Laboratory model of adaptive radiation: a selection experiment in the bank vole

In a laboratory colony of a wild rodent, the bank vole Myodes (=Clethrionomys) glareolus, a multiway artificial selection experiment was applied to mimic evolution toward high aerobic metabolism achieved during locomotor activity, predatory behavior, and ability to cope with herbivorous diet. Four lines for each of the selection directions and four unselected control lines have been maintained. After three generations of within-family selection, the maximum rate of oxygen consumption achieved during swimming was 15% higher in the selected than in the control lines (least square means, adjusted for body mass: 252.0 vs. 218.6 mL O(2)/h, P = 0.0001). When fed a low-quality diet made of dried grass, voles from the lines selected for ability to cope with herbivorous diet lost about 0.7 g less mass than voles from the control lines (-2.44 vs. -3.16 g/4 d, P = 0.008). In lines selected for predatory behavior toward crickets, proportion of "predatory" individuals was higher than in the control lines (43.6% vs. 24.9%; P = 0.045), but "time to capture" calculated for the successful trials did not differ between the lines. The experiment continues, and the selected lines of voles will provide a unique model for testing hypotheses concerning correlated evolution of complex traits.     

EVOLUTION OF BRASSICA RAPA L. (CRUCIFERAE) POPULATIONS IN INTRA- AND INTERSPECIFIC COMPETITION

Populations of Brassica rapa were grown for three generations in each of two environments: intraspecific competition, with four surrounding Brassica rapa neighbors per pot, and interspecific competition, with two Raphanus sativus neighbors per pot. In each environment, the largest (by flower number) 10% of the plants were outcrossed and provided seeds for the next generation. As a control, a randomly chosen 10% of the plants in each environment were outcrossed to produce seed for the next generation. Each of these four treatments, the selected lines in intra- and interspecific competition and the corresponding control lines, was maintained for three generations. After a single generation of growth in a common, no-competition environment, replicate plants from each treatment were grown with no competition and with intra- and interspecific competition for determination of growth responses. After two generations of selection, flower number in the intraspecific-selection line had increased by more than 50% over that in the control line and by more than 19% over that under interspecific selection. After a common-environment generation, plants from the intraspecific-selection line were shown to have significantly faster growth in height and flower number as seedlings. Plants in the interspecific-selection line showed similar but nonsignificant trends. No differences in seed mass, emergence time, or photosynthetic rate were found between control and selected lines in either intra- or interspecific competition. Some differences between control and selected lines were noted in biomass allocation related to differences in phenology. The results demonstrate that performance in competitive environments can evolve through changes in plant development but that rates of evolution will differ in intra- and interspecific competition.     

Direct Response to Selection for Postweaning Gain in the Rat

The effectiveness of selection for 3–9-week gain was examined in a population of rats with a history of past selection for high 3–9-week gain. Lines were selected for high (U line) and low (D line) 3–9-week gain with two replicates of each line. Two randomly selected lines were also kept, one originating from the same base population as the two selected lines (R line) and the other originating from a population that had been randomly mated for the previous 27 generations (C line). Two replicates of each of these lines were kept. After seven generations of selection, a randomly selected line (relaxed line) was formed from each of the two upward- and each of the two downward-selected lines. Results have been presented for 13 generations of selection. The environmental trend for 3–9-week gain, as indicated by the randomly selected R and C lines, was consistently negative in all four lines. Realized heritabilities calculated by deviating the response to selection from the trend in the R or C lines resulted in non-significantly higher values in the D lines than the U lines. Six generations of relaxation of selection indicated no effect of natural selection in the U lines or the D lines. The relative magnitude of the drift, error and common environmental variances were estimated by the methods given by Hill (1971). The estimates of these parameters then led to calculation of the degree of bias in the sampling variances of the realized heritability estimates. As was predicted by Hill (1971), estimates of the variance of realized heritabilities obtained by using standard regression techniques were less than those obtained using Hill''s formulae. The results are discussed in relation to other similar studies with rats and mice.     

Effect of directional selection on spontaneous male recombination in Drosophila ananassae.

