Biomedical subjects

Christopher Wills

Publications and source records attributed to Christopher Wills.

6 recordsLinked to original sources

PubMed2006

Nonrandom processes maintain diversity in tropical forests.

An ecological community's species diversity tends to erode through time as a result of stochastic extinction, competitive exclusion, and unstable host-enemy dynamics. This erosion of diversity can be prevented over the short term if recruits are highly diverse as a result of preferential recruitment of rare species or, alternatively, if rare species survive preferentially, which increases diversity as the ages of the individuals increase. Here, we present census data from seven New and Old World tropical forest dynamics plots that all show the latter pattern. Within local areas, the trees that survived were as a group more diverse than those that were recruited or those that died. The larger (and therefore on average older) survivors were more diverse within local areas than the smaller survivors. When species were rare in a local area, they had a higher survival rate than when they were common, resulting in enrichment for rare species and increasing diversity with age and size class in these complex ecosystems.

Biodiversity↗

PubMed2006

Testing metabolic ecology theory for allometric scaling of tree size, growth and mortality in tropical forests.

The theory of metabolic ecology predicts specific relationships among tree stem diameter, biomass, height, growth and mortality. As demographic rates are important to estimates of carbon fluxes in forests, this theory might offer important insights into the global carbon budget, and deserves careful assessment. We assembled data from 10 old-growth tropical forests encompassing censuses of 367 ha and > 1.7 million trees to test the theory's predictions. We also developed a set of alternative predictions that retained some assumptions of metabolic ecology while also considering how availability of a key limiting resource, light, changes with tree size. Our results show that there are no universal scaling relationships of growth or mortality with size among trees in tropical forests. Observed patterns were consistent with our alternative model in the one site where we had the data necessary to evaluate it, and were inconsistent with the predictions of metabolic ecology in all forests.

Biometry↗

PubMed2006

Comparing tropical forest tree size distributions with the predictions of metabolic ecology and equilibrium models.

Tropical forests vary substantially in the densities of trees of different sizes and thus in above-ground biomass and carbon stores. However, these tree size distributions show fundamental similarities suggestive of underlying general principles. The theory of metabolic ecology predicts that tree abundances will scale as the -2 power of diameter. Demographic equilibrium theory explains tree abundances in terms of the scaling of growth and mortality. We use demographic equilibrium theory to derive analytic predictions for tree size distributions corresponding to different growth and mortality functions. We test both sets of predictions using data from 14 large-scale tropical forest plots encompassing censuses of 473 ha and > 2 million trees. The data are uniformly inconsistent with the predictions of metabolic ecology. In most forests, size distributions are much closer to the predictions of demographic equilibrium, and thus, intersite variation in size distributions is explained partly by intersite variation in growth and mortality.

Biomass↗

PubMed2004

Molecular analysis of adenovirus isolates from vaccinated and unvaccinated young adults.

Infections of adenovirus type 4 (Ad4) and Ad7 were discovered among previously vaccinated individuals through febrile respiratory illness surveillance at military recruit camps. Genetic analysis was performed on these isolates and a sample of adenovirus isolates from unvaccinated patients. Antigenic regions of the adenovirus hexon gene from 21 vaccinated and 31 unvaccinated patients were sequenced and compared to homologous regions of Ad4 and Ad7 vaccine strains and of other representative hexon sequences archived in GenBank. The phylogenetic distribution of sequences from vaccinated individuals closely resembled those from unvaccinated individuals. The most common Ad7 strain was the Ad7d2 hexon genotype, and the most common Ad4 strain was a genotype nearly identical to the recently discovered Z-G 95-873 Ad4 variant. Near exclusive isolation of Ad4 since 1999 indicates that the Ad4 variant is currently responsible for the vast majority of adenovirus morbidity in military recruit camps. Different ratios of nonsynonymous to synonymous nucleotide substitution rates in known antigenic regions compared to nonantigenic regions indicated positive selection for diversity in the antigenic regions and purifying selection in the nonantigenic regions.

Adenovirus Infections, Human↗

PubMed2002

Domain-level differences in microsatellite distribution and content result from different relative rates of insertion and deletion mutations.

Microsatellites (short tandem polynucleotide repeats) are found throughout eukaryotic genomes at frequencies many orders of magnitude higher than the frequencies predicted to occur by chance. Most of these microsatellites appear to have evolved in a generally neutral manner. In contrast, microsatellites are generally absent from bacterial genomes except in locations where they provide adaptive functional variability, and these appear to have evolved under selection. We demonstrate a mutational bias towards deletion (repeat contraction) in a native chromosomal microsatellite of the bacterium Mycoplasma gallisepticum, through the collection and analysis of independent mutations in the absence of natural selection. Using this and similar existing data from two other bacterial species and four eukaryotic species, we find strong evidence that deletion biases resulting in repeat contraction are common in bacteria, while eukaryotic microsatellites generally experience unbiased mutation or a bias towards insertion (repeat expansion). This difference in mutational bias suggests that eukaryotic microsatellites should generally expand wherever selection does not exclude them, whereas bacterial microsatellites should be driven to extinction by mutational pressure wherever they are not maintained by selection. This is consistent with observed bacterial and eukaryotic microsatellite distributions. Hence, mutational biases that differ between eukaryotes and bacteria can account for many of the observed differences in microsatellite DNA content and distribution found in these two groups of organisms.

Chromosome Deletion↗

PubMed2002

Fatal Saccharomyces cerevisiae aortic graft infection.

Saccharomyces cerevisiae is a yeast commonly used in baking and a frequent colonizer of human mucosal surfaces. It is considered relatively nonpathogenic in immunocompetent adults (J. N. Aucott, J. Fayan, H. Grossnicklas, A. Morrissey, M. M. Lederman, and R. A. Salata, Rev. Infect. Dis. 12:406-411, 1990). We present a case of S. cerevisiae fungemia and aortic graft infection in an immunocompetent adult. This is the first reported case of S. cerevisiae fungemia where the identity of the pathogen was confirmed by rRNA sequencing.

Aorta, Abdominal↗