Seasonality ofOrmia depletaand Limits to Its Spread

Ormia depleta(Wiedemann) (Diptera: Tachinidae), a South American fly that homes on the calling songs of its hosts, was brought from Piracicaba, Brazil; colonized; and released in Florida for the biological control ofScapteriscusspp. mole crickets. It became established at some release sites and rapidly spread through most of peninsular Florida. The seasonal distributions of phonotactic females in Florida differed from the single summer peak recorded at Rio Claro and Piracicaba, Brazil (23° S). Near Bradenton (27° N), spring and fall peaks with a summer hiatus were quickly established and remain evident. At two sites near Gainesville (30° N), a strong fall peak and a modest-to-none spring peak developed. Peaks of phonotactic females seem to follow times of peak host availability and sometimes coincide with minima of host availability. Limits to geographical expansion ofO. depletaare poorly understood. At Gainesville, the site of earliest introduction, establishment was rapid and soundtrap catches increased for the first 3 years, exceeding 1000 per year at one site. Catches then declined for 3 years, almost reaching zero in 1994. On the other hand, annual sound-trap catches ofO. depletaat Bradenton exceeded 1000 in <2 years and have remained above that level for 5 years. Winters in Gainesville are more severe than those in Bradenton, but minimum winter temperatures in Gainesville were substantially lower duringO. depleta's increase than during its decline. Perhaps mild winters cause the flies to become active early—only to starve for lack of adequate winter nectar sources.O. depletamay yet spread to the northern limits of itsScapteriscushosts as present populations adapt to new environments. Or farther spread may require that new strains be introduced from more southerly sites in South America.     

Uptake of saline groundwater by plants: An analytical model for semi-arid and arid areas

An analytical model, based on unsaturated zone water and solute balances, was developed to describe the uptake of saline groundwater by plants in dry regions. It was assumed that: i. initially, the profile had low water and salt contents to some depth; ii. both water and solutes move upwards from the water table by piston flow due only to plant water extraction; iii. the uptake of water concentrates solutes in the soil solution until some threshold salinity is reached, above which plants can no longer extract water due to osmotic effects; iv. uptake of the groundwater does not affect the water table level; and v. uptake of groundwater is only limited by transmission of groundwater through the soil. Model predictions were compared with measurements of groundwater uptake made over 15 months at five sites in aEucalyptus forest in a semi-arid area, using independently measured model parameters. Depth and salinity of groundwater, and soil type varied greatly between sites. Predicted groundwater uptake rates were close to measured values, generally being within ∼ 0.1 mm day^-1. Sensitivity analysis showed that groundwater depth and salinity were the main controls on uptake of groundwater, while soil properties appeared to have a lesser effect. The model showed that uptake of groundwater would result in complete salinisation of the soil profile within 4 to 30 yr at the sites studied, unless salts were leached from the soil by rainfall or flood waters. However, a relatively small amount of annual leaching may be sufficient to allow groundwater uptake to continue. Thus groundwaters, even when saline, may be important sources of water to plants in arid and semi-arid areas.     

Abundance of phytoseiid mites on Vitis species: effects of leaf hairs,domatia, prey abundance and plant phylogeny

We observed the number of predatory mites (Phytoseiidae:Typhlodromus caudiglans) on the foliage of 20 North American species of grapes (Vitis spp) plus the domesticated EuropeanVitis vinifera, all grown in a common garden. We found relatively few phytophagous mites. The numbers of phytophagous mites were not correlated with the plant characteristics that we measured. We found approximately five times as many predatory mites as phytophagous mites and the numbers of these phytoseiid predators were not affected by the availability of prey. Similarly, numbers of phytoseiids were unaffected by plant gender and, hence, the availability of pollen, another source of food. The numbers of phytoseiids were not clustered according to the taxonomic grouping of the tested plant species. Leaf surface characteristics explained over 25% of the variance in the numbers of phytoseiids. Numbers of phytoseiids were positively associated with the density of vein hairs, the density of bristles in leaf axils, and the presence of leaf domatia. These results suggest that sheltered habitats rather than food availability may limit the numbers of phytoseiid mites on grapevines.     

An experimental assessment of the temperature responses of two sympatric seagrasses,Amphibolis antarctica and Amphibolis griffithii,in relation to their biogeography

Walker, D. I. and Cambridge, M. L. 1994. An experimental assessment of the temperature responses of two sympatric seagrasses, Amphibolis antarctica and Amphibolis griffithii, in relation to their biogeography.Seedlings of the viviparous seagrasses, Amphibolis antarctica (Labill.) Sonder & Aschers. and Amphibolis griffithii (Black) den Hartog, were grown in seawater cultures at temperatures of 10–30 °C. This temperature range exceeded the range of temperatures occurring in habitats where Amphibolis grows.All seedlings of both species survived at 15 °C, and all A. antarctica at 10 and 20 °C. There was some mortality at 25 °C, but more in A. griffithii than in A. antarctica. All seedlings showed marked senescence at 30 °C within 2 weeks, and all seedlings of both species were dead at this temperature in 6 weeks. Leaf production rates were different at different temperatures for each species, but were high across the 15–25 °C temperature range for both species. Given the time of release of seedlings from parent plants (winter), these results are consistent with the observed distribution of adult plants, and so the more restricted range of A. griffithii can be explained partially by its' response to temperature in culture. These results do not explain the absence of both Amphibolis species from the east coast of Australia, which may be a consequence of habitat availability.     