Artificial selection was carried out for high and low spontaneous male recombination values in D. ananassae for nine generations by using cu b se marker (second chromosome) and wild stocks which were free from heterozygous chromosome inversions. The mean crossing-over frequency of nine generations was 2.22, 0.70 and 1.20% in high, low and control lines respectively. The values of regression coefficient and realized heritability also indicated that male recombination was affected by selection. However, response to selection was more pronounced in high line as compared to low line. This provides evidence that spontaneous male crossing-over in D. ananassae is under polygenic control.     

Environmental sensitivity differs between rabbit lines selected for reproductive intensity and longevity

To better understand the mechanisms that allow some animals to sustain their productive effort in harsh environmental conditions, rabbit does from two selection lines (LP and V) were housed in normal (NC), nutritional (NF) or heat (HC) challenging environmental conditions from first to third partum. The LP line (n=85) was founded on reproductive longevity criteria by selecting does from commercial farms that had a minimum of 25 partum with more than 7.5 kits born alive per parity. Line V (n=79) was constituted from four specialised maternal lines into a composite synthetic line and then selected by litter size at weaning for 36 generations. Female rabbits in NC and NF environments were housed at normal room temperature (18°C to 24°C) and fed with control [11.6 MJ digestible energy (DE)/kg dry matter (DM)] or low-energy diets (9.1 MJ DE/kg DM). HC does were housed at high room temperatures (25°C to 35°C) and fed the control diet. Female rabbits in the HC and NF environments ingested 11.5% and 6% less DE than NC does, respectively (P<0.05). These differences between environments occurred in both lines, with the differences being higher for LP than for V does (+6%; P<0.05). Milk yield responses followed those of energy intake also being higher for LP does (+21.3 g/day; P<0.05). The environmental conditions did not affect the perirenal fat thickness (PFT), but a genotype by environment interaction was observed. In NC and HC, the PFT was higher for line V (+0.23 and +0.35 mm, respectively; P<0.05) than for LP does, but this was not the case at NF (−0.01 mm). Moreover, the PFT evolution was different between them. In the NC environment, LP does used the accreted PFT in late lactation (−0.29 mm), whereas V does did not (−0.08 mm). Conversely, in the HC environment, LP does showed a flat PFT evolution in late lactation, whereas V does accumulated PFT. In the NF environment, LP and V does had a similar PFT evolution. There was also a litter size reduction for V does of −2.59 kits total born in HC and −1.78 kits total born in NF environments, whereas this was not observed for LP does. The results for LP does indicate a direct use of DE ingested for reproduction with little PFT change, whereas V does actively use the PFT reserves for reproduction.     

Genomic convergence toward diploidy in Saccharomyces cerevisiae

Genome size, a fundamental aspect of any organism, is subject to a variety of mutational and selection pressures. We investigated genome size evolution in haploid, diploid, and tetraploid initially isogenic lines of the yeast Saccharomyces cerevisiae. Over the course of ~1,800 generations of mitotic division, we observed convergence toward diploid DNA content in all replicate lines. This convergence was observed in both unstressful and stressful environments, although the rate of convergence was dependent on initial ploidy and evolutionary environment. Comparative genomic hybridization with microarrays revealed nearly euploid DNA content by the end of the experiment. As the vegetative life cycle of S. cerevisiae is predominantly diploid, this experiment provides evidence that genome size evolution is constrained, with selection favouring the genomic content typical of the yeast's evolutionary past.     

Developmental Stability of DROSOPHILA MELANOGASTER under Artificial and Natural Selection in Constant and Fluctuating Environments

Populations of Drosophila melanogaster in constant 25° and fluctuating 20/29° environments showed increases in developmental stability, indicated by decreases in bilateral asymmetry of sterno-pleural chaeta number. In both environments, rates of decrease in asymmetry were greater under natural selection (control lines) than under artificial stabilizing selection. Overall mean asymmetry was greater in the fluctuating environment.—There was no evidence that decreased asymmetry was due to heterozygosity, and the decline in asymmetry was not explained by the decline in chaeta number in the lines under only natural selection. However, the decline was consistent with changes in total phenotypic variance and environmental variance.—The divergence between lines after 39 generations of selection was seen in differences in asymmetry and also in the genotype-environment interaction expressed in cross-culturing experiments.     

