THE EFFECT OF REPEATED SPRAYING OF INSECTS IN INCREASING THEIR RESISTANCE TO INSECTICIDES

During the selection of a stock of Drosophila melanogaster for resistance to DDT, in which carbon dioxide was used for purposes of anaesthesis, a sensitivity to this gas developed. The phenomena closely paralleled those shown by the CO₂-sensitive ebony stock isolated by L'Héritier and his co-workers. An experimental analysis of its effect upon DDT sensitivity was made. It was found that a stock selected for CO₂ resistance gave the same probit regression line as the original stock. A CO₂-sensitive stock, whether anaesthetized with nitrogen or carbon dioxide, gave the same regression line at a temperature of 25° C. at which CO₂ sensitivity disappeared, or at 15° C. if adjustment to the proportion of deaths in the control was made. The effect of CO₂ was therefore to limit the population from which selection is made for DDT resistance, rather than to alter the distribution of DDT resistance within the stock.     

Agrobacterium tumefaciens-mediated transformation and regeneration of herbicide resistant onion (Allium cepa) plants

Transgenic onion plants (Allium cepa) tolerant to herbicides containing active ingredients glyphosate and phosphinothricin were recovered from immature embryos of open pollinated and hybrid parent onion lines at a maximum transformation frequency of 0.9%. Transformants of different onion cultivars, grown on different selective agents and confirmed by Southern analysis, thrived with no apparent ill effects when sprayed with the respective herbicides at double the recommended field dosage for weed eradication. This study demonstrates that the transformation process described previously can be used with different selective agents and is cultivar independent.     

An analysis of population genetic differentiation and genotype–phenotype association across the hybrid zone of carrion and hooded crows using microsatellites and MC1R

The all black carrion crow ( Corvus corone corone ) and the grey and black hooded crow ( Corvus   corone cornix ) meet in a narrow hybrid zone across Europe. To evaluate the degree of genetic differentiation over the hybrid zone, we genotyped crows from the centre and edges of the zone, and from allopatric populations in northern (Scotland–Denmark–Sweden) and southern Europe (western–central northern Italy), at 18 microsatellites and at a plumage candidate gene, the MC1R gene. Allopatric and edge populations were significantly differentiated on microsatellites, and populations were isolated by distance over the hybrid zone in Italy. Single-locus analyses showed that one locus, CmeH9, differentiated populations on different sides of the zone at the same time as showing only weak separation of populations on the same side of the zone. Within the hybrid zone there was no differentiation of phenotypes at CmeH9 or at the set of microsatellites, no excess of heterozygotes among hybrids and low levels of linkage disequilibrium between markers. We did not detect any association between phenotypes and nucleotide variation at MC1R , and the two most common haplotypes occurred in very similar frequencies in carrion and hooded crows. That we found a similar degree of genetic differentiation between allopatric and edge populations irrespectively of their location in relation to the hybrid zone, no differentiation between phenotypes within the hybrid zone, and neither heterozygote excess nor consistent linkage disequilibrium in the hybrid zone, is striking considering that carrion and hooded crows are phenotypically distinct and sometimes recognised as separate species.     

Where temperate meets tropical: multi-factorial effects of elevated CO2, nitrogen enrichment, and competition on a mangrove-salt marsh community

Our understanding of how elevated CO₂ and interactions with other factors will affect coastal plant communities is limited. Such information is particularly needed for transitional communities where major vegetation types converge. Tropical mangroves (Avicennia germinans) intergrade with temperate salt marshes (Spartina alterniflora) in the northern Gulf of Mexico, and this transitional community represents an important experimental system to test hypotheses about global change impacts on critical ecosystems. We examined the responses of A. germinans (C₃) and S. alterniflora (C₄), grown in monoculture and mixture in mesocosms for 18 months, to interactive effects of atmospheric CO₂ and pore water nitrogen (N) concentrations typical of these marshes. A. germinans, grown without competition from S. alterniflora, increased final biomass (35%) under elevated CO₂ treatment and higher N availability. Growth of A. germinans was severely curtailed, however, when grown in mixture with S. alterniflora, and enrichment with CO₂ and N could not reverse this growth suppression. A field experiment using mangrove seedlings produced by CO₂‐ and N‐enriched trees confirmed that competition from S. alterniflora suppressed growth under natural conditions and further showed that herbivory greatly reduced survival of all seedlings. Thus, mangroves will not supplant marsh vegetation due to elevated CO₂ alone, but instead will require changes in climate, environmental stress, or disturbance to alter the competitive balance between these species. However, where competition and herbivory are low, elevated CO₂ may accelerate mangrove transition from the seedling to sapling stage and also increase above‐ and belowground production of existing mangrove stands, particularly in combination with higher soil N.     

