The evolution of intermediate castration virulence and ant coexistence in a spatially structured environment

Theory suggests that spatial structuring should select for intermediate levels of virulence in parasites, but empirical tests are rare and have never been conducted with castration (sterilizing) parasites. To test this theory in a natural landscape, we construct a spatially explicit model of the symbiosis between the ant-plant Cordia nodosa and its two, protecting ant symbionts, Allomerus and Azteca . Allomerus is also a castration parasite, preventing fruiting to increase colony fecundity. Limiting the dispersal of Allomerus and host plant selects for intermediate castration virulence. Increasing the frequency of the mutualist, Azteca , selects for higher castration virulence in Allomerus , because seeds from Azteca -inhabited plants are a public good that Allomerus exploits. These results are consistent with field observations and, to our knowledge, provide the first empirical evidence supporting the hypothesis that spatial structure can reduce castration virulence and the first such evidence in a natural landscape for either mortality or castration virulence.     

Breeding biology, resource partitioning and reproductive effort of a dioecious shrub, Clutia pulchella L. (Euphorbiaceae)

 The flowering of Clutia pulchella was studied in a coastal scarp forest community in southern KwaZulu-Natal, South Africa. Sex dependent differences in the frequency and magnitude of floral output were recorded, with male individuals producing 4.32 times more flowers per leaf axis than females. Increased node production, as a manifestation of significantly increased branching, allowed for prolific flower production in males. Energy investment per flower was marginally greater in females (0.045 J/mg), despite copious pollen production and glandular secretion in males (0.035 J/mg). Differential flower production, reproductive organ dry mass and an unbalanced branching ratio revealed that “per plant” expenditure towards flowering was more pronounced in males (♀ flowering × 5.04). The elevated cost of fruit production (♀ flowering × 5.5), inclusive of imperfect fruit set, resulted in overall reproductive expense being slightly higher for females. Such sex-dependent reproductive investment was shown to have minimal influence on the population sex ratio, with sex frequencies and nearest neighbor distances yielding a non-significant male bias of 1.16. No evidence of spatial asymmetry was found. Received December 1, 1999 Accepted September 27, 2000     

Electric membrane properties of adult mouse DRG neurons and the effect of culture duration

The electrical membrane properties (EMP) of adult mouse dorsal root ganglion (DRG) neurons were characterized by an extensive electrophysiological investigation of 450 cells. The neurons were divided into two types: an M-type having an action potential with monophasic falling phase and a B-type with a more complex biphasic or triphasic falling phase. Compared to M-type, B-type were “slow” neurons with a higher specific membrane resistance (R_m), and a longer time constant (τ), duration of action potential (Δt), and absolute refractory period (ARP). B-type also had a larger amplitude action potential, afterhyperpolarization and positive overshoot. The action potential of the M-type neuron had only a Na⁺ component while that of the B-type had both a Na⁺ and a Ca²⁺ component. After two days in culture, M-type neurons exhibited phase bright cytoplasmic granules, which were seldom observed for B-type neurons. Although neuron survival remained constant during the first six days in culture (DIV), the relative frequency of occurrence of the M-type decreased from 82 to 50%. Thereafter, it decreased more gradually to a final value of approximately 20% after 40 DIV. It was concluded that at least during the first 6 DIV and possibly through to 40 DIV, M-type neurons transformed into B-type. Both M- and B-type neurons showed significant and similar changes in their EMP with increasing DIV (up to 40 DIV). For M- and B-types combined, R_m increased approximately 142%, τ by 204%, and no significant change in specific membrane capacitance was observed. Rheobasic threshold depolarization decreased 58%, while the resting membrane potential decreased by only 19%. These changes in the EMP of adult neurons are strikingly similar to changes in EMP observed in adult denervated muscle and in cultures of either embryonic nerve or muscle. This similarity suggested that the adult DRG neurons in cell culture undergo progressive dedifferentiation because of isolation from their usual trophic interactions. Determination of neuronal membrane electrical characteristics provides a new method for evaluating the effects of various possible trophic agents, e.g., hormones and tissue extracts, on the state of differentiation of neurons in cell culture.     

The Saccharomyces cerevisiae ARO1 gene. An example of the co-ordinate regulation of five enzymes on a single biosynthetic pathway

The ARO1 gene of Saccharomyces cerevisiae encodes the arom multifunctional enzyme. Specific inhibitors of amino acid biosynthesis have been used to obtain evidence that expression of a cloned ARO1 gene is regulated in response to amino acid limitation. Northern blot analysis and sequence studies indicate that ARO1 is regulated by the well characterised S. cerevisiae 'general control' mechanism. This provides a very economical means of simultaneously tailoring the synthesis of five shikimate pathway enzymes to the needs of the cell.     

The effect of dietary precursors on the excretion of amines and their metabolites in the rat

The influence of diet on the excretion of catecholamines, some of their metabolites, and pHPG, an octopamine metabolite, was examined. Two groups of rats were fed either a cereal-containing standard laboratory Purina rat chow or a cereal-free casein diet. Use of the standard chow resulted in significant increases in the urinary values for total MHPG, pHPG, DHPE, MHPE, and free and total DOPAC by the seventh day in comparison to the casein diet. No changes were noted in the excretion of free and total NE, DA, and HVA. The data indicate that it is necessary to place the animals on the casein diet several days before determining the excretion of the aforementioned metabolites.     

Mapping Brucellosis Increases Relative to Elk Density Using Hierarchical Bayesian Models

The relationship between host density and parasite transmission is central to the effectiveness of many disease management strategies. Few studies, however, have empirically estimated this relationship particularly in large mammals. We applied hierarchical Bayesian methods to a 19-year dataset of over 6400 brucellosis tests of adult female elk (Cervus elaphus) in northwestern Wyoming. Management captures that occurred from January to March were over two times more likely to be seropositive than hunted elk that were killed in September to December, while accounting for site and year effects. Areas with supplemental feeding grounds for elk had higher seroprevalence in 1991 than other regions, but by 2009 many areas distant from the feeding grounds were of comparable seroprevalence. The increases in brucellosis seroprevalence were correlated with elk densities at the elk management unit, or hunt area, scale (mean 2070 km²; range  = [95–10237]). The data, however, could not differentiate among linear and non-linear effects of host density. Therefore, control efforts that focus on reducing elk densities at a broad spatial scale were only weakly supported. Additional research on how a few, large groups within a region may be driving disease dynamics is needed for more targeted and effective management interventions. Brucellosis appears to be expanding its range into new regions and elk populations, which is likely to further complicate the United States brucellosis eradication program. This study is an example of how the dynamics of host populations can affect their ability to serve as disease reservoirs.