Experimental adaptive radiation in Pseudomonas

We studied the importance of selection and constraint in determining the limits of adaptive radiation and the consequences of adaptive radiation in an experimental system. We propagated four replicate lines of the bacterium Pseudomonas fluorescens derived from a single ancestral clone in 95 environments, where growth was limited by the availability of a single carbon source for 1,000 generations. We then assayed the growth of the ancestral clone and the evolved lines in all 95 environments. Evolved lines increased their performance in almost every selection environment and invaded 70% of the novel environments as a direct response to selection. Direct responses tended to be larger in environments where growth was initially poor. Although evolved lines lost the ability to grow on about three substrates that their ancestor could readily grow on, the correlated response to selection was, on average, positive. The correlated response allowed all of our evolved populations to expand their niches and to occupy collectively the remaining novel habitats. This is inconsistent with classical theories of niche evolution. In the most extreme cases, adaptation occurred through "roundabout selection": lineages became adapted to an environment through selection in another environment but not through selection in the environment itself. Our results indicate that mutation accumulation by neutral drift was responsible for generating the majority of costs of adaptation.     

Fitness costs of chemical defense in Plantago lanceolata L.: effects of nutrient and competition stress

Fitness costs of defense are often invoked to explain the maintenance of genetic variation in levels of chemical defense compounds in natural plant populations. We investigated fitness costs of iridoid glycosides (IGs), terpenoid compounds that strongly deter generalist insect herbivores, in ribwort plantain (Plantago lanceolata L.) using lines that had been artificially selected for high and low leaf IG concentrations for four generations. Twelve maternal half-sib families from each selection line were grown in four environments, consisting of two nutrient and two competition treatments. We tested whether: (1) in the absence of herbivores and pathogens, plants from lines selected for high IG levels have a lower fitness than plants selected for low IG levels; and (2) costs of chemical defense increase with environmental stress. Vegetative biomass did not differ between selection lines, but plants selected for high IG levels produced fewer inflorescences and had a significantly lower reproductive dry weight than plants selected for low IG levels, indicating a fitness cost of IG production. Line-by-nutrient and line-by-competition interactions were not significant for any of the fitness-related traits. Hence, there was no evidence that fitness costs increased with environmental stress. Two factors may have contributed to the absence of higher costs under environmental stress. First, IGs are carbon-based chemicals. Under nutrient limitation, the relative carbon excess may result in the production of IGs without imposing a further constraint on growth and reproduction. Second, correlated responses to selection on IG levels indicate the existence of a positive genetic association between IG level and cotyledon size. At low nutrient level, a path analysis based on family means revealed that in the presence of competitors, the negative direct effect of a high IG level on aboveground plant dry weight was partly offset by a positive direct effect of the associated larger cotyledon size. This indicates that fitness costs of defense may be modulated by environment-specific fitness effects of genetically associated traits.     

Environmentally Induced Heritable Changes in Flax

Some flax varieties respond to nutrient stress by modifying their genome and these modifications can be inherited through many generations. Also associated with these genomic changes are heritable phenotypic variations ^1,2. The flax variety Stormont Cirrus (Pl) when grown under three different nutrient conditions can either remain inducible (under the control conditions), or become stably modified to either the large or small genotroph by growth under high or low nutrient conditions respectively. The lines resulting from the initial growth under each of these conditions appear to grow better when grown under the same conditions in subsequent generations, notably the Pl line grows best under the control treatment indicating that the plants growing under both the high and low nutrients are under stress. One of the genomic changes that are associated with the induction of heritable changes is the appearance of an insertion element (LIS-1) ^{3, 4} while the plants are growing under the nutrient stress. With respect to this insertion event, the flax variety Stormont Cirrus (Pl) when grown under three different nutrient conditions can either remain unchanged (under the control conditions), have the insertion appear in all the plants (under low nutrients) and have this transmitted to the next generation, or have the insertion (or parts of it) appear but not be transmitted through generations (under high nutrients) ⁴. The frequency of the appearance of this insertion indicates that it is under positive selection, which is also consistent with the growth response in subsequent generations. Leaves or meristems harvested at various stages of growth are used for DNA and RNA isolation. The RNA is used to identify variation in expression associated with the various growth environments and/or t he presence/absence of LIS-1. The isolated DNA is used to identify those plants in which the insertion has occurred.     