Monitoring rangeland biodiversity: Plants as indicators

Abstract As well as being important components of biodiversity in their own right, plants reflect the physical environment, are the primary target of many of the pressures acting on rangelands, and are relatively amenable to measurement. Hence, measurements based on plants have considerable potential to be efficient indicators of the response of rangeland biodiversity to land use. A recent report commissioned by the National Land and Water Resources Audit recommended a core set of 11 indicators, six of which relied on measurements of plants. These were trends in (i) the extent of clearing; (ii) the cover of native perennial ground‐layer vegetation; (iii) the distribution and abundance of exotic plant species; (iv) the distribution and abundance of fire‐sensitive species; (v) the distribution and abundance of grazing‐sensitive species; and (vi) the distribution and abundance of listed threatened entities. Most indicated responses of plants to pressures acting on them. Only two (clearing and exotic plants) related to pressures. We recommend that the set be expanded to include two additional pressure indicators, one for grazing and another for fire, in recognition of their extent and potential influence on rangeland biodiversity. We also recommend that benchmark sites be included in all ground‐based monitoring programmes to provide reference standards for those biotic indicators about which little is known. Assessments of the current state of knowledge about these indicators for two case‐study regions, the Gascoyne–Murchison strategy area and Cape York Peninsula, have shown that it would be possible to monitor most of them directly at regional scales, but that current monitoring programmes fall short of achieving this.     

The Effect of Drying Rate on the Survival of Three Desiccation-tolerant Angiosperm Species

The effect of drying rate on the survival of three angiospermresurrection plants, Craterostigma wilmsii (homoiochlorophyllous),Xerophyta humilis (poikilochlorophyllous) and Myrothamnus flabellifolius(homoiochlorophyllous) was examined. All species survived slowdrying, but only C. wilmsii was able to survive rapid drying.C. wilmsii was rapidly able to induce protection mechanismssuch as folding of cell walls to prevent mechanical stress andcurling of leaves to minimize light stress, and thus survivedfast drying. Rapid drying of X. humilis andM. flabellifoliusappeared to allow insufficient time for complete induction ofprotection mechanisms. In X. humilis, there was incomplete replacementof water in vacuoles, the photosynthetic apparatus was not dismantled,plasma membrane disruption occurred and quantum efficiency ofphotosystem II (F_V/F_M) did not recover on rehydration. Rapidlydried leaves of M. flabellifolius did not fold tightly againstthe stem and F_V/F_Mdid not recover. Ultrastructural studies showedthat subcellular damage incurred during drying was exacerbatedon rehydration. The three species co-occur in environments inwhich they experience high desiccation pressures. C. wilmsiihas few features to retard water loss and thus the ability forrapid induction of subcellular protection is vital to survival.X. humilis and M. flabellifolius are able to retard water lossand protection is acquired relatively slowly. Copyright 1999Annals of Botany Company Chlorophyll fluorescence, Craterostigma wilmsii, drying rate, Myrothamnus flabellifolius, resurrection plant, ultrastructure, Xerophyta humilis.     

Differentiation of metallicolous and non‐metallicolous Salix caprea populations based on phenotypic characteristics and nuclear microsatellite (SSR) markers

The Salicaceae family comprises a large number of high‐biomass species with remarkable genetic variability and adaptation to ecological niches. Salix caprea survives in heavy metal contaminated areas, translocates and accumulates Zn/Cd in leaves. To reveal potential selective effects of long‐term heavy metal contaminations on the genetic structure and Zn/Cd accumulation capacity, 170 S. caprea isolates of four metal‐contaminated and three non‐contaminated middle European sites were analysed with microsatellite markers using Wright's F statistics. The differentiation of populations North of the Alps are more pronounced compared to the Southern ones. By grouping the isolates based on their contamination status, a weak but significant differentiation was calculated between Northern metallicolous and non‐metallicolous populations. To quantify if the contamination and genetic status of the populations correlate with Zn/Cd tolerance and the accumulation capacity, the S. caprea isolates were exposed to elevated Cd/Zn concentrations in perlite‐based cultures. Consistent with the genetic data nested anova analyses for the physiological traits find a significant difference in the Cd accumulation capacity between the Northern and Southern populations. Our data suggest that natural populations are a profitable source to uncover genetic mechanisms of heavy metal accumulation and biomass production, traits that are essential for improving phytoextraction strategies.