Hypothesis formulation and testing in aquatic bioassays: a deterministic model approach

The paper examines the significance of toxicant kinetics information obtained from aquatic toxicity bioassays and bioconcentration tests. The data, bioconcentration kinetics and acute mortality versus exposure-duration information for juvenile American flagfish (Jordanella foridae) exposed to 1,4-dichlorobenzene, are interpreted in terms of a one-compartment, first-order kinetics model. The output of the model is used to formulate a testable hypothesis regarding the comparison of toxicant kinetics derived from both bioconcentration test exposures and toxicity bioassays. The model's estimates of the toxicant body burden attained at mortality are compared with theoretical and observed body burdens from literature sources. The use of a simple, deterministic residue-based, one-compartment, first-order kinetics model to evaluate existing data, as well as to formulate hypotheses to direct experimental designs, is examined.     

Recognizing the forest for the trees: testing temporal patterns of cladogenesis using a null model of stochastic diversification

Computer simulations are developed and employed to examine the expected temporal distributions of nodes under a null model of stochastic lineage bifurcation and extinction. These Markovian models of phylogenetic process were constructed so as to permit direct comparisons against empirical phylogenetic trees generated from molecular or other information available solely from extant species. For replicate simulated phylads with n extant species, cumulative distribution functions (cdf's) of branching times were calculated, and compared (using the Kolmogorov-Smirnov test statistic D) to those from three published empirical trees. Molecular phylogenies for columbine plants and avian cranes showed statistically significant departures from the null expectations, in directions indicating recent and ancient species' radiations, respectively, whereas a molecular phylogeny for the Drosophila virilis species group showed no apparent historical clustering of branching events. Effects of outgroup choice and phylogenetic frame of reference were investigated for the columbines and found to have a predictable influence on the types of conclusions to be drawn from such analyses. To enable other investigators to statistically test for nonrandomness in temporal cladogenetic pattern in empirical trees generated from data on extant species, we present tables of mean cdf's and associated probabilities under the null model for expected branching times in phylads of varying size. The approaches developed in this report complement and extend those of other recent methods for employing null models to assess the statistical significance of pattern in evolutionary trees.      

The role of metallothionein synthesis in cadmium- and zinc-resistant CHO-K1M cells

In order to probe the mechanism of survival at high concentrations of cadmium, a population of Cd²⁺-resistant Chinese hamster ovary cells (CHO-K1M), was obtained by selective pressure. This stable population of cells were characterized as tolerant to 200 μM Cd²⁺. In addition to the acquired resistance to Cd²⁺, the CHO-K1M cells also demonstrated resistance to 2 mM Zn²⁺. The CHO-K1M cells exhibit a diminished capacity to accumulate Cd²⁺ at low concentration (0.5 or 1.0 μM), which is not evident at high Cd²⁺ concentration. CHO-K1M cells demonstrated an induced synthesis of metallothionein as defined by physical characteristics and cysteine incorporation. The CHO-K1M cells cultured in the presence of 200 μM Cd²⁺ were determined to have an intracellular concentration of metallothionein representing more than 50-fold that observed in the wild-type cells. These results suggest that in CHO-K1M cells, the induction of metallothionein synthesis represents the important parameter involved in the determination of resistance to high levels of Cd²⁺ and Zn²⁺.     

Resource availability, breeding site selection, and reproductive success of red-winged blackbirds

Red-winged blackbirds are polygynous and show strong breeding site preferences, but it is unclear which environmental factors regulate their reproductive success and are ultimately responsible for shaping their patterns of habitat selection and their mating system. We evaluated the effect of variation in insect emergence rates on the reproductive success of male and female redwings nesting on replicate ponds. The number of male and female redwings that settled on a pond varied two- to three-fold among ponds, but was not related to insect emergence rates. Insect emergence rates had a positive effect on the number of nestlings successfully fledged by females, the number of nestlings fledged from male territories, and on the mass of nestlings at fledging. Typha stem density also varied widely among ponds, and was positively related to male and female settling density and mass of nestlings at fledging, but not to the number of nestlings fledged by females or males. We conclude that alternative breeding sites differ in their ability to support redwing reproduction, and that the availability of emerging odonates is an important environmental factor influencing the reproductive success of both male and female red-winged blackbirds. Received: 31 March 1997 / Accepted: 3 July 1997     

Growth of Lactic Acid Bacteria in Waste Waters of Vegetable-Processing Plants

Waters used in washing, blanching, cooling, and conveying vegetables during processing for freezing were filtered, sterilized, and inoculated with Streptococcus faecalis, S. lactis, or Lactobacillus plantarum. The contents of total nitrogen and total solids were determined, and ninhydrin tests and Benedict's tests for reducing sugars were performed. Substances positive to the ninyhydrin tests and also capable of supporting the growth of the bacteria to high levels of population were found in waters used to blanch cut green beans, but not in the cooling or conveying waters. They were found only in waters following slicing of blanched whole beans. They were also present in waters used in processing purple hull peas at all stages, but only in the waters used to blanch and cool lima beans. The substances were present in waters used to wash and blanch squash, but only in the waters used to blanch greens; they were not found in the cooling waters during the handling of either vegetable. No waters used in the processing of okra yielded a positive ninhydrin test, nor did they support the growth of the lactic acid bacteria.