Genetic and environmental components of chiasma control

A disruptive selection programme for high and low chiasma frequency over four generations has produced two lines of Schistocerca gregaria with mean values of 21.6 and 18.0, respectively. The mean of the base population was 20.4. Realised heritability estimates for both the high and low lines were 27.4% and 48.8% respectively; only the latter value is significant. The non-significant value in the high line has resulted either from unidirectional dominance or because the genes responsible for high chiasma values themselves undergo a higher recombination rate in high frequency individuals and thus reduce the response to selection. — It is suggested that the genes governing chiasma frequency can be regarded as mainly neutral due to the large additive genetic component, at least in low chiasma frequency types. Accordingly, chiasma frequency variation and its adaptive significance may not be such an important component of fitness as has been proposed in the past.     

Reproductive robustness differs between generalist and specialist maternal rabbit lines: the role of acquisition and allocation of resources

Background

Farm animals are normally selected under highly controlled, non-limiting conditions to favour the expression of their genetic potential. Selection strategies can also focus on a single trait to favour the most ‘specialized’ animals. Theoretically, if the environment provides enough resources, the selection strategy should not lead to changes in the interactions between life functions such as reproduction and survival. However, highly ‘specialized’ farm animals can be required for breeding under conditions that differ largely from selection conditions. The consequence is a degraded ability of ‘specialized’ animals to sustain reproduction, production and health, which leads to a reduced lifespan. This study was designed to address this issue using maternal rabbit lines. A highly specialized line with respect to numerical productivity at weaning (called V) and a generalist line that originated from females with a long reproductive life (called LP) were used to study the strategies that these lines develop to acquire and use the available resources when housed in different environments. In addition, two generations of line V, generations 16 and 36, were available simultaneously, which contributed to better understand how selection criteria applied in a specific environment changed the interplay between functions related to reproduction and survival.

Results

We show that, under constrained conditions, line LP has a greater capacity for resource acquisition than line V, which prevents excessive mobilization of body reserves. However, 20 generations of selection for litter size at weaning did not lead to an increased capacity of nutrient (or resource) acquisition. For the two generations of line V, the partitioning of resources between milk production, body reserves preservation or repletion or foetal growth differed.

Conclusions

Combining foundational and selection criteria with a specific selection environment resulted in female rabbits that had a different capacity to deal with environmental constraints. An increased robustness was considered as an emergent property of combining a multiple trait foundational criterion with a wide range of environmental conditions. Since such a strategy was successful to increase the robustness of female rabbits without impairing their productivity, there is no reason that it should not be applied in other livestock species.     

Evolution in stressful environments. I. Phenotypic variability, phenotypic selection, and response to selection in five distinct environmental stresses

Considerable debate has accompanied efforts to integrate the selective impacts of environmental stresses into models of life-history evolution. This study was designed to determine if different environmental stresses have consistent phenotypic effects on life-history characters and whether selection under different stresses leads to consistent evolutionary responses. We created lineages of a wild mustard (Sinapis arvensis) that were selected for three generations under five stress regimes (high boron, high salt, low light, low water, or low nutrients) or under near-optimal conditions (control). Full-sibling families from the six selection histories were divided among the same six experimental treatments. In that test generation, lifetime plant fecundity and six phenotypic traits were measured for each plant. Throughout this greenhouse study, plants were grown individually and stresses were applied from the early seedling stage through senescence. Although all stresses consistently reduced lifetime fecundity and most size- and growth-related traits, different stresses had contrasting effects on flowering time. On average, stress delayed flowering compared to favorable conditions, although plants experiencing low nutrient stress flowered earliest and those experiencing low light flowered latest. Contrary to expectations of Grime's triangle model of life-history evolution, this ruderal species does not respond phenotypically to poor environments by flowering earlier. Most stresses enhanced the evolutionary potential of the study population. Compared with near-optimal conditions, stresses tended to increase the opportunity for selection as well as phenotypic variance, although both of these quantities were reduced in some stresses. Rather than favoring traits characteristic of stress tolerance, such as slow growth and delayed reproduction, phenotypic selection favored stress-avoidance traits: earlier flowering in all five stress regimes and faster seedling height growth in three stresses. Phenotypic correlations reinforced direct selection on these traits under stress, leading to predicted phenotypic change under stress, but no significant selection in the control environment. As a result of these factors, selection under stress resulted in an evolutionary shift toward earlier flowering. Environmental stresses may drive populations of ruderal plant species like S. arvensis toward a stress-avoidance strategy, rather than toward stress tolerance. Further studies will be needed to determine when selection in stressful environments leads to these alternative life-history strategies.     

