mature leaves. Colobus polykomosalso prefer liane to tree leaves, despite the relatively high quality of mature tree foliage at the Tiwai site. Analysis of protein, fiber, and energy values of foods selected and items available, but not eaten, suggests that preference is related to protein and energy maximization. Leguminous plants, especially Papilionaceae and Mimosaceae, are highlighted as important food sources for C. polykomos;seeds and leaves from these families have a high nitrogen content, and the protein content of leguminous seeds often equals or exceeds that found in leaves. It is predicted, therefore, that colobines living in habitats with a high density of legumes will feed heavily on seeds, subject to constraints such as seasonal availability. Where suitable leguminous species are less common, a mixture of fruits, seeds, and young or mature leaves or both is likely to be selected. The results of this and other recent studies of colobines do not support the notion that colobines are specialist folivores.     

Selection by wild birds on artificial dimorphic prey on varied backgrounds

Our aim was to test the effects of prey frequency and background composition on selection by free-ranging birds. We did three series of experiments with populations of grey and orange pastry prey scattered among coloured stones that made the prey either conspicuous or inconspicuous. Series 1 tested whether the predicted equilibrium frequency of the two prey types was influenced by the frequency of matching grey and orange stones. Birds at a single site were given a random sequence of combinations of prey frequency and stone frequency. Selection was dependent on background and the effect of prey frequency also varied with background. In series 2, we explored the frequency-independent effect of background: birds at five sites were given equal numbers of the two prey in three frequencies of matching stones and two of non-matching. There was a higher risk of predation for prey that matched rarer stones. In series 3 we attempted to measure, at a single site, the actual equilibrium prey frequencies in three different backgrounds: two extreme stone frequencies and one intermediate. Each experiment started with a population of equal numbers of grey and orange prey. After half the prey had been eaten we calculated the frequencies of the survivors and presented a new population of the original size but with the new prey frequencies; each experiment ran for 25 such 'generations'. The results suggested that at equilibrium the commoner 'morph' was the one that resembled the commoner colour of stone. Overall, our findings support the idea that visual selection can result in morph frequencies becoming related to the proportions of their matching background components and that this equilibrium will 'track' temporal or spatial changes in the background.     

Effects of predatory risk and resource renewal on the timing of foraging activity in a gerbil community

The foraging decisions of animals are often influenced by risk of predation and by the renewal of resources. For example, seed-eating gerbils on sand dunes in the Negev Desert of Israel prefer to forage in the bush microhabitat and during darker hours due to risk of predation. Also, daily renewal of seed resource patches and timing of nightly foraging activity in a depleting environment play important roles in species coexistence. We examined how these factors influence the timing of gerbil foraging by quantifying foraging activity in seed resource patches that we experimentally renewed hourly during the night. As in previous work, gerbils showed strong preference for the safe bush microhabitat and foraged less in response to high levels of illumination from natural moon light and from artificial sources. We demonstrate here for the first time that gerbils also responded to temporal and spatial heterogeneity in predatory risk through their timing of activity over the course of each night. Typically, gerbils concentrated their activity early in the night, but this changed with moon phase and in response to added illumination. These results can be understood in terms of the nature of patch exploitation by gerbils and the role played by the marginal value of energy in determining the cost of predation. They further show the dynamic nature of gerbil foraging decisions, with animals altering foraging efforts in response to time, microhabital, moon phase, illumination, and resource availability.