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Results for “artificial selection”

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PubMed2004

The genetic architecture of response to long-term artificial selection for oil concentration in the maize kernel.

In one of the longest-running experiments in biology, researchers at the University of Illinois have selected for altered composition of the maize kernel since 1896. Here we use an association study to infer the genetic basis of dramatic changes that occurred in response to selection for changes in oil concentration. The study population was produced by a cross between the high- and low-selection lines at generation 70, followed by 10 generations of random mating and the derivation of 500 lines by selfing. These lines were genotyped for 488 genetic markers and the oil concentration was evaluated in replicated field trials. Three methods of analysis were tested in simulations for ability to detect quantitative trait loci (QTL). The most effective method was model selection in multiple regression. This method detected approximately 50 QTL accounting for approximately 50% of the genetic variance, suggesting that >50 QTL are involved. The QTL effect estimates are small and largely additive. About 20% of the QTL have negative effects (i.e., not predicted by the parental difference), which is consistent with hitchhiking and small population size during selection. The large number of QTL detected accounts for the smooth and sustained response to selection throughout the twentieth century.

Computer Simulation↗
PubMed2004

A resource allocation model describing consequences of artificial selection under metabolic stress.

Long-term selection on production results in increased environmental sensitivity. This often is expressed through decreased fertility and increased health problems. The phenomenon has been described in all common farm animal species. One theory is that potential resource intake is insufficient to express production potential. Additional resources are drawn away from fitness-related traits, such as fertility and health, to further increase observed production. In addition, resources for maintaining fitness depend on the demands by the environment. In a harsh environment, more resources are required for fitness-related traits than in an optimal environment. Literature results show that selection in an optimal environment will increase sensitivity to less optimal environments. The objectives of this paper were to increase understanding of the underlying mechanism behind the development of environmental sensitivity and to gain insight into correlated response(s) when selection is on observed production. A resource allocation model was defined where observed production depended on production potential, resource intake potential, and the allocation of resources to production or fitness, including maintenance, health, and reproduction. Penalties for reproductive performance and probability of survival were included when the proportion of resources assigned to fitness dropped below a certain, environment-related, threshold. Mass selection was practiced on observed production during 40 generations using stochastic simulation. Depending on the heritabilities of the underlying components and on the environment, selection on observed production resulted in a decrease in reproductive rate and in the development of environmental sensitivity when resource intake becomes limiting. Correlations of observed production with underlying components changed across generations, following a nonlinear pattern. The proposed model is simple, but increases the understanding of underlying mechanisms and consequences of selection for production when resources are limiting.

Animal Husbandry↗
PubMed1988

Comparison of pedigreed and nonpedigreed randombred control systems for use with artificial selection in the Japanese quail.

The performance characteristics of six replicate sublines of a randombred control, three pedigreed (P) and three nonpedigreed (NP), were studied to see if the NP mating system could be used as a method for maintaining a randombred control for selection experiments. Each subline was maintained with 32 single-pair matings, with 64 offspring placed each generation for each line. Brother X sister matings were avoided in the P but not in the NP system. The NP chicks were pedigreed in order to determine the exact numbers of brother X sister matings selected in that system, but pedigrees were not considered during selection. Records were collected on the numbers of matings producing offspring, the expected rate of inbreeding based on the effective population size, hatchability, egg production, egg weight, and the body weights of the males and females. Replicate lines in the P system had more matings that produced viable offspring, which resulted in larger effective population sizes, lower rates of inbreeding, and smaller average family sizes than those in the NP system. A number of significant differences were found between means of the replicate lines within system and between regressions of the traits measured in the individual lines on generation number. Most of these differences disappeared when the data from the three replicates within mating system were pooled and compared, suggesting that differences found between lines within system were due to random genetic drift due to restrictions in population size. It is suggested that the NP system can be used as an effective control procedure for Japanese quail breeding experiments, but that more than 32 mating pairs should be used for its maintenance in order to reduce the amount of genetic drift within the control population.

Animals↗
PubMed2001

Phylogenetic comparison and artificial selection. Two approaches in evolutionary physiology.

