This study concerns a seasonal succession of a large herbivore, Daphnia pulex, by a smaller one, Ceriodaphnia reticulata, in a Minnesota pond. Based on a description of events in the pond and results of several field experiments, one can explain this replacement in terms of a coexploitative interaction between these 2 species. The field experiments, in which pairs of cladocerans were isolated for 4 wk, gave several results which are inconsistent with the assumptions of contemporary competition theory. In the 1st experiment, Daphnia survived when solitary but went extinct when paired with Ceriodaphnia, regardless of starting densities or age structure. At the same, Daphnia was declining in the pond while Ceriodaphnia was increasing. Four wk later, Daphnia coexisted in pairs with both Ceriodaphnia and Bosmina longirostris, and at much greater densities than in its controls. Not only did the competitive abilities of these species shift over this very short time period, but the densities which they attained in control enclosures dramatically declined. Thus, static measures of competition (such as alphas) and carrying capacities are inappropriate for these species. Furthermore, these interactions can be understood only in the context of a framework which considers age structure. Generally, juveniles were more sensitive to coexploitative interactions while adults were unaffected and sometimes even benefited. Since their resources interact, coexploitative interactions between these species are not simply a result of resource depression but of resource alteration. Despite the fact that they overlap extensively in resource utilization, an appropriate modification of the resource base by 1 species can lead to enhanced conditions for a 2nd. I suggest that such facilitation may be an important process in these communities which merits further investigation.
Because of their large population sizes, short generation times, and clonal mode of propagation, microorganisms should often be the first members of a community to respond evolutionarily to temporal changes in the environment. Because the planktonic microbial community directly or indirectly influences all other members of aquatic ecosystems, it is useful to have a general theory for the magnitude and limits of such response. Models are presented for the expected dynamics of evolutionary change for the mean and variance of a quantitative character under natural selection toward a fixed or a moving optimum. It is also shown how the rate of population growth is related to the phenotypic composition of the population and the selective aspects of the environment. These models, which lead to the identification of extinction thresholds for the rate of environmental change beyond which a population cannot maintain itself, provide a heuristic basis for understanding the response of ecosystems to environmental perturbations. The analyses also indicate that clones of microorganisms isolated into novel laboratory environments are likely to undergo substantial evolutionary change over periods of a few hundred days, which raises questions about the utility of such cultures for inferring ecological properties of natural populations.
Because of the current low capital gains rates, many speculative investors are selling large parcels of undeveloped or partially developed real estate. The IRS has sought to tax such sales at the higher ordinary income rates. According to the IRS, if such sales are frequent or substantial, if the property has been improved too much by the seller, or if the seller is merely an "agent" of the buyer, then the IRS will deny capital gains treatment. Investor status benefits all taxpayers. Individual taxpayers are subject to a maximum 15% tax rate on capital gains resulting from the sale of property that has been held for a period of greater than one year. Taxpayers assume a degree of risk in claiming investor status because such related-party transactions receiving capital gains treatment are frequently contested by the IRS. Taxpayers wishing to minimize the risk of an audit may seek a private letter ruling from the IRS.
Gene duplication has generally been viewed as a necessary source of material for the origin of evolutionary novelties, but it is unclear how often gene duplicates arise and how frequently they evolve new functions. Observations from the genomic databases for several eukaryotic species suggest that duplicate genes arise at a very high rate, on average 0.01 per gene per million years. Most duplicated genes experience a brief period of relaxed selection early in their history, with a moderate fraction of them evolving in an effectively neutral manner during this period. However, the vast majority of gene duplicates are silenced within a few million years, with the few survivors subsequently experiencing strong purifying selection. Although duplicate genes may only rarely evolve new functions, the stochastic silencing of such genes may play a significant role in the passive origin of new species.
ABSTRACT The causes and consequences of spatiotemporal variation in mutation rates remains to be explored in nearly all organisms. Here we examine relationships between local mutation rates and replication timing in three bacterial species whose genomes have multiple chromosomes: Vibrio fischeri, Vibrio cholerae , and Burkholderia cenocepacia . Following five evolution experiments with these bacteria conducted in the near-absence of natural selection, the genomes of clones from each lineage were sequenced and analyzed to identify variation in mutation rates and spectra. In lineages lacking mismatch repair, base-substitution mutation rates vary in a mirrored wave-like pattern on opposing replichores of the large chromosome of V. fischeri and V. cholerae , where concurrently replicated regions experience similar base-substitution mutation rates. The base-substitution mutation rates on the small chromosome are less variable in both species but occur at similar rates as the concurrently replicated regions of the large chromosome. Neither nucleotide composition nor frequency of nucleotide motifs differed among regions experiencing high and low base-substitution rates, which along with the inferred ~800 Kb wave period suggests that the source of the periodicity is not sequence-specific but rather a systematic process related to the cell cycle. These results support the notion that base-substitution mutation rates are likely to vary systematically across many bacterial genomes, which exposes certain genes to elevated deleterious mutational load.
Our observations 1) shed light on the evolution of a putative regulatory motif across large phylogenetic distances; 2) are expected to facilitate the understanding of the modulation of ribosomal genes expression in Paramecium; and 3) reveal a largely unexplored--and presumably not restricted to Paramecium--association between the presence/absence of a DNA motif and the evolutionary fate of its host genes.
SUMMARY Foodborne outbreaks from contaminated fresh produce have been increasingly recognized in many parts of the world. This reflects a convergence of increasing consumption of fresh produce, changes in production and distribution, and a growing awareness of the problem on the part of public health officials. The complex biology of pathogen contamination and survival on plant materials is beginning to be explained. Adhesion of pathogens to surfaces and internalization of pathogens limits the usefulness of conventional processing and chemical sanitizing methods in preventing transmission from contaminated produce. Better methods of preventing contamination on the farm, or during packing or processing, or use of a terminal control such as irradiation could reduce the burden of disease transmission from fresh produce. Outbreak investigations represent important opportunities to evaluate contamination at the farm level and along the farm-to-fork continuum. More complete and timely environmental assessments of these events and more research into the biology and ecology of pathogen-produce interactions are needed to identify better prevention strategies.
ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTChemical-Biological Activities: A Computer-produced Express Digest.G. Malcolm Dyson and Michael F. LynchCite this: J. Chem. Doc. 1963, 3, 2, 81–85Publication Date (Print):April 1, 1963Publication History Published online1 May 2002Published inissue 1 April 1963https://pubs.acs.org/doi/10.1021/c160009a011https://doi.org/10.1021/c160009a011research-articleACS PublicationsRequest reuse permissionsArticle Views20Altmetric-Citations4LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access options Get e-Alerts
Abstract Although various empirical studies have reported a positive correlation between the specific growth rate and cell size across bacteria, it is currently unclear what causes this relationship. We conjecture that such scaling occurs because smaller cells have a larger surface-to-volume ratio and thus have to allocate a greater fraction of the total resources to the production of the cell envelope, leaving fewer resources for other biosynthetic processes. To test this theory, we developed a coarse-grained model of bacterial physiology composed of the proteome that converts nutrients into biomass, with the cell envelope acting as a resource sink. Assuming resources are partitioned to maximize the growth rate, the model predicts that the growth rate and ribosomal mass fraction scale negatively, while the mass fraction of envelope-producing enzymes scales positively with surface-to-volume. These relationships are compatible with growth measurements and quantitative proteomics data reported in the literature.