Do mites evolving in alternating host plants adapt to host switch?

A fluctuating environment may be perceived as a composition of different environments, or as an environment per se, in which it is the fluctuation itself that poses a selection pressure. If so, then organisms may adapt to this alternation. We tested this using experimental populations of spider mites that have been evolving for 45 generations in a homogeneous environment (pepper or tomato plants), or in a heterogeneous environment composed of an alternation of these two plants approximately at each generation. The performance (daily oviposition rate and juvenile survival) of individuals from these populations was tested in each of the homogeneous environments, and in two alternating environments, one every 3 days and the other between generations. To discriminate between potential genetic interactions between alleles conferring adaptation to each host plant and environmental effects of evolving in a fluctuating environment, we compared the performance of all lines with that of a cross between tomato and pepper lines. As a control, two lines within each selection regime were also crossed. We found that crosses between alternating lines and between pepper and tomato lines performed worse than crosses between lines evolving in homogeneous environments when tested in that environment. In contrast, alternating lines performed either better or similarly to lines evolving in homogeneous environments when tested in a fluctuating environment. Our results suggest that fluctuating environments are more than the juxtaposition of two environments. Hence, tests for adaptation of organisms evolving in such environments should be carried out in fluctuating conditions.     

Prolonged negative selection of Drosophila melanogaster for a character of adaptive significance disturbs stress reactivity.

The metabolism of juvenile hormone by JH-esterase and JH-epoxide hydrolase, and octopamine by tyrosine decarboxylase were studied under normal and stress conditions in flies of two related lines of D. melanogaster. One was selected for high (HA line) and another for low (LA line) male sexual activity for more than 700 generations. It was demonstrated that prolonged selection for low male sexual activity results in considerable changes in both systems. Tyrosine decarboxylase activity in males and females of the LA line was sharply reduced as compared with those of the HA and control Canton-S lines; JH-esterase and JH-epoxide hydrolase activities were decreased in females, and not in males, of the LA line. It was demonstrated that the response of both metabolic systems to heat stress is impaired in individuals of the LA line: the system of juvenile hormone metabolism does not respond to stress, and that of octopamine metabolism is decelerated. The role of juvenile hormone metabolism in male reproductive function is discussed.     

Responses to divergent selection for plasma concentrations of insulin-like growth factor-1 in mice

A divergent selection experiment with mice, using plasma concentrations of insulin-like growth factor-1 (IGF-1) at 42 days of age as the selection criterion, was undertaken for 7 generations. Lines were not replicated. To obtain sufficient plasma for the IGF-1 assay, blood from four individuals was volumetrically bulked to obtain a litter mean IGF-1 concentration. This necessitated the use of between family selection. Although inbreeding accumulated in a linear fashion in each of the high, control and low lines, the rates were different for each line (3.6, 1.6 and 5.3% per generation for the high, control and low lines, respectively). As a consequence, the effects of selection and inbreeding are confounded in this experiment. Divergence between the high and low lines in plasma concentrations of IGF-1 continued steadily until generation 5. In generations 6 and 7, there was a reduced degree of divergence and this contributed towards the low realized heritability value of 0.15 +/- 0.12. Six-week liveweight showed a steady positive correlated response to selection for or against plasma concentrations of IGF-1 until generation 4 (high-low difference = 1.7 g = 12%). In generation 5, a substantial drop in 6-week liveweight in the low line relative to both the high and control lines occurred (high-low difference, 3.9; g, 25%). This difference was maintained until generation 7.(ABSTRACT TRUNCATED AT 250 WORDS)     

Response to Divergent Selection for Nesting Behavior in MUS MUSCULUS

Replicated bidirectional selection (with control lines) for nest-building behavior in Mus musculus, where nesting scores consisted of the total weight of cotton pulled through the cage lid during four days of testing, yielded an eight-fold difference between high and low lines after 15 generations of selection. The overall realized heritability pooled across lines and replicates was 0.18 ± 0.02 (0.15 ± 0.03 for high nesting scores and 0.23 ± 0.04 for low nesting scores), or 0.28 ± 0.05 when adjusted for within-family selection. Across the 15 generations and the entire experiment, average body weight and number of infertile matings increased, while average litter size decreased, although these changes were not consistent across lines. Inbreeding could account for average decreases in the fertility traits, but there was also a correlated response to selection, since both high lines showed increased litter size and decreased infertile matings.     