The Rhizobium meliloti exoK gene and prsD/prsE/exsH genes are components of independent degradative pathways which contribute to production of low-molecular-weight succinoglycan

When grown on medium supplemented with the succinoglycan-binding dye, Calcofluor, and visualized under UV light, colonies of Rhizobium meliloti (Sinorhizobium meliloti) exoK mutants produce a fluorescent halo with a delayed onset relative to wild-type colonies. By conducting transposon mutagenesis of exoK mutants of R. meliloti and screening for colonies with even more severe delays in production of these fluorescent halos, we identified three genes, designated prsD, prsE, and exsH, which are required for the eventual production of fluorescent halos by exoK colonies. Nucleotide sequence indicates that the prsD and prsE genes encode homologues of ABC transporters and membrane fusion proteins of Type I secretion systems, respectively, whereas exsH encodes a homologue of endo-1,3-1,4-beta-glycanases with glycine-rich nonameric repeats typical of proteins secreted by Type I secretion systems. The exoK gene and the prsD/prsE/exsH genes were shown to be components of independent pathways for production of extracellular succinoglycan degrading activities and for production of low-molecular-weight succinoglycan by R. meliloti. Based on these results, we propose that ExsH is a succinoglycan depolymerase secreted by a Type I secretion system composed of PrsD and PrsE, and that the ExsH and ExoK glycanases contribute to production of low-molecular-weight succinoglycan.     

Mutational analysis of the RecA protein L1 region identifies this area as a probable part of the co-protease substrate binding site

Previous mutational analysis of the L1 region of the RecA protein suggested that Gly-157 and Glu-158 are 'hot-spots' for the occurrence of constitutive LexA co-protease mutants (coprt^c). In the present study, we clearly establish that position 157 is a hot-spot for the occurrence of such mutants, as 12 of 14 and 10 of 14 substitutions result in this phenotype for UmuD and LexA cleavage respectively. The frequency of such mutations at position 158 is somewhat lower, 8 of 13 and 5 of 13 for UmuD and LexA respectively. Comparison of the UmuD vs. LexA co-protease activity for all single mutants with substitutions at positions 154, 155, 156, 157 and 158 (47 in total) reveals that, although there is good agreement among most mutants regarding their ability to cleave both LexA and UmuD, there are two in particular (Glu-154→Asp and Glu-154→Gln) that show a clear preference for cleavage of UmuD. We also show that three second-site mutations that completely suppress coprt^c activity toward LexA have little or no effect on the coprt^c activity of the primary mutant toward UmuD. In addition, we observe a high frequency of second-site suppressor mutations, suggesting a functional interaction among side-chains in this region. Together, these results support the idea that the L1 region of RecA makes up part of the co-protease substrate-binding site.     

Influence of muscle activity on the forces in the femur: An in vivo study

Experiments were performed on two patients with custom-made instrumented massive proximal femoral prostheses implanted after tumour resection. In vivo axial forces transmitted along the prostheses were telemetered during level walking, single- and double-leg stance, and isometric exercises of the hip muscles. These activities varied the lever arms available to the external loads: minimum for double-leg stance and maximum for hip isometric exercises. Kinematic, force plate, EMG and telemetered force data were recorded simultaneously. The force magnification ration (FMR; the ratio of the telemetered axial force to the external force) was calculated. The FMRs ranged from 1.3 (during double-leg stance) to 29.8 (during abductors test), indicating that a major part of the axial force in the long bones is a response to muscle activity, the strength of which depends on the lever arms available to the external loads. From these results, it was shown that the bulk of the bending moment along limbs is transmitted by a combination of tensile forces in muscles and compressive forces in bones, so moments transmitted by the bones are smaller than the limb moments. It was concluded that appropriate simulation of muscle forces is important in experimental or theoretical studies of load transmission along bones.     

The Actions of Calmodulin Antagonists W-7 and TFP and of Calcium on the Gating Kinetics of the Calcium-activated Large Conductance Potassium Channel of the Chara Protoplasmic Drop: A Substate-sensitive Analysis

The effects of the calmodulin antagonists W-7 and trifluoperazine have been measured on the Ca²⁺-activated potassium channel in the membrane surrounding protoplasmic drops expressed from internodal cells of charophyte plants. The large-conductance (170 pS), voltage- and Ca²⁺-dependent gating, and prominent conductance substrate of this channel shows a strong kinetic resemblance to those of the Maxi-K channel from animal cells. This is the first study of the action of calmodulin antagonists which measures their effects on the most populated substates as well as the closed and main open states of Maxi-K channels. The substate analysis provides new evidence for different modes of action of- and different bindings sites for these calmodulin antagonists. Neither antagonist produces the simple closure of the channel reported previously as its effect on the Maxi-K channel, though both do induce flicker-block, reducing the mean current to near zero at high concentrations following an inverted Michaelis-Menten curve. W-7 reduces residence time in the fully open state, thus raising, in the same proportions, the probabilities of finding the channel in the closed state or a pre-existing substate. Its binding to the channel is voltage- and calcium-dependent. In contrast, trifluoperazine reduces residence in the open state and promotes an apparently new substate which overlaps the closed state at −50 mV but is distinguishable from it at voltages more negative than −100 mV. This substate may represent times that trifluoperazine is bound to the channel. Both antagonists have effects clearly distinguishable from that of withdrawing calcium from the channel, which does not affect open state residence time but increases closed state residence time. Thus neither antagonist reverses the activating effect of Ca²⁻. This is good kinetic evidence against the view that the channel is activated by Ca²⁺-calmodulin and that the effect of a calmodulin antagonist is to reverse this process by making Ca²⁻-calmodulin less available. Received: 26 August 1996/Revised: 7 October 1996