Interspecific comparison has a long and productive history in physiology. Conceptual and statistical advances over the last 15 years have demonstrated several ways in which comparisons can be enhanced by consideration of phylogenetic information, i.e., empirical estimates of the ways in which organisms are related (evolutionary trees). Choice of species to be compared should be informed by phylogenetic information. For example, a comparison of three species that inhabit high altitude with three that live at low altitude would be suspect if each of the two groups were composed of closely-related species (e.g., within single genera). To avoid such "phylogenetic pseudoreplication," one might instead study species from three different genera, each containing one high-altitude and one low-altitude inhabitant. Unfortunately, many studies have not been so carefully designed, sometimes because organisms were not accessible or because the studies incorporated data from the literature. Fortunately, several new statistical methods correct for problems caused by phylogenetic relatedness and descent with modification, the most common being phylogenetically independent contrasts. Another tool that can be used in comparative physiology is selective breeding, which has been practiced for millennia and applied in scientific contexts for over a century. In the last 20 years, ecological and evolutionary physiologists have begun using selection experiments to study processes of genetic adaptation in physiological and behavioral traits. For example, house mice have been maintained in the cold for multiple generations to see what adaptations may occur naturally in response to reduced ambient temperature ("laboratory natural selection"). Our own laboratory has used selective breeding to create four replicate lines of mice that exhibit high levels of voluntary wheel-running behavior, as well as various morphological and physiological characteristics that cause or allow the elevated locomotor activity. Similar experiments could be used to study adaptation to hypoxia.

Animals↗
PubMed1989

[Artificial selection for increased metastatic potential in cell population of transplanted rhabdomyosarcoma RA-2 in rats].

The rhabdomyosarcoma was induced with 20-methylcholanthrene in unbred white rat; it was selected for the metastatic potential (MP)--the ability to form experimental lung metastasis after i. v. injection of tumor cells. After 10-15 cycles of selection there was 100-fold increase in the level of MP, all the tumor cells acquired simultaneously the affinity to the lung tissue. After that the tempo of selection decreased. From 112-124 to 177-183 cycles of selection the frequencies of MP-cells in the population were increased only 10-fold (from 0.007 to 0.06). High level of karyotypic and phenotypic heterogeneity persisted in the selected cell population, the differences for MP between clones were great. It was shown the heritable instability of the character "the MP-level" in the selected cell population.

Animals↗
PubMed1985

The effect of section thickness and embedding media on the observed S-phase labelling index of artificially selected cell populations from neonatal mouse liver and spleen.

Following an intraperitoneal injection of tritiated thymidine to neonatal mice, livers and spleens were removed and their labelling indices were derived autoradiographically. This was done in a number of ways: (1) from tissue imprints on gelatinised glass slides; (2) from tissue embedded in JB4 plastic sectioned at thicknesses of 2, 5 and 7 micron; and (3) from tissue embedded in paraffin wax and sectioned at 7 micron. The results show that the indices from the JB4 embedded sections increase as the section thickness decreases, and that this relationship persists down to the notional section thickness of zero in the tissue imprints (in which all the cells are in contact with the autoradiographic emulsion). Indices from the 7 micron paraffin wax embedded sections are surprisingly close to the values from the imprints, are higher than indices from the 5 and 7 micron JB4 embedded sections, and are not significantly different (at the 2% level) from those from 2 micron JB4 embedded sections. Possible reasons for these results are discussed in respect of the autoradiographic process and in relationship to various mathematical correction factors which have been proposed to take account of beta-particle self-absorption in thick sections. It is concluded that none of these correction factors is of value and that the embedding medium has an important effect on the observed labelling indices. Comparisons between labelling indices, therefore, should be made only when they are derived from similarly embedded material at the same section thickness.

Animals↗
PubMed1983

Constitutional biases in early perceptual learning II. Visual preferences in artificially selected, visually naive and imprinted quail chicks (C. coturnix japonica).

Approach choices were tested in genetically and environmentally manipulated quail chicks with pairs of stimuli identical in size and luminance but different in color, flicker, or both color and flicker. Data indicated comparable flicker and vastly different color preferences in subjects that were bidirectionally selected for color choices. In the choices between composite stimuli, flicker effects dominated over color effects in genetic controls, and color effects over flicker effects in selected subjects. Imprinting to colors modified color preferences, but imprinting to white or colored flicker did not change, or only marginally changed, flicker preferences. Flicker in testing stimuli, however, influenced the phenotypic expression of acquired color preferences. The data are examined for implications about the nature of constitutional biases and constitution--environment interactions in early perception and perceptual learning.

Animals↗
PubMed2005

Processes that constrain and facilitate the evolution of sexual dimorphism.

Sexual dimorphism, or differences between the sexes, is pervasive in both plants and animals despite genetic and developmental constraints on its evolution. This special issue of the American Naturalist, which is based on the annual Vice Presidential Symposium, documents how the underlying processes responsible for the presence and extent of sexual dimorphism can be qualified and quantified by a variety of approaches. These include estimates of the G matrix and phenotypic selection, artificial selection, phenotypic manipulation of hormones, estimates of sex-differential sensitivity to maternal effects, among-population and phenotypic plasticity studies, and the mapping of sexual dimorphism onto a phylogeny. The questions addressed in the articles in this issue vary depending on the motivation for the studies and the taxa being investigated, but taken together, they show how the integration of genetic, developmental, physiological, ecological, and phylogenetic approaches can illuminate the processes underlying the evolution of sexual dimorphism.

Animals↗