Biomedical subjects

S M Sait

Publications and source records attributed to S M Sait.

5 recordsLinked to original sources

PubMed2005

Phenotypic and genotypic characterisation of persistent baculovirus infections in populations of the cabbage moth (Mamestra brassicae) within the British Isles.

The genotypic relatedness of persistent baculovirus infections within UK populations of Mamestra brassicae was assessed by sequencing amplified regions from polyhedrin and ie1. Most populations harboured Mamestra brassicae (Mb) nucleopolyhedrosis virus (NPV) which showed very little genotypic variation between populations. However, one population harboured a virus that closely resembled a baculovirus found previously only in Pine Beauty Moth (Panolis flammea) populations in Scotland. Persistent baculoviruses that had emerged spontaneously as lethal, overt infections from two of the insect populations were compared with the type strain of MbNPV and a mixture of P. flammea (Pafl) NPV strains, isolated from a single host, by bioassay in virus-free Spodoptera exigua larvae. Reactivated baculoviruses were as pathogenic as the stock virus and showed phenotypic characteristics closest to the type strain they most resembled genetically. Sequence data from the insect host cytochrome oxidase genes were compared and showed a high degree of sequence conservation between populations and it was not possible to determine whether the persistent baculovirus infections had arisen on many occasions or whether they represented a single initial infection that had spread with the host. However, the presence of two distinct virus genotypes in separate M. brassicae populations suggests multiple colonisations of the host are a possibility.

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PubMed2002

Vertical transmission of sublethal granulovirus infection in the Indian meal moth, Plodia interpunctella.

Knowledge of the mechanisms of pathogen persistence in relation to fluctuations in host density is crucial to our understanding of disease dynamics. In the case of insect baculoviruses, which are typically transmitted horizontally via a lifestage that can persist outside the host, a key issue that remains to be elucidated is whether the virus can also be transmitted vertically as a sublethal infection. We show that RNA transcripts for the Plodia interpunctella GV granulin gene are present in a high proportion of P. interpunctella insects that survive virus challenge. Granulin is a late-expressed gene that is only transcribed after viral genome replication, its presence thus strongly indicates that viral genome replication has occurred. Almost all insects surviving the virus challenge tested positive for viral RNA in the larval and pupal stage. However, this proportion declined in the emerging adults. Granulin mRNA was also detected in both the ovaries and testes, which may represent a putative mechanism by which reduced fecundity in sublethally affected hosts might be manifested. RNA transcripts were also detected in 60-80% of second-generation larvae that were derived from mating surviving adults, but there was no difference between the sexes, with both males and females capable of transmitting a sublethal infection to their offspring. The data indicate that low-level persistent infection, with at least limited gene expression, can occur in P. interpunctella following survival of a granulovirus challenge. We believe that this is the first demonstration of a persistent, sublethal infection by a baculovirus to be initiated by a sublethal virus dose. We hypothesize that the 'latent' baculovirus infections frequently referred to in the literature may also be low level persistent, sublethal infections resulting from survival from initial baculovirus exposure.

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PubMed2001

The impact of specialized enemies on the dimensionality of host dynamics.

Although individual species persist within a web of interactions with other species, data are usually gathered only from the focal species itself. We ask whether evidence of a species' interactions be detected and understood from patterns in the dynamics of that species alone. Theory predicts that strong coupling between a prey and a specialist predator/parasite should lead to an increase in the dimensionality of the prey's dynamics, whereas weak coupling should not. Here we describe a rare test of this prediction. Two natural enemies were added separately to replicate populations of a moth. For biological reasons that we identify here, the prediction of increased dimensionality was confirmed when a parasitoid wasp was added (although this increase had subtleties not previously appreciated), but the prediction failed for an added virus. Thus, an imprint of the interactions may be discerned within time-series data from component species of a system.

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PubMed2001

Behavior of a recombinant baculovirus in lepidopteran hosts with different susceptibilities.

Insect pathogens, such as baculoviruses, that are used as microbial insecticides have been genetically modified to increase their speed of action. Nontarget species will often be exposed to these pathogens, and it is important to know the consequences of infection in hosts across the whole spectrum of susceptibility. Two key parameters, speed of kill and pathogen yield, are compared here for two baculoviruses, a wild-type Autographa californica nucleopolyhedrovirus (AcNPV), AcNPV clone C6, and a genetically modified AcNPV which expresses an insect-selective toxin, AcNPV-ST3, for two lepidopteran hosts which differ in susceptibility. The pathogenicity of the two viruses was equal in the less-susceptible host, Mamestra brassicae, but the recombinant was more pathogenic than the wild-type virus in the susceptible species, Trichoplusia ni. Both viruses took longer to kill the larvae of M. brassicae than to kill those of T. ni. However, whereas the larvae of T. ni were killed more quickly by the recombinant virus, the reverse was found to be true for the larvae of M. brassicae. Both viruses produced a greater yield in M. brassicae, and the yield of the recombinant was significantly lower than that of the wild type in both species. The virus yield increased linearly with the time taken for the insects to die. However, despite the more rapid speed of kill of the wild-type AcNPV in M. brassicae, the yield was significantly lower for the recombinant virus at any given time to death. A lower yield for the recombinant virus could be the result of a reduction in replication rate. This was investigated by comparing determinations of the virus yield per unit of weight of insect cadaver. The response of the two species (to both viruses) was very different: the yield per unit of weight decreased over time for M. brassicae but increased for T. ni. The implications of these data for risk assessment of wild-type and genetically modified baculoviruses are discussed.

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PubMed2000

Invasion sequence affects predator-prey dynamics in a multi-species interaction.

Ecologists seek to understand the rules that govern the assembly, coexistence and persistence of communities of interacting species. There is, however, a variety of sequences in which a multi-species community can be assembled--unlike more familiar one- and two-species systems. Ecological systems can exhibit contrasting dynamics depending on initial conditions, but studies have been focused on simple communities initiated at different densities, not on multi-species communities constructed in different sequences. Investigations of permanence and convergence in ecological communities have been concerned with the flux of whole species (presence or absence) but have not addressed the central issues concerning the dynamics exhibited by individual species in particular interactions. Here we examine data for replicated three-species systems and demonstrate that the dynamic trajectories of both a predator and its prey within the system are determined by the sequence in which it is constructed, and that for one construction-sequence alternative dynamic patterns are possible.

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