The epidemiology of maize streak disease in relation to population densities of Cicadulina spp

The effect of population density of Cicadulina spp. in maize on the epidemiology of maize streak disease was investigated for irrigated crops planted in March or July at Salisbury, Rhodesia, from 1967 to 1969. Experimental plots of maize were invaded for 2–3 months following germination by Cicadulina adults leaving drying veld grasses. When there were few immigrant leaf-hoppers, incidence of maize streak increased arithmetically through transient settling and feeding of previously infective adults. Infection rates increased when infected plants were removed daily, presumably because of the disturbance of vectors.
By comparison, large immigration populations (one leaf-hopper per three maize plants) resulted in an exponential increase in the incidence of maize streak infection.
Aldicarb granules (10%) at 38 g per 12 m row length, applied into a furrow before planting the maize, gave almost complete protection from infection.     

Nicotiana tabacum callus studies. X. ABA increases resistance to cold damage

The effects of cis. trans abscisic acid on response to chilling was investigated in callused Nicotiana tabacum L. pith explants. Explants pretreated with 10^-4M ABA underwent approximately 50% less cellular leakage when chilled at 2°C under short-day conditions for 10 d than the comparable non-treated tissue. Growth in terms of fresh and dry weights, although poor in comparison to non-chilled (20°C, long days) treatments, was more than twice that of the non-ABA-treated material. On an absolute dry weight basis proline content increased on chilling from 0.7 to 3.4 mg g^-1 in non-ABA-treated explants, but rose to nearly 17 mg g^-1 in the tissue treated with ABA. Only in the case of cold-hardened. ABA-treated tissue could some cells survive subzero temperatures and regenerate callus again. It is suggested that at least part of the ameliorating effects of ABA result from an increase in the level of proline.     

CADMIUM, LEAD AND COPPER IN FRESHWATER MUSSELS FROM THE ASSINIBOINE RIVER, MANITOBA, CANADA

Cadmium, lead and copper levels were examined in adductor muscle,foot, gonad, mantle, gill, and heart of 9 species of freshwatermussels from the Assini-boine River, using anodic strippingvoltametry. Metal levels in most organs were lower than in bottomsediments. The metals generally showed significantly higherconcentrations in heart and hemolymph samples than in the othertissues examined. However in four species, gill tissues showedthe highest values forcopper, and Quadrula quadnda and Ligumiarecta accumulated the greatest amounts of copper overall. Cadmiumconcentrations were particularly high in heart and hemolymphof Lampsilis radiata sili-quoidea, L. vcntricosa, and Ligumiarecta. Metal concentrations in tissues of a given species showedvarious relationships to weight of soft parts, shell length,and shell weight, but inverse correlations predominated, indicatingthat metals in many organs were less concentrated in older individuals.In L. radiatasiliquoidea and L. vcntricosa, copper concentrationswere higher in males than in females. Lead levels were alsohigher in L. vcntricosamales. (Received 24 June 1994; accepted 8 September 1994)     

The role of major histocompatibility complex diversity in vigour of fish males (Abramis brama L.) and parasite selection

The major histocompatibility complex (MHC) presents a group of genes with highly polymorphic loci involved in specific immune responses. The factors maintaining extensive MHC polymorphism have been questioned, considering three possible hypotheses of parasite‐mediated selection driving an extensive MHC diversity (i.e. heterozygote advantage, rare‐allele advantage, and favouring optimal MHC diversity). The patterns of MHC diversity of class IIB genes were investigated following two noncontradicting hypotheses, parasite‐driven selection and MHC‐based mating preferences, using males of common bream collected in the spawning period. Two allelic groups DAB1 and DAB3 were recognized from the phylogenetic analyses. Individuals expressed one or two alleles of the same or different allelic groups. Several individuals shared identical alleles; however, the presence of parasite species was not associated with the occurrence of a particular allele. The presence of different allelic groups (only DAB1, only DAB3, or both DAB1 and DAB3) in individuals was not associated with parasite presence or diversity. The expression of two DAB1 alleles was associated with higher endoparasite abundance. Moreover, nucleotide diversity in individuals expressing a single type of alleles (DAB1 or DAB3) increased with the abundance of ectoparasitic Dactylogyrus spp. (Monogenea) and Ergasilus sp. (Crustacea). This suggests that the expression of two alleles of a single allelic type is related to high metazoan parasite infection whereas no significant influence of parasitism on the combined allelic form (the presence of both DAB1 and DAB3 alleles) was found. Moreover, the expression of two alleles of a single allelic type was related to decreased immunocompetence measured by spleen size. The condition factor was higher in fish expressing the combined allelic type. Thus, the presence of alleles of different lineages in individuals appears to be advantageous for individual male fitness. The expression of a single allelic type was related to higher sexual ornamentation, which could support the role of MHC in the hypothesis of the sexual selection of ‘good genes’. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 90 , 525–538.     

