Co-metabolism of fluorobenzoates by natural microbial populations.

Co-metabolic degradation of monofluorobenzoates was carried out by a mixed soil population in a basal salts medium. The monofluorobenzoates did not support growth of microorganisms but were shown to be subject to ring cleavage as a result of microbial activity. Rate of ring cleavage was increased by use of the co-substrate enrichment technique using glucose as the co-substrate. Results indicate that the monofluorobenzoates were subject to an initial co-metabolic attack with glucose, providing the energy necessary for co-metabolism to proceed to a point where complete metabolism became possible.     

Heterogeneity of human natural killer cell populations.

Natural killing (NK) in human donors was determined by the ability of peripheral blood subpopulations to lyse the myeloid target, K562, in a 2 to 4 hr ⁵¹Cr release assay. The most active cell was a non-T cell which passed through nylon columns (representing 10 to 25% of column passed cells). A second column passed cell population, with characteristics of T lymphocytes (75 to 90% of column passed cells), was also capable of mediating natural killing. Non-T cells which were retained by the nylon columns (45 to 55% of adherent cells) lacked NK activity. However, nylon adherent T cells (45 to 55% of adherent cells) were consistently active in NK assays, illustrating an important subset of NK effector cell often overlooked. Both column passed and adherent T cells were further separated according to their ability to bind IgG or IgM immune complexes, showing that those mediating NK have receptors for IgG (Tγ+) but not for IgM (Tμ+).     

A germination model for natural seed populations.

A general model for determining time and rate of germination under continuously changing temperatures and other environmental parameters is proposed. The seed population may vary in size and density. The principle of the model is based upon the adherence of seed germination to an Arrhenius type system. The model, with slight modifications, may be applied to many temperature or other environmentally dependent biological systems.     

Secondary theorem of natural selection in biocultural populations.

The "Secondary Theorem of Natural Selection," an extension of Fisher's fundamental theorem, states that the rate of change in the mean of an arbitrary character in response to selection is proportional to the additive genetic covariance between the character and fitness. Here I derive an expression for the change in the mean value of a trait subject to both genetic and cultural transmission. I start with the one-locus case under generalized mating and cultural transmission from parents to offspring, then proceed to the two-locus case. My results support previous work on the effects of nongenetic inheritance by showing that (i) cultural transmission introduces a timelag in the population response to selection; (ii) with cultural transmission the effects of selection persist even after selection is relaxed; and (iii) cultural transmission can either enhance or retard phenotypic evolution relative to that obtained under purely genetic transmission.     

Dynamics of natural rotifer populations

New tools for analyzing ecological time series have permitted the construction of rigorous models from relatively short series. We have applied these techniques to abundance data for nine natural rotifer populations to construct realistic models of their dynamics. Species included are Asplanchna girodi, Filinia pejleri, Keratella tropica, Monostyla bulla, Brachionus rotundiformis, and four other Brachionus species. The overall shapes of the time series were similar with an initial peak followed by oscillations of varying amplitude around a mean of lower population density. Auto correlation functions (ACF) for all populations were positive at small time lags and decayed rapidly to zero. This suggest that these are stationary, exponentially damped time series, fluctuating arround a constant mean with constant variance. The rapid decay of the ACFs indicates that the effect of a perturbation on these populations is quickly removed in one or two days. Phase portrait plots of log current population density vs log lagged density indicate that the time series are stable and non-chaotic. One type of model yielded the highest R² for four of the nine species and was designated the consensus model. The mean R² of this model for all nine species was 0.53 with a coefficient of variation of 38%. Lyapanov exponents were strongly negative, indicating that these populations rapidly return to equilibrium after an exogenous perturbation. Rotifer populations appear to be tracking very recent perturbations and their dynamics cannot be predicted from perturbations in the more distant past. We investigated the effects of increasing the level of stochasticity in the consensus model on the length of the growing season and resting egg production. Increasing stochastic variance increased the probability of extremely low population densities, shortening the growing season. In shorter growing seasons, fewer resting eggs were produced, other factors being equal. Counteracting this negative effect, was an increased probability of extremely high populations densities which increased mixis and resting egg production. Constructing models accurately depicting the dynamics of natural zooplankton populations should improve aquatic ecosystem models. This revised version was published online in July 2006 with corrections to the Cover Date.     

Factors affecting levels of genetic diversity in natural populations.

Genetic variability is the clay of evolution, providing the base material on which adaptation and speciation depend. It is often assumed that most interspecific differences in variability are due primarily to population size effects, with bottlenecked populations carrying less variability than those of stable size. However, we show that population bottlenecks are unlikely to be the only factor, even in classic case studies such as the northern elephant seal and the cheetah, where genetic polymorphism is virtually absent. Instead, we suggest that the low levels of variability observed in endangered populations are more likely to result from a combination of publication biases, which tend to inflate the level of variability which is considered ''normal'', and inbreeding effects, which may hasten loss of variability due to drift. To account for species with large population sizes but low variability we advance three hypotheses. First, it is known that certain metapopulation structures can result in effective population sizes far below the census size. Second, there is increasing evidence that heterozygous sites mutate more frequently than equivalent homozygous sites, plausibly because mismatch repair between homologous chromosomes during meiosis provides extra opportunities to mutate. Such a mechanism would undermine the simple relationship between heterozygosity and effective population size. Third, the fact that related species that differ greatly in variability implies that large amounts of variability can be gained or lost rapidly. We argue that such cases are best explained by rapid loss through a genome-wide selective sweep, and suggest a mechanism by which this could come about, based on forced changes to a control gene inducing coevolution in the genes it controls. Our model, based on meiotic drive in mammals, but easily extended to other systems, would tend to facilitate population isolation by generating molecular incompatabilities. Circumstances can even be envisioned in which the process could provide intrinsic impetus to speciation.     

