Determining the effectiveness of resistance to subterranean clover mottle sobemovirus in different genotypes of subterranean clover in the field using the grazing animal as virus vector

The effectiveness of resistance to subterranean clover mottle sobemovirus (SCMoV) previously identified in different genotypes of subterranean clover (Trifolium subterraneum) inoculated with infective sap in the glasshouse, was tested in two field experiments which used the grazing animal as virus vector. Replicated plots each consisting of paired test rows of 20 different genotypes were used. Clover plants infected with SCMoV were transplanted in between the paired test rows and these acted as sources of the virus for spread by grazing sheep. Although used in different years at different sites with different virus isolates, the field exposure methodology employed produced consistent results. The genotypes each behaved similarly in both experiments as regards the relative extents of SCMoV infection that developed, levels ranging from 0–98%. The previously identified resistance in six ‘highly resistant’ and three ‘partially resistant’ cultivars was effective under field conditions. However, the ‘partial resistance’ in three others was overcome, cvs Green Range and Mt Barker developing levels of infection approaching those in ‘susceptible’ cultivars, while an intermediate infection level developed in cv. Karridale. The three cultivars in which partial resistance was not effective all belonged to ssp. subterraneum. In subterranean clover breeding programmes, field screening using the grazing animal as a vector is advisable to determine whether SCMoV resistance found by sap inoculation is still effective under field conditions.     

Suppressing spread of alfalfa mosaic virus in grazed legume pasture swards using insecticides and admixture with grass, and effects of insecticides on numbers of aphids and three other pasture pests

Field experiments were sown with alfalfa mosaic virus (AMV)‐infected or healthy seed of burr medic (Medicago polymorpha) and grazed by sheep. Seed‐infected plants acted as primary sources for virus spread by naturally occurring aphids. Admixture with annual ryegrass (Lolium rigidum), a non‐host of AMV, and different insecticides were used in attempts to suppress virus spread. Sowing swards to provide the ratios 1 : 4 and 1 : 13 of medic:ryegrass plants diminished AMV spread in medic plants by 23% and 45% respectively. Applications of organophosphorus (demeton‐s‐methyl), carbamate (pirimicarb) and newer generation synthetic pyrethroid (alpha‐cypermethrin) insecticides, all significantly decreased final AMV incidence. Alpha‐cypermethrin was the most effective, suppressing AMV incidence by 87% (two sprays), 79% (one late spray) and 65% (one early spray). Two sprays of demeton‐s‐methyl decreased incidence by only 36%, while two and 2 weekly applications of pirimicarb diminished it by 29–65% and 35–70% respectively. AMV infection of medic seed harvested decreased by up to 76% in sprayed plots. Insecticide treatment did not prevent winged aphids from landing but numbers of wingless Acyrthosiphon kondoi colonising swards were suppressed by up to 92% by spraying with pirimicarb and up to 96% by alpha‐cypermethrin. A. kondoi were much slower to recover with alpha‐cypermethrin than with pirimicarb, the former still significantly diminishing its numbers 35 days after spraying. Alpha‐cypermethrin was also very effective at suppressing Halotydeus destructor and Penthaleus major but not Sminthurus viridis. Greater effectiveness of insecticides in controlling spread of AMV in pasture than has been found previously with non‐persistently aphid‐transmitted viruses in annual crops seems due to the key role played by wingless aphids as virus vectors.     

Role of hydrology in development of a vernal clear water phase in an urban impoundment

1. Grazer control of phytoplankton and water transparency was responsive to interannual variations in river discharge and corresponding water retention time in Ford Lake, Michigan, U.S.A. 2. A simple mathematical model was developed to separate the effects of washout from in situ zooplankton population dynamics. The threshold reservoir flushing time for development of a large Daphnia population and corresponding clear water phase during May was about 20 days. Predation by Leptodora was insufficient to cause the subsequent decline of the Daphnia population. 3. Recruitment success by the zebra mussel Dreissena polymorpha appeared responsive to flushing time indirectly, through hydrological effects on the magnitude of the spring diatom crop.     

The experimental silicification of Aquificales and their role in hot spring sinter formation

Archean microfossils provide some of the earliest physical evidence for life on Earth, yet there remains a great deal of uncertainty regarding which micro‐organisms were actually preserved. Because of the limited cellular detail remaining, interpretation of those microfossils has been based solely on size and morphology. This has led to significant controversy surrounding the presence or absence of cyanobacteria as early as 3.5 billion years. Accordingly, there has been an experimental bias towards studying their silicification. Here we report the very first findings on thermophilic bacteria–silica interactions, and investigate how Sulfurihydrogenibium azorense, a representative of the Aquificales often found as prominent members of modern hot spring vent communities, interacts with highly siliceous hydrothermal fluids. We show that adsorption of silica is limited to silica polymers and colloids, and that the magnitude of silica adsorption is dependent on its chemolithoautotrophic pathway. Intriguingly, when S. azorense is grown as a H₂‐oxidizer, it responds to increasing silica concentrations by producing a protein‐rich biofilm that may afford the cells protection against cell wall silicification. Although the biofilms of Aquificales could potentially contribute to or accelerate siliceous sinter formation under certain growth conditions, the cells themselves show a low preservation potential and are unlikely to have been preserved in the ancient rock record, despite phylogenetic analyses suggesting that they represent one of the most primordial life forms.     

Chloroplast DNA phylogeography of Alnus glutinosa (L.) Gaertn.

Traditionally, information on the postglacial history of plant species has been gained from the analysis of fossil pollen data. More recently, surveys of present patterns of genetic variation have given valuable insights into species phylogeography. The genus Alnus , based on fossil data, is known to have had at least four glacial refugia. A survey of chloroplast DNA (cpDNA) diversity in populations of black alder ( A. glutinosa ) was undertaken in order to gain more insight into its postglacial history. This revealed a high degree of structuring of 13 cpDNA haplotypes on a European scale which indicated that most of northern and central Europe was colonized from a refuge in the Carpathian Mountains. Based on the distribution of two common cpDNA haplotypes, colonization routes from this refuge can be determined. The locations of other previously identified refugia are confirmed and two formerly unconfirmed refugial areas for alder (southern Spain and Turkey) are proposed.