Epidemic strains of Salmonella agona differentiated by phage restriction and u. v.-protection markers on Collb plasmids

Genes for phage restriction and u. v.-protection, carried by some Coll plasmids, are useful markers of plasmids carried by host bacteria. Colicinogeny, with associated marker characters, may prove useful for strain differentiation as it did, in this study, with strains of Salmonella agona involved in an outbreak.     

Phylogeography and ecological niche modelling implicate coastal refugia and trans‐alpine dispersal of a New Zealand fungus beetle

The Last Glacial Maximum (LGM) severely restricted forest ecosystems on New Zealand’s South Island, but the extent of LGM distribution for forest species is still poorly understood. We used mitochondrial DNA phylogeography (COI) and ecological niche modelling (ENM) to identify LGM refugia for the mycophagous beetle Agyrtodes labralis (Leiodidae), a forest edge species widely distributed in the South Island. Both the phylogenetic analyses and the ENM indicate that A. labralis refuged in Kaikoura, Nelson, and along much of the South Island’s west coast. Phylogeography of this species indicates that recolonization of the largely deforested east and southeast South Island occurred in a west–east direction, with populations moving through the Southern Alps, and that the northern refugia participated little in interglacial population expansion. This contradicts published studies of other New Zealand species, in which recolonization occurs in a north–south fashion from many of the same refugia.     

Restricted distribution of potato leafroll virus antigen in resistant potato genotypes and its effect on transmission of the virus by aphids

Enzyme-linked immunosorbent assay was used to measure the concentration of potato leafroll virus (PLRV) antigen in different parts of field-grown secondarily infected plants of three potato genotypes known to differ in resistance to infection. The antigen concentration in leaves of cv. Maris Piper (susceptible) was 10–30 times greater than that in cv. Pentland Crown or G 7445(1), a breeder's line (both resistant). Differences between genotypes in antigen concentration were smaller in petioles and tubers (5–10-fold) and in above-ground stems (about 4-fold), and were least in below-ground stems, stolons and roots (about 2-fold). PLRV antigen, detected by fluorescent antibody staining of tissue sections, was confined to phloem companion cells. In Pentland Crown, the decrease in PLRV antigen concentration in leaf mid-veins and petioles, relative to that in Maris Piper, was proportional to the decrease in number of PLRV-containing companion cells; this decrease was greater in the external phloem than in the internal phloem. The spread of PLRV infection within the phloem system seems to be impaired in the resistant genotypes. Green peach aphids (Myzuspersicae) acquired < 2800 pg PLRV/aphid when fed for 4 days on infected field-grown Maris Piper plants and < 58% of such aphids transmitted the virus to Physalis floridana test plants. In contrast, aphids fed on infected Pentland Crown plants acquired <120 pg PLRV/aphid and <3% transmitted the virus to P. floridana. The ease with which M. persicae acquired and transmitted PLRV from field-grown Maris Piper plants decreased greatly after the end of June without a proportionate drop in PLRV concentration. Spread of PLRV in potato crops should be substantially decreased by growing cultivars in which the virus multiplies to only a limited extent.     

Resistance to potato leafroll luteovirus can be greatly improved by combining two independent types of heritable resistance

Twelve potato clones were exposed to infection by aphids with potato leafroll luteovirus (PLRV) in three field trials in order to assess their resistance to infection. Up to 92% of the plants of some clones became infected, although other clones were relatively resistant to infection and one clone remained virus-free in all three trials. The resistance of the same 12 clones to PLRV multiplication was assessed in glasshouse-grown plant: lants were graft-inoculated and their daughter tubers were used to grow plants with secondary infection. High concentrations of PLRV were found in some clones (c. 1700 ng/g leaf) while in others much less virus accumulated (as little as 60 ng/g leaf). However, clones in which little virus accumulated were not necessarily those which were most resistant to infection in the field, and there was no association between the two types of resistance. Nevertheless, both types of resistance were found in some clones. The clone G8107(1), which remained virus-free in all the field exposure trials, was also the most resistant to PLRV multiplication. The combination of these two types of resistance in cultivars should help to eliminate the spread of PLRV in crops.