Rapid surveying of DNA sequence variation in natural populations.

DNA sequencing can be costly and time consuming for population studies because of the relative rarity of variation along exons. These problems can be substantially reduced by the use of the polymerase chain reaction on introns using primers from the exon region. These problems can be further reduced by the use of denaturing gradient gel electrophoresis to identify those alleles in need of sequencing.     

Standard reference strains of Escherichia coli from natural populations.

A set of 72 reference strains of Escherichia coli isolated from a variety of hosts and geographical locations has been established for use in studies of variation and genetic structure in natural populations. The strains, which have been characterized by multilocus enzyme electrophoresis, are representative of the range of genotypic variation in the species as a whole.     

Cytogenetics of natural populations of Drosophila

D. bipectinata and D. malerkotliana differ from each other by three overlapping inversions in IIL, two included inversions in IIIL and two overlapping inversions in IIIR. These inversions were analysed on the basis of the salivary chromosome maps of D. malerkotliana. Bock's (1971) data revealed that the four members of the bipectinata species complex differ from each other with respect to overlapping inversions. The reason why the ancestral population which may have been heterozygous for common inversions split into at least four groups, each leading to the formation of a new species, and the possible mechanism of the origin of sexual isolation between the groups is discussed.     

Epigenetics in natural animal populations

Phenotypic plasticity is an important mechanism for populations to buffer themselves from environmental change. While it has long been appreciated that natural populations possess genetic variation in the extent of plasticity, a surge of recent evidence suggests that epigenetic variation could also play an important role in shaping phenotypic responses. Compared with genetic variation, epigenetic variation is more likely to have higher spontaneous rates of mutation and a more sensitive reaction to environmental inputs. In our review, we first provide an overview of recent studies on epigenetically encoded thermal plasticity in animals to illustrate environmentally‐mediated epigenetic effects within and across generations. Second, we discuss the role of epigenetic effects during adaptation by exploring population epigenetics in natural animal populations. Finally, we evaluate the evolutionary potential of epigenetic variation depending on its autonomy from genetic variation and its transgenerational stability. Although many of the causal links between epigenetic variation and phenotypic plasticity remain elusive, new data has explored the role of epigenetic variation in facilitating evolution in natural populations. This recent progress in ecological epigenetics will be helpful for generating predictive models of the capacity of organisms to adapt to changing climates.     

Adult neurogenesis in natural populations

The dogma that the adult brain produces no new neurons has been overturned, but the critics are still asking, so what? Is adult neurogenesis a biologically relevant phenomenon, or is it perhaps harmful because it disrupts the existing neuronal circuitry? Considering that the phenomenon is evolutionarily conserved in all mammalian species examined to date and that its relevance has been well documented in non-mammalian species, it seems self-evident that neurogenesis in adult mammals must have a role. In birds, it has been established that neurogenesis varies dramatically with seasonal changes in song production. In chickadees, the learning behaviour related to finding stored food is also correlated with seasonal adult neurogenesis. Such studies are still nonexistent in mammals, but the related evidence suggests that neurogenesis does vary seasonally in hamsters and shows sexual differences in meadow voles. To promote studies on natural populations asking fundamental questions of the purpose and function of neurogenesis, we organized a Workshop on "Hippocampal Neurogenesis in Natural Populations" in Toronto in May 2000. The Workshop highlighted recent discoveries in neurogenesis from the lab, and focused on its functional consequences. The consensus at the Workshop was that demonstration of a role for neurogenesis in natural behaviours will ultimately be essential if we are to understand the purpose and function of neurogenesis in humans.     

滇北球花报春天然群体表型变异研究

滇北球花报春(Primula denticulata ssp. sinodenticulata)的早春蓝紫色球状花序具有独特的观赏价值, 但由于多种因素的影响, 其天然群体正在逐渐减少。为评价其遗传多样性水平, 我们对滇北球花报春10个天然群体进行了表型性状变异研究。结果表明: 滇北球花报春8个表型性状在群体间和群体内均存在极显著差异, 变异系数(CV)在4.73–9.90%之间, 表型分化系数(V_st)在0.1541–0.4069之间, 平均值为0.2854, 群体内的变异是表型变异的主要来源。根据8个表型性状的UPGMA聚类分析将10个天然群体划分为3类。依本研究结果, 在对滇北球花报春种质资源保护时进行就地保护是十分必要的, 并且应该优先保护表型变异较为丰富的天然群体。     

Competition in natural populations of Daphnia

I investigated the competitive relationships between two species of Daphnia, D. galeata and D. cucullata, and their interspecific hybrid. The term hemispecific competition was introduced to describe competition between parental species and hybrids. In eutrophic Tjeukemeer both parental species were found to compete with the hybrid, whereas competition between D. galeata and D. cucullata seemed limited. Although the effect of competition on life history traits of daphnids may be profound, the influence of the competitors on the seasonal dynamics of the Daphnia species seems limited.