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J Bergelson

Publications and source records attributed to J Bergelson.

24 records · Page 2Linked to original sources
PubMed1998

Genetic variation within and among populations of Arabidopsis thaliana.

We investigated levels of nucleotide polymorphism within and among populations of the highly self-fertilizing Brassicaceous species, Arabidopsis thaliana. Four-cutter RFLP data were collected at one mitochondrial and three nuclear loci from 115 isolines representing 11 worldwide population collections, as well as from seven commonly used ecotypes. The collections include multiple populations from North America and Eurasia, as well as two pairs of collections from locally proximate sites, and thus allow a hierarchical geographic analysis of polymorphism. We found no variation at the mitochondrial locus Nad5 and very low levels of intrapopulation nucleotide diversity at Adh, Dhs1, and Gpa1. Interpopulation nucleotide diversity was also consistently low among the loci, averaging 0.0014. gst, a measure of population differentiation, was estimated to be 0.643. Interestingly, we found no association between geographical distance between populations and genetic distance. Most haplotypes have a worldwide distribution, suggesting a recent expansion of the species or long-distance gene flow. The low level of polymorphism found in this study is consistent with theoretical models of neutral mutations and background selection in highly self-fertilizing species.

Arabidopsis↗
PubMed1997

Fitness consequences of genetically engineered herbicide and antibiotic resistance in Arabidopsis thaliana.

Researchers have often invoked the concept of metabolic drain to explain the lower growth rates of bacteria containing plasmids that confer antibiotic resistance. This idea posits that the energetic input needed to produce detoxifying enzymes diverts resources from clonal reproduction. In this paper we examine whether the concept of metabolic drain can be applied successfully to plants that differ from bacteria in several key aspects including their relative genome size and reproductive rate. We have conducted a field experiment using mutant and transgenic Arabidopsis thaliana that allows the comparison of genotypes differing by a single gene conferring resistance to either the herbicide chlorsulfuron or the antibiotic kanamycin. In addition to testing whether these traits reduce fitness, this experiment was conducted at two levels of resource availability to examine whether costs of resistance are sensitive to environmental quality. We found that herbicide-resistant individuals produced 26% fewer seeds than susceptible counterparts. However, contrasting published results in bacterial systems, the fecundity of individuals was completely unaffected by the expression of an introduced antibiotic resistance gene. The fitness cost associated with chlorsulfuron resistance was greater in nutrient-poor conditions relative to nutrient-rich conditions for comparisons involving mutant, but not transgenic, genotypes.

Arabidopsis↗
PubMed1996

Costs of resistance: a test using transgenic Arabidopsis thaliana.

Evolutionary biologists have long attributed polymorphisms in resistance status to fitness costs of resistance traits. Nevertheless, pleiotropic fitness costs of resistance have been notoriously difficult to detect. We have transformed Arabidopsis thaliana with a mutant acetolactate synthase gene that confers resistance to the herbicide, chlorsulfuron. Our experiment revealed a 34% reduction in the lifetime seed production of transgenic, herbicide resistant Arabidopsis thaliana relative to their susceptible null segregants. Our experimental design allows us to conclude that this fitness cost of resistance is caused by the pleiotropic effect of the introduced acetolactate synthase gene rather than other potential costs associated with the plasmid or mutational changes induced by plant transformation. In addition, we can attribute the cost of resistance to the presence of the resistance gene rather than an increase in gene dosages. The implications of these results for the risk assessment of transgenic crops are discussed.

Acetolactate Synthase↗
PubMed1990

Statistical genetics of an annual plant, Impatiens capensis. I. Genetic basis of quantitative variation.

Analysis of quantitative genetics in natural populations has been hindered by computational and methodological problems in statistical analysis. We developed and validated a jackknife procedure to test for existence of broad sense heritabilities and dominance or maternal effects influencing quantitative characters in Impatiens capensis. Early life cycle characters showed evidence of dominance and/or maternal effects, while later characters exhibited predominantly environmental variation. Monte Carlo simulations demonstrate that these jackknife tests of variance components are extremely robust to heterogeneous error variances. Statistical methods from human genetics provide evidence for either a major locus influencing germination date, or genes that affect phenotypic variability per se. We urge explicit consideration of statistical behavior of estimation and testing procedures for proper biological interpretation of statistical results.

Genetic Variation↗
PubMed1990

Statistical genetics of an annual plant, Impatiens capensis. II. Natural selection.

Measurement of natural selection on correlated characters provides valuable information on fitness surfaces, patterns of directional, stabilizing, or disruptive selection, mechanisms of fitness variation operating in nature, and possible spatial variation in selective pressures. We examined effects of seed weight, germination date, plant size, early growth, and late growth on individual fitness. Path analysis showed that most characters had direct or indirect effects on individual fitness, indicating directional selection. For most early life-cycle characters, indirect effects via later characters exceed the direct causal effect on fitness. Selection gradients were uniform across the experimental site. There was no evidence for stabilizing or disruptive selection. We discuss several definitions of stabilizing and disruptive selection. Although early events in the life of an individual have important causal effects on subsequent characters and fitness, there is no detectable genetic variance for most of these characters, so little or no genetic response to natural selection is expected.

Least-Squares Analysis↗