PubMed Health⌕ Search

Biomedical subjects

H D Loxdale

Publications and source records attributed to H D Loxdale.

10 recordsLinked to original sources
PubMed2006

Do distantly related parasites rely on the same proximate factors to alter the behaviour of their hosts?

Phylogenetically unrelated parasites often increase the chances of their transmission by inducing similar phenotypic changes in their hosts. However, it is not known whether these convergent strategies rely on the same biochemical precursors. In this paper, we explored such aspects by studying two gammarid species (Gammarus insensibilis and Gammarus pulex; Crustacea: Amphipoda: Gammaridae) serving as intermediate hosts in the life cycle of two distantly related parasites: the trematode, Microphallus papillorobustus and the acanthocephalan, Polymorphus minutus. Both these parasite species are known to manipulate the behaviour of their amphipod hosts, bringing them towards the water surface, where they are preferentially eaten by aquatic birds (definitive hosts). By studying and comparing the brains of infected G. insensibilis and G. pulex with proteomics tools, we have elucidated some of the proximate causes involved in the parasite-induced alterations of host behaviour for each system. Protein identifications suggest that altered physiological compartments in hosts can be similar (e.g. immunoneural connexions) or different (e.g. vision process), and hence specific to the host-parasite association considered. Moreover, proteins required to alter the same physiological compartment can be specific or conversely common in both systems, illustrating in the latter case a molecular convergence in the proximate mechanisms of manipulation.

Acanthocephala↗
PubMed2005

Behavioural manipulation in a grasshopper harbouring hairworm: a proteomics approach.

The parasitic Nematomorph hairworm, Spinochordodes tellinii (Camerano) develops inside the terrestrial grasshopper, Meconema thalassinum (De Geer) (Orthoptera: Tettigoniidae), changing the insect's responses to water. The resulting aberrant behaviour makes infected insects more likely to jump into an aquatic environment where the adult parasite reproduces. We used proteomics tools (i.e. two-dimensional gel electrophoresis (2-DE), computer assisted comparative analysis of host and parasite protein spots and MALDI-TOF mass spectrometry) to identify these proteins and to explore the mechanisms underlying this subtle behavioural modification. We characterized simultaneously the host (brain) and the parasite proteomes at three stages of the manipulative process, i.e. before, during and after manipulation. For the host, there was a differential proteomic expression in relation to different effects such as the circadian cycle, the parasitic status, the manipulative period itself, and worm emergence. For the parasite, a differential proteomics expression allowed characterization of the parasitic and the free-living stages, the manipulative period and the emergence of the worm from the host. The findings suggest that the adult worm alters the normal functions of the grasshopper's central nervous system (CNS) by producing certain 'effective' molecules. In addition, in the brain of manipulated insects, there was found to be a differential expression of proteins specifically linked to neurotransmitter activities. The evidence obtained also suggested that the parasite produces molecules from the family Wnt acting directly on the development of the CNS. These proteins show important similarities with those known in other insects, suggesting a case of molecular mimicry. Finally, we found many proteins in the host's CNS as well as in the parasite for which the function(s) are still unknown in the published literature (www) protein databases. These results support the hypothesis that host behavioural changes are mediated by a mix of direct and indirect chemical manipulation.

Animals↗
PubMed2004

Evidence for gene flow and local clonal selection in field populations of the grain aphid (Sitobion avenae) in Britain revealed using microsatellites.

Samples of the grain aphid, Sitobion avenae (F.), a major European pest of cereals, were collected in June and July 1997 from fields sown with winter wheat in a rough transect south-west of Rothamsted, UK. These aphids were genotyped at four microsatellite loci known from previous studies to be highly polymorphic. Allelic frequencies were similar between samples collected in the fields and in the 12.2 m high suction trap at Rothamsted, and there were many widespread genotypes (clones), providing evidence that the species is highly migratory. However, field samples were found to display a high level of genotypic heterogeneity (= variable clonal composition), most probably the result of clonal selection. The suction trap genotypes sample were slightly different from the field samples, indicative of the inclusion of genotypes from plant hosts (cereals and grasses, Poaceae) other than winter wheat and/or genotype-biased emigration from the field. The relevance of these data to modelling of aphid outbreaks is briefly discussed.