Response to Selection at Two Temperatures for Fast and Slow Growth from Five to Nine Weeks of Age in Poultry

Cornell Control White Leghorn chicks were grown in a common environment to five weeks of age and selected for fast and slow gain in body weight from five to nine weeks of age at two temperatures, 21.1° (cold) and 32.2° (hot), during which time a constant 50% relative humidity was maintained. All lines were tested each generation in both temperature environments. Selection continued for four generations, with a second replicate started six weeks after the first replicate in each generation. In the hot environment, a 20% reduction (104 g) in five-to-nine-week weight gain was found. The responses to selection for fast and slow growth were symmetrical except in the first generation, when an outbreak of bronchitis confounded selection for body weight with selection for disease resistance and allowed little gain in the slow lines. No genotype-by-environment interactions were found, indicating that selection in either direction in either selection temperature produced equal responses in either test temperature. This suggests that any interactions observed between the growth of strains in tropical vs. temperate climates must be due to some difference between these environments other than the temperature differences studied.     

Additive, dominant, and epistatic effects for maize grain yield in acid and non-acid soils

Acid soils severely reduce maize (Zea mays L.) yield in the tropics. Breeding for tolerance to soil acidity provides a permanent, environmentally friendly, and inexpensive solution to the problem. This study was carried out to determine the relative importance of additive, dominant, and epistatic effects on maize grain-yields in different tropical genotypes. Divergent selection in three populations (SA4, SA5, and SA7) provided inbred lines tolerant or sensitive to acid soils. The tolerant and sensitive lines from each population were used to obtain the F₁, F₂, F₃, back-crosses, second back-crosses, and selfed back-cross generations. In addition, the tolerant lines from SA4 and SA5 were crossed with a sensitive line from the Tuxpeño Sequía population, from which the same generations were also derived. All generations from each of the five sets of crosses were evaluated in three acid-soil environments and one non-acid-soil environment. A generation-mean analysis was performed on each set for yield. The sequential sum of squares associated with additive, dominance, and digenic epistatic effects were used to estimate the relative importance of each genetic effect. Epistasis was not important in any set in the non-acid-soil environment, with dominance accounting for 80.76% of the total variation among generation means across sets. In acid-soil environments, epistasis was more important. The relative importance of digenic epistasis was greater in those evaluations with large experimental errors. The tolerant line from population SA5 was prone to severe root lodging, suggesting a very poor root system. Apparently, the tolerance to soil acidity in this line is not associated with a large root system.     

A selection strategy to accommodate genotype-by-environment interaction for grain yield of wheat: managed-environments for selection among genotypes

Selection for grain yield among wheat lines is complicated by large line-by-environment (L × E) interactions in Queensland, Australia. Early generation selection is based on an evaluation of many lines in a few environments. The small sample of environments, together with the large L × E interaction, reduces the realised response to selection. Definition of a series of managed-environments which provides discrimination among lines, which is relevant to the target production-environments, and can be repeated over years, would facilitate early generation selection. Two series of managed-environments were conducted. Eighteen managed-environments were generated in Series-1 by manipulating nitrogen and water availability, together with the sowing date, at three locations. Nine managed-environments based on those from Series-1 were generated in Series-2. Line discrimination for grain yield in the managed-environments was compared to that in a series of 16 random production-environments. The genetic correlation between line discrimination in the managed-environments and that in the production-environments was influenced by the number and combination of managed-environments. Two managed-environment selection regimes, which gave a high genetic correlation in both Series-1 and 2, were identified. The first used three managed-environments, a high input (low water and nitrogen stress) environment with early sowing at three locations. The second used six managed-environments, a combination of a high input (low water and nitrogen stress) and medium input (water and nitrogen stress) with early sowing at three locations. The opportunities for using managed-environments to provide more reliable selection among lines in the Queensland wheat breeding programme and its potential limitations are discussed.