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Biomedical subjects

G D Hurst

Publications and source records attributed to G D Hurst.

At least 19 recordsLinked to original sources
PubMed2001

Recombination confounds interpretations of Wolbachia evolution.

Wolbachia are vertically transmitted bacteria known from arthropods and nematode worms, which are maintained in host populations because they either physiologically benefit infected individuals or parasitically manipulate their reproduction. The different manipulation phenotypes are scattered across the Wolbachia phylogeny, suggesting that there have been multiple evolutions of similar phenotypes. This conclusion relies on the assumption of an absence of recombination between bacterial strains, so that the gene used to reconstruct the phylogeny reflects the evolutionary history of the genes involved in the trait. We tested for recombination by reconstructing the phylogeny of two Wolbachia genes from seven B-subdivision strains. The two genes produced mutually incompatible topologies, indicating that these lineages are subject to genetic recombination. This means that many evolutionary patterns inferred from Wolbachia phylogenies must be re-evaluated. Furthermore, recombination may be an important feature both in the evolution of the manipulation phenotypes and avoidance of Müller's ratchet. Finally, we discuss the implications of recombination for attempts to genetically engineer Wolbachia for use in the control of crop pests and human pathogens.

Biological Evolution↗
PubMed2001

How many species are infected with Wolbachia? Cryptic sex ratio distorters revealed to be common by intensive sampling.

Inherited bacterial symbionts from the genus Wolbachia have attracted much attention by virtue of their ability to manipulate the reproduction of their arthropod hosts. The potential importance of these bacteria has been underlined by surveys, which have estimated that 17% of insect species are infected. We examined whether these surveys have systematically underestimated the proportion of infected species through failing to detect the low-prevalence infections that are expected when Wolbachia distorts the sex ratio. We estimated the proportion of species infected with Wolbachia within Acraea butterflies by testing large collections of each species for infection. Seven out of 24 species of Acraea were infected with Wolbachia. Four of these were infected with Wolbachia at high prevalence, a figure compatible with previous broad-scale surveys, whilst three carried low-prevalence infections that would have had a very low likelihood of being detected by previous sampling methods. This led us to conclude that sex-ratio-distorting Wolbachia may be common in insects that have an ecology and/or genetics that permit the invasion of these parasites and that previous surveys may have seriously underestimated the proportion of species that are infected.

Animals↗
PubMed2001

Inherited microorganisms, sex-specific virulence and reproductive parasitism.

Parasites show an amazing repertoire of adaptations, highlighted by complex life cycles that allow both survival in the host and transmission among hosts. However, there is one heterogeneous group of microorganisms whose adaptations are perhaps even more surprising: parthenogenesis induction, feminization of genetic males, killing of male hosts and sperm-mediated sterilization of uninfected eggs. The common feature of these microorganisms is their mode of transmission: inheritance from mother to offspring. Here, we present an introduction to hereditary symbiosis, focusing on microsporidia and bacteria that manipulate host reproduction in arthropods (reproductive parasites). We also discuss the implications of one of these microorganisms, Wolbachia, for the control of arthropod pests and vectors and for the therapy of filarial diseases. Finally, we discuss whether some parasites of vertebrates might show sex-specific virulence.

Animals↗
PubMed2001

The role of selfish genetic elements in eukaryotic evolution.

'Selfish genetic elements', such as transposons, homing endonucleases, meiotic drive chromosomes and heritable microorganisms, are common features of eukaryotes. However, their importance in the evolution of eukaryotic genomes is still controversial. In this review, we discuss these diverse elements and their potential importance in the evolution of genetic systems, adaptation, and the extinction and birth of species.

Animals↗
PubMed2001

Two male-killing Wolbachia strains coexist within a population of the butterfly Acraea encedon.

