A bioassay for screening materials influencing feeding in the field slug Deroceras reticulatum (Müller) (Mollusca: Pulmonata)

Test materials were incorporated in wheatflour pellets to measure their effect on slug feeding. The dry weights of individual pellets were measured before and after exposure for 24 h to starved slugs. The method gave consistent results, and can accommodate test materials with different physical properties. The phagostimulant effect of sucrose was confirmed. An extract of the herb tarragon (Artemisia dracunculus) was the most effective antifeedant of the materials used at the concentrations tested. The method is compared with other published techniques.     

Small mammal abundance in relation to microhabitat in a dry sub-humid grassland in Kenya

Six species of rodents and two species of insectivores, live-trapped on a 1 ha grid over a 27 month period, were studied in relation to microhabitat factors in a dry sub-humid grassland in Kenya. Wet season peaks of small mammal species were followed by low numbers trapped during the dry seasons. Species distributions were correlated with different microhabitat parameters, and the separation of species with apparently similar environmental requirements could be explained largely in terms of current knowledge of the biology of the species. Correlation analyses indicated an ecological separation of two closely-related Mus species and of two Crocidura shrews. Overlap indices were extremely high, ranging from 0.556 to 0.877 for all combinations of pairs of species. It seems likely that neither food nor micro-habitat partitioning completely explains the coexistence of the small mammal species and it is suggested that populations are highly transient, moving from refuge areas into temporarily favourable areas during the wet season.     

Age-specific patterns of reproduction in White-tailed and Willow Ptarmigan Lagopus leucurus and L. lagopus

Although many studies report a difference in reproductive success between old and young birds, little is known about how, why and when productivity changes as individuals age. We examined age-dependent reproduction in two bird species that inhabit harsh tundra environments: White-tailed Ptarmigan Lagopus leucurus in alpine areas and Willow Ptarmigan Lagopus lagopus in subarctic Canada. We evaluated reproductive performance in the light of three hypotheses: constraint, restraint and selection. Using cross-sectional and longitudinal data, we observed significant age effects in seven of the eight life history and behavioural traits examined for the two species. However, the pattern of age effects variedconsiderably across life history stages; younger birds generally had smaller clutches, later laying dates and poorer spring body condition, but the nesting success did not vary with age. Brood-rearing and renesting abilities were greater for older parents. The oldest age class of White-tailed Ptarmigan showed reproductive senescence for laying date and clutch size but fledged such a large proportion of the brood that they had the highest overall production of any class. It thus appears that parental experience can compensate for reduced physical ability to produce eggs. Annual mortality rates for breeding females were U-shaped for White-tailed Ptarmigan, with higher rates for young and old birds, but mortality did not change with age in Willow Ptarmigan. Overall, the two species differed in the presence of age dependence for only two traits (renesting ability and annual survival). Age-dependent effects were generally greater for White-tailed Ptarmigan than for Willow Ptarmigan. The patterns of mortality and fecundity we observed in ptarmigan provide general support for the constraint hypothesis of reproductive performance. By examining discrete stages of reproduction, we identified the life history stages where age effects occur and propose proximate mechanisms responsible for these effects.     

DspA/E, a type III effector of Erwinia amylovora, is required for early rapid growth in Nicotiana benthamiana and causes NbSGT1-dependent cell death

DspA/E is a pathogenicity factor of Erwinia amylovora that is translocated into the plant cell cytoplasm through an Hrp type III secretion system. Transient expression of dspA/E in Nicotiana benthamiana or yeast induced cell death, as it does in N. tabacum and apple as described previously. DspA/E-induced cell death in N. benthamiana was not inhibited by coexpression of AvrPtoB of Pseudomonas syringae pv. tomato , which inhibits programmed cell death (PCD) induced by several other elicitors in plants. Silencing of NbSGT1 , the expression of which is required for PCD mediated by several resistance proteins of plants, prevented DspA/E-induced cell death in N. benthamiana. However, silencing of NbRAR1 , or two MAP kinase kinase genes, which are required for PCD associated with many resistance genes in plants, did not prevent cell death induced by DspA/E. Silencing of NbSGT1 also compromised non-host resistance against E. amylovora . E. amylovora grew rapidly within the first 24 h after infiltration in N. benthamiana , and DspA/E was required for this early rapid growth. However, bacterial cell numbers decreased after 24 h in TRV-vector-transformed plants, whereas a dspA/E mutant strain grew to high populations in NbSGT1 -silenced plants. Our results indicate that DspA/E enhances virulence of E. amylovora in N. benthamiana, but the bacteria are then recognized by the plant, resulting in PCD and death of bacterial cells or restriction of bacterial cell growth.     

Direct evidence for biased gene diversity estimates from dominant random amplified polymorphic DNA (RAPD) fingerprints

The relevance of using dominant random amplified polymorphic DNA (RAPD) fingerprints for estimating population differentiation was investigated when typically small population sample sizes were used. Haploid sexual tissues were first used to determine genotypes at RAPD loci for 75 eastern white pines ( Pinus strobus L.) representing five populations. Dominant RAPD fingerprints were then inferred from genotypic data for each individual at each locus, and gene diversity estimates from both sources of data were compared. Genotypic information at RAPD loci indicated little or no differentiation among populations, similar to allozyme loci. However, estimates of population differentiation derived from dominant RAPD fingerprints according to various common methods of analysis were generally inflated, especially when all fragments were considered. Simulations showed that an increase in loci sampling and population sample sizes did not significantly alleviate the biases observed.