Animals↗
PubMed2003

Migration and genetic structure of the grain aphid (Sitobion avenae) in Britain related to climate and clonal fluctuation as revealed using microsatellites.

Genetic structuring of populations reflects the interaction of genetic drift, mutation, migration and selection, with influences from life history. Aphids are interesting in this regard as they have the potential for unusually high levels of dispersal and natural selection, which typically counter each other. In the present study, winged grain aphids Sitobion avenae (F.) were collected in four 12.2-m high suction traps along a north-south transect in Britain in order to eliminate sampling bias from plant hosts (cereals and grasses; Poaceae), it being known that these insects show host adaptation demonstrable using molecular markers. Samples were analysed at four polymorphic microsatellite loci over two consecutive years. Population allele frequencies were similar nationally during the two years, although clonal diversity varied greatly between sites and years. In the first sampling year following a harsh winter, diversity was found to display a latitudinal clinal trend: the proportion of unique clones (genotypes) increased with latitude. However, this pattern was less apparent the following year, after a milder winter. Nonetheless, overall FST analysis showed that there was little spatial genetic structuring in either sampling year. These data support the view that the insect is highly migratory and also support a theoretical model and previous data suggesting that the reproductive mode is clinal in S. avenae. This appears to be because natural selection (reduced reproductive success of asexual genotypes under cold conditions) is sufficiently powerful to overcome the homogenizing effects of strong migration. There was no clear evidence for isolation by distance for the genetic data obtained. The data are compared with similar data from other aphid species and other insects. Only by the collection of such data sets can an accurate picture be built up relating genetic variability to flight behaviour, including migratory ambit in this group of insects since, due to their small size and rapid dilution in the air, other marking approaches are impracticable over large geographical distances.

Animal Migration↗
PubMed2003

Genetic variation and founder effects in the parasitoid wasp, Diaeretiella rapae (M'intosh) (Hymenoptera: Braconidae: Aphidiidae), affecting its potential as a biological control agent.

The effects of recent colonization on the aphid parasitoid, Diaeretiella rapae (M'Intosh) (Hymenoptera: Braconidae: Aphidiidae), in Western Australia were investigated. When compared with populations from the Old World, the results of a microsatellite analysis show that the insects have low allelic length and low allele frequency variation, revealing that these individuals experienced a significant founder effect. Marked genetic differentiation between populations was also revealed, which has potentially important implications for host utilization in this species when introduced to a new geographical area(s). Low genetic variation and gene flow in a founder population could limit evolutionary potential in Australia, including the ability of a population to mount a response to newly introduced hosts, such as the Russian wheat aphid, Diuraphis noxia (Mordvilko). Although the actual importance of genetic diversity in the success of biological control agents is unclear, current theory concerning the potential impact of genetic bottlenecks on additive genetic variance is discussed.

Animals↗
PubMed2002

Host-based genotype variation in insects revisited.

Elucidation of the genetic variability of a model insect species, the grain aphid, Sitobion avenae (Fabricius), a predominantly asexual herbivore within the temperate agro-ecosystem tested, was initiated using molecular DNA markers (RAPDs). This revealed genetic profiles that appeared related to host adaptation at the specific level amongst the natural populations colonizing different grasses and cereals (Poaceae) within the same geographic location. These profiles were recorded either as 'specialist' genotypes found on specific grasses, or as 'generalist' genotypes colonizing several host types including cultivated cereals or native grasses. These findings are compared with analogous systems found amongst insect species, including at a higher trophic level, i.e. interactions between hymenopterous aphid parasitoids. As the aphids and their respective plant hosts occur in the same geographical region at the same time, this appears to be a rare example of the evolutionary transition leading to sympatric speciation in insects. Hence, this study highlights the importance of understanding not only the demographic parameters to genetic diversity, but also the more intricate correlation of genetic diversity to host types in agricultural environments.

Animals↗
PubMed1997

Genotypic variation among different phenotypes within aphid clones.

Most aphid species Hemiptera: Aphididae are parthenogenetic between periods of sexual reproduction. They are also highly polyphenic, with different adult morphs occurring in the life cycle, piz. winged, wingless, asexual and sexual. It is assumed that aphids born in a parthenogenetic clonal lineage are genetically identical regardless of the final adult form with the exception of sexual forms). Using the randomly amplified polymorphic DNA-polymerase chain reaction (RAPD-PCR) we have found that different asexual adult phenotypes winged and wingless of some clones of two cereal aphid species (the grain aphid, Sitobion avenae (F.) and the bird-cherry aphid. Rhopalosiphum padi (L.) may be distinguished by the presence or absence of one or more RAPD-PCR bands. In three of nine clones examined, such differences were found, and Southern blotting and hybridization of the discriminating bands confirmed these to be of aphid origin, rather than due to endosymbiotic bacteria or contaminating fungi. The main 248 and 296 bp bands, in the two species respectively, were sequenced and found to be A/T rich. The smaller band showed 57% homology with white striated muscle over a stretch of 90 bp. Genomic DNA treated with dimethyl sulphoxide to remove secondary structures still showed differences in RAPD-PCR profiles between winged and wingless morphs within the unusual clones. This discovery may be widespread and therefore it is important to understand the phenomenon in relation to clonal organisms.

Animals↗
PubMed1985

Dissociation between mechanical performance and the cost of isometric tension maintenance in Lethocerus flight muscle.

A method for the continuous measurement of ATP hydrolysis (ATPase) by demembranated muscle fibres has been applied to isometrically held, glycerol-extracted flight muscle fibres from the water-bug Lethocerus, under conditions of high MgATP, neutral pH, and varying ionic strength, Ca2+ and extension. These variables caused parallel changes in isometric tension and ATPase. The slope of ATPase upon tension (delta ATPase/delta Tension; incremental tension cost) remained the same upon extension at either different Ca2+ concentrations or different ionic strength. Isometric activation by Ca2+ gave a higher incremental tension cost. The calculated mechanical rate constant of the work-producing process, measured by the small-amplitude behaviour, was increased by either Ca2+ or ionic strength, and little changed by extension; there was therefore a dissociation between its value and that of the incremental tension cost. The results appear to exclude a two-state crossbridge model for fibrillar insect flight muscle.

Adenosine Triphosphate↗
PubMed1983

Generation of tension by glycerol-extracted vertebrate skeletal muscle fibres in the absence of calcium.

When a small bundle of glycerol-extracted fibres from either frog, tortoise or rabbit skeletal muscle was first exposed to high MgATP (5 mM) in the absence of Ca2+ (less than 1 nM) and at low ionic strength (less than 0.11) at 20 degrees C, it produced a single sharp transient contraction followed by a lower maintained isometric tension. The maintained tension was investigated further in rabbit psoas fibres. Ca2+-free tension was dependent on the ionic strength in the range 0.04-0.10, on the temperature in the range 6-20 degrees C and the free Mg2+ in the range 0-6 mM. It was promoted by low ionic strength, low Mg2+ and high temperature, and was unaffected by varying the MgATP2- in the range 0.4-4 mM and by adding ATP regenerating components. A separate regime of tension generation was detected at MgATP2- concentrations of less than 0.1 mM, in which MgATP2- concentration was critical. The results are interpreted on the assumption that binding of Mg2+ to some component of the regulatory system is necessary to maintain its inhibitory effect in the absence of Ca2+. Ionic strength and temperature, on the other hand, may affect actomyosin directly.

Adenosine Triphosphatases↗