Impact of Livestock Husbandry on Small- and Medium-Sized Carnivores in Kalahari Savannah Rangelands

ABSTRACT We analyzed relative sensitivities of small- and medium-sized carnivores to livestock husbandry (stocking rates and predator control) in Kalahari, South Africa, rangelands at a regional scale. We monitored small carnivores using track counts on 22 Kalahari farms across a land-use gradient ranging from low to high stocking rates and also interviewed each farm manager to identify farmers' perception of small carnivores as potential predators for livestock. We recorded 12 species of small- and medium-sized carnivores across 22 Kalahari farms. Stocking rate was the most important driving variable for local carnivore abundance. Abundance of all species was lowest on farms where stocking rate was high. Most farm managers perceived medium-sized carnivores, in particular, African wildcat (Felis silvestris lybica), black-backed jackal (Canis mesomelas), and caracal (Caracal caracal), as potential predators of livestock. Multiple regression analysis shows that black-backed jackal, African wildcat, and caracal were negatively affected by predator control measures, whereas bat-eared fox (Otocyon megalotis), cape fox (Vulpes chama), and small-spotted genet (Genetta genetta) were positively affected. Our results show a need for expanding research and conservation activities toward small- and medium-sized carnivores in southern African savannah rangelands. We, therefore, suggest developing a monitoring program combining passive tracking with indigenous knowledge of local Khoisan Bushmen to monitor carnivore populations, and we recommend additional predator removal experiments that manipulate predator densities.     

Impact of habitat fragmentation on the genetics of populations in dendritic landscapes

1. The effect of habitat fragmentation on freshwater species has been addressed using brown trout Salmo trutta L. as a model species with a dendritic population structure. 2. Microsatellite loci were employed as molecular markers. Levels of gene flow and population subdivision were determined in more than 1200 brown trout individuals inhabiting four south European rivers with contrasting patterns of fragmentation, defined by the presence of barriers. 3. The genetic units in the four rivers were restricted by artificial barriers, and gene flow among samples within each river was associated with the level of fragmentation of the river. 4. Loss of genetic diversity and dislocation of the dendritic model have been detected in fragmented rivers. These results emphasise the importance of mitigating the impact of dams by constructing passages to restore gene flow along the river, for fish and other migratory species, as well as the need for caution in relation to stocking in isolated areas to avoid problems of inbreeding.     

The influence of large wood on brown trout (Salmo trutta) behaviour and surface foraging

1. Changes in riparian vegetation owing to forest harvesting may affect the input of large wood, a major structural element, to streams. Studies of large wood impacts on stream fish have focused on population‐level responses, whereas little attention has been given to how wood affects fish behaviour. 2. In a laboratory stream experiment, we tested how two size classes of brown trout, Salmo trutta, (mean size of 85 and 125 mm), alone and together, responded to a gradient of large wood in terms of activity, foraging on terrestrial drift and interactions between conspecifics. 3. The results showed that the presence of large wood significantly reduced the overall activity of the fish, the number of agonistic interactions between individuals and the proportion of captured prey. However, activity decreased relatively more than the proportion of captured prey, resulting in a significant positive net effect of wood on the number of prey captures per time spent active (PTA). This indicates that trout living in habitats with high wood density may have a higher net energy gain than trout living in habitats with less wood. 4. There were no observable size‐class differences in the benefits of large wood or in the utilisation of surface‐drifting terrestrial prey. 5. These results suggest that the presence of large wood may be an important factor shaping stream communities and that a lack of structural complexity may decrease energy gain, increase agonistic interactions and, consequently, lower the production of brown trout.