Inherited bacteria that kill male hosts early in their development are known from five insect orders. We ask to what extent the incidence of male-killers might be restricted by the rate at which new host-parasite interactions arise, by testing whether multiple male-killers have invaded a single host species. In Uganda, the butterflies Acraea encedon and A. encedana are both infected by the same strain of male-killing Wolbachia and there was no evidence of variation within the population. In Tanzanian A. encedon however, two phylogenetically distinct strains of male-killing Wolbachia were found within the same population. If this pattern of male-killer polymorphism is found to be general across infected species, it suggests that new male-killing infections arise frequently on an evolutionary time scale. Whether this polymorphism is stable, and what forces may be maintaining it, are unknown.

Animals↗
PubMed2001

What causes inefficient transmission of male-killing Wolbachia in Drosophila?

Selfish genetic elements that distort the sex ratio are common in arthropods. Theory predicts they will invade and spread to fixation if they are vertically transmitted with perfect fidelity, potentially leading to host extinction. For inherited microorganisms that distort the sex ratio, inefficient vertical transmission or incomplete sex ratio distorting ability is required for host persistence. However, the relative roles of genetic and environmental factors in permitting the survival of male hosts and preventing parasite transmission are poorly understood. We examined the causes of transmission infidelity and male survival for a male-killing Wolbachia strain in Drosophila bifasciata. Under standard laboratory conditions (18 degrees C), in its standard genetic background, males are produced very rarely, and no case of reversion has been observed in 20 generations of laboratory rearing. To investigate the role of host genetic factors, Wolbachia was crossed into 27 different inbred lines of D. bifasciata, but in no case was reversion observed at preferred environmental temperatures. The role of elevated temperature in inducing inefficient transmission was examined. Whilst vertical transmission was perfect over three generations of maintenance at 23.5 degrees, transmission infidelity was observed at 25 degrees. We conclude that there is no evidence for the presence of either fixed or polymorphic host genes that repress transmission at standard environmental temperatures. However, severe temperature treatment does make vertical transmission imperfect. We suggest that the case of Wolbachia in D. bifasciata is one that is naturally balanced, the population being maintained polymorphic without the evolution of host resistance genes.

Animals↗
PubMed2001

Extreme length and length variation in the first ribosomal internal transcribed spacer of ladybird beetles (Coleoptera: Coccinellidae).

DNA sequences of the first ribosomal internal transcribed spacer (ITS1) were isolated from 10 ladybird beetle species (Coleoptera: Coccinellidae) representing four subfamilies (Coccinellinae, Chilocorinae, Scymninae, and Coccidulinae). The spacers ranged in length from 791 to 2,572 bp, thereby including one of the longest ITS1s and exhibiting one of the most extreme cases of ITS1 size variation in eukaryotes recorded to date. The causes of length variation were therefore analyzed. Almost no putatively homologous sequence similarities were identified for the taxa included. The only exception was for the subfamily Coccinellinae, which yielded sequence similarities in six regions of approximately 550 nucleotide positions, primarily at the 5' and 3' ends of ITS1. The majority of differences in ITS1 length between taxa could be attributed to the presence of repetitive elements with comparatively long repeat units. Repetition arose several times independently and was confined to the middle of the spacer which, in contrast to the 5' and 3' ends, had not been inferred in previous studies to be subject to functional constraints. These elements were characterized by high rates of evolutionary change, most likely as a result of high substitution rates in combination with inefficient homogenization across repeats. The repeated origin and subsequent divergence of "long" repetitive elements should thus be assumed to be an important factor in the evolution of coccinellid ITS1.

Amino Acid Sequence↗
PubMed2001

Incidence of male-killing Rickettsia spp. (alpha-proteobacteria) in the ten-spot ladybird beetle Adalia decempunctata L. (Coleoptera: Coccinellidae).

The diversity of endosymbiotic bacteria that kill male host offspring during embryogenesis and their frequencies in certain groups of host taxa suggest that the evolution of male killing and the subsequent spread of male-killing symbionts are primarily determined by host life history characteristics. We studied the 10-spot ladybird beetle, Adalia decempunctata L. (Coleoptera: Coccinellidae), in which male killing has not been recorded previously, to test this hypothesis, and we also assessed the evolution of the male killer identified by DNA sequence analysis. Our results show that A. decempunctata harbors male-killing Rickettsia (alpha-proteobacteria). Male-killing bacteria belonging to the genus Rickettsia have previously been reported only for the congeneric two-spot ladybird beetle, Adalia bipunctata L. Phylogenetic analysis of Rickettsia DNA sequences isolated from different populations of the two host species revealed a single origin of male killing in the genus Rickettsia. The data also indicated possible horizontal transfer of symbionts between host species. In addition, A. bipunctata is known to bear at least four different male-killing symbionts in its geographic range two of which coexist in the two locations from which A. decempunctata specimens were obtained for the present study. Since only a single male-killing taxon was found in A. decempunctata, we assume that the two closely related ladybird beetle species must differ in the number and/or geographic distribution of male killers. We discuss the importance of these findings to our understanding of the evolution and dynamics of symbiotic associations between male-killing bacteria and their insect hosts.

Animals↗
PubMed2000

Sex-ratio-distorting Wolbachia causes sex-role reversal in its butterfly host.

Sex-role-reversed mating systems in which females compete for males and males may be choosy are usually associated with males investing more than females in offspring. We report that sex-role reversal may also be caused by selfish genetic elements which distort the sex ratio towards females. Some populations of the butterflies Acraea encedon and Acraea encedana are extremely female biased because over 90% of females are infected with a Wolbachia bacterium that is maternally inherited and kills male embryos. Many females in these populations are virgins suggesting that their reproductive success may be limited by access to males. These females form lekking swarms at landmarks in which females exhibit behaviours which we interpret as functioning to solicit matings from males. The hypothesis that female A. encedon swarm in order to mate is supported by the finding that, in release recapture experiments, mated females tend to leave the swarm while unmated females remained. This behaviour is a sex-role-reversed form of a common mating system in insects in which males form lekking swarms at landmarks and compete for females. Female lekking swarms are absent from less female-biased populations and here the butterflies are instead associated with resources in the form of the larval food plant.

Animals↗
PubMed2000

Evolutionary lability of context-dependent codon bias in bacteria.

In bacteria, synonymous codon usage can be considerably affected by base composition at neighboring sites. Such context-dependent biases may be caused by either selection against specific nucleotide motifs or context-dependent mutation biases. Here we consider the evolutionary conservation of context-dependent codon bias across 11 completely sequenced bacterial genomes. In particular, we focus on two contextual biases previously identified in Escherichia coli; the avoidance of out-of-frame stop codons and AGG motifs. By identifying homologues of E. coli genes, we also investigate the effect of gene expression level in Haemophilus influenzae and Mycoplasma genitalium. We find that while context-dependent codon biases are widespread in bacteria, few are conserved across all species considered. Avoidance of out-of-frame stop codons does not apply to all stop codons or amino acids in E. coli, does not hold for different species, does not increase with gene expression level, and is not relaxed in Mycoplasma spp., in which the canonical stop codon, TGA, is recognized as tryptophan. Avoidance of AGG motifs shows some evolutionary conservation and increases with gene expression level in E. coli, suggestive of the action of selection, but the cause of the bias differs between species. These results demonstrate that strong context-dependent forces, both selective and mutational, operate on synonymous codon usage but that these differ considerably between genomes.

Codon↗
PubMed2000

The butterfly Danaus chrysippus is infected by a male-killing Spiroplasma bacterium.

Many insects carry maternally inherited bacteria which kill male offspring. Such bacteria will spread if male death benefits the female siblings who transmit the bacterium, and they are therefore expected in insects with antagonistic sibling interactions. We report that the butterfly Danaus chrysippus is host to a maternally inherited male-killing bacterium. Using diagnostic PCR and rDNA sequence, the bacterium was identified as a Spiroplasma closely related to 2 ladybird beetle male-killers and the tick symbiont Spiroplasma ixodetis. The male-killer was found to have a geographically restricted distribution, with up to 40% of females being infected in East Africa, but no detectable infection in small samples from other populations. Danaus chrysippus is a surprising host for a male-killer as its eggs are laid singly. This suggests that the ecological conditions permitting male-killers to invade may be more widespread than previously realized.

Animals↗
PubMed2000

Male-killing Wolbachia in Drosophila: a temperature-sensitive trait with a threshold bacterial density.

Inherited microorganisms that disturb the reproduction of their host have been characterized from a number of host taxa. To understand the general principles underlying the genetic and mechanistic basis of interactions, study of different agents in model host species is required. To this end, the nature and genetics of the maternally inherited sex-ratio trait of Drosophila bifasciata were investigated. Successful curing of affected lines with antibiotics demonstrated this trait was associated with the presence of a bacterium, and molecular systematic analysis demonstrated an association between the presence of the trait and infection with an A group Wolbachia. The penetrance and heritability of the trait did not vary with maternal age. Exposure to elevated temperatures did reduce trait penetrance but did not affect heritability. Examination of the effect of temperature on bacterial density in eggs revealed a decrease in bacterial density following exposure of the parent to elevated temperature, consistent with the hypothesis that male killing in D. bifasciata requires a threshold density of Wolbachia within eggs. The male offspring produced following exposure to elevated temperatures were infected with Wolbachia on emergence as adults. Crossing studies demonstrated a weak cytoplasmic incompatibility phenotype exhibited by Wolbachia in these males. The results are discussed with respect to the incidence of male killing within the clade Wolbachia, the general nature of Wolbachia-host interactions, and the prospects for using this association to investigate the mechanism of male killing.

Animals↗
PubMed2000

Molecular evolution and phylogenetic utility of Wolbachia ftsZ and wsp gene sequences with special reference to the origin of male-killing.

A detailed assessment of the evolution and phylogenetic utility of two genes, ftsZ and wsp, was used to investigate the origin of male-killing Wolbachia, previously isolated from the ladybird Adalia bipunctata and the butterfly Acraea encedon. The analysis included almost all available sequences of B-group Wolbachia and two outgroup taxa and showed that (1) the two gene regions differ in phylogenetic utility, (2) sequence variation is here correlated with phylogenetic information content, (3) both genes show significant rate heterogeneity between lineages, (4) increased substitution rates are associated with homoplasy in the data, (5) wsp sequences of some taxa appear to be subject to positive selection, and (6) only a limited number of clades can be inferred with confidence due to either lack of phylogenetic information or the presence of homoplasy. With respect to the evolution of male-killing, the two genes nevertheless seemed to provide unbiased information. However, they consistently produce contradictory results. Current data therefore do not permit clarification of the origin of this behavior. In addition, A. bipunctata was found to be a host to two recently diverged strains of male-killing Wolbachia that showed increased substitution rates for both genes. Moreover, the wsp gene, which codes for an outer membrane protein, was found to be subject to positive selection in these taxa. These findings were postulated to be the product of high selection pressures due to antagonistic host-symbiont interactions in this ladybird species. In conclusion, our study demonstrates that the results of a detailed phylogenetic analysis, including characterization of the limitations of such an approach, can serve as a valuable basis for an understanding of the evolution of Wolbachia bacteria. Moreover, particular features of gene evolution, such as elevated substitution rates or the presence of positive selection, may provide information about the dynamics of Wolbachia-host associations.

Bacterial Outer Membrane Proteins↗
PubMed1999

Adonia variegata (Coleoptera: Coccinellidae) bears maternally inherited flavobacteria that kill males only.

Inherited bacteria that parasitically distort the pattern of sex allocation of their host, biasing allocation towards female progeny, are found in many arthