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Biomedical subjects

J D Bever

Publications and source records attributed to J D Bever.

6 recordsLinked to original sources
PubMed2001

Coexistence under positive frequency dependence.

Negative frequency dependence resulting from interspecific interactions is considered a driving force in allowing the coexistence of competitors. While interactions between species and genotypes can also result in positive frequency dependence, positive frequency dependence has usually been credited with hastening the extinction of rare types and is not thought to contribute to coexistence. In the present paper, we develop a stochastic cellular automata model that allows us to vary the scale of frequency dependence and the scale of dispersal. The results of this model indicate that positive frequency dependence will allow the coexistence of two species at a greater rate than would be expected from chance. This coexistence arises from the generation of banding patterns that will be stable over long time-periods. As a result, we found that positive frequency-dependent interactions over local spatial scales promote coexistence over neutral interactions. This result was robust to variation in boundary conditions within the simulation and to variation in levels of disturbance. Under all conditions, coexistence is enhanced as the strength of positive frequency-dependent interactions is increased.

Animals↗
PubMed2000

Evolution of nitrogen fixation in spatially structured populations of Rhizobium.

Symbiosis between legumes and nitrogen-fixing bacteria is thought to bring mutual benefit to each participant. However, it is not known how rhizobia benefit from nodulation of legume hosts because they fix nitrogen only after differentiating into bacteroids, terminally differentiated cells that cannot reproduce. Because free-living rhizobia can reproduce, and may benefit from the increase of plant root exudates stimulated by nodulation, evolution of symbiotic nitrogen fixation may depend upon kin selection. However, unrelated nonmutualists may also benefit from increased plant exudates and nitrogen-fixing populations are therefore vulnerable to invasion by nonfixing, saprophytic Rhizobium. The access of nonfixing Rhizobium to the plant exudates associated with nodules depends upon the spatial structure of the Rhizobium populations within the soil. We investigate the influence of spatial structure on the evolution of N-fixation within a Rhizobium population using a mathematical model. Our model demonstrates that spatial structure is necessary for the evolution of N-fixation and that N-fixation is more likely to evolve with increasing degrees of spatial structure. In fact, we identify three dynamic outcomes that depend upon the relative strength of the costs of N-fixation relative to the degree of spatial structure and benefits resulting from nodulations. If the costs are relatively high, N-fixation will not evolve; if the costs are relatively low, N-fixing genes will fix in the population, but at intermediate conditions, a stable mixture of N-fixing bacteria and nonfixing bacteria will be maintained. The conditions for coexistence of N-fixing bacteria and nonfixing bacteria expand under a saturating relationship between nodule numbers and N-fixing genotype frequency.

Biological Evolution↗
PubMed2000

Genetic variation and evolutionary trade-offs for sexual and asexual reproductive modes in Allium vineale (Liliaceae).

Populations of Allium vineale commonly include individuals with very different allocation patterns to three modes of reproduction: sexual flowers, aerially produced asexual bulbils, and belowground asexual offsets. If selection is currently acting to maintain these different allocation patterns there must be a genetic basis for variation in allocation to these three reproductive modes. In addition, negative genetic correlations between reproductive traits would imply evolutionary trade-offs among reproductive strategies. We evaluated the heritability of these allocation patterns by growing 16 clones from a single population in the greenhouse at two levels of fertilization. Bulb mass and the mass and number of bulbils, offsets, and flowers were used as response variables, in addition to the proportion allocated to each reproductive mode. We found evidence of substantial heritable variation in allocation to sexual reproduction and in allocation within the two modes of asexual reproduction, indicating high sensitivity of these allocation patterns to natural selection. We also found evidence of strong negative genetic correlations between bulbil and flower traits, as well as between bulbil and offset traits, with one group of genotypes allocating greater resources to aerial asexual bulbils and the second group allocating more resources to belowground asexual offsets and aerial flowers. Phenotypic plasticity in allocation to above- vs. belowground asexual reproduction and sexual vs. asexual aerial reproduction was limited, indicating that plants are unlikely to change reproductive mode in response to nutrient availability. Together, then, we have demonstrated strong heritability for, and trade-offs in, the reproductive allocation patterns within this plant population.

Journal Article↗
PubMed1999

Mycorrhizal status of the genus Carex (Cyperaceae).

The Cyperaceae have generally been considered nonmycorrhizal, although recent evidence suggests that mycotrophy may be considerably more widespread among sedges than was previously realized. This study surveyed 23 species of Carex occurring in upland and wetland habitats in northeastern Illinois. Mycorrhizal infection by arbuscular fungi was found in the roots of 16 species of Carex and appears to occur in response to many factors, both environmental and phylogenetic. While some species appear to be obligately nonmycorrhizal, edaphic influences may be responsible for infection in others. In five of the seven Carex species that were nonmycorrhizal, a novel root character, the presence of bulbous-based root hairs, was identified. The taxonomically patchy distribution of the distinctive root hair trait suggests that these structures may have evolved several times within the genus. Evidence of multiple independent origins of the root hair trait lends support to the hypothesis that root hairs represent an adaptation to nonmycotrophy. Although taxonomic position does seem to be of importance in determining the mycorrhizal dependence of sedges, the pattern may be a patchwork of both mycorrhizal clades and clades that have adapted to the nonmycorrhizal state.

Journal Article↗
PubMed1999

Heritable variation and mechanisms of inheritance of spore shape within a population of Scutellospora pellucida, an arbuscular mycorrhizal fungus.

Substantial variation was found among single-spore cultures established from a single population of the arbuscular mycorrhizal fungus Scutellospora pellucida. A common environment experiment demonstrated that five single-spore cultures differed in their average spore shape (as measured by length:width ratios) and size (volume) with interisolate heritabilities of offspring mean values of 0.96 and 0.87, respectively (0.66 and 0.43 for the shape and size of individual spores). The distribution of offspring spore shapes also differed in levels of variance, skewness, and kurtosis. In fact, these aspects of the distributions shifted with mean spore shape as predicted by the binomial distribution-the distribution expected due to the segregation of genetically diverse nuclei through dividing hyphae. Thus, the original parental spores generating these cultures appear to have contained genetically variable nuclei, which then segregate into the offspring spores to generate consistent differences in the mean, variance, skewness, and kurtosis of the distribution of offspring spore shapes. This nuclear segregation may be followed by the assemblage of novel combinations of nuclei through hyphal fusion. Together these processes are rarely considered mechanisms for the creation of novel genetic combinations and may contribute to the maintenance of the high level of heritable variation observed in this study.

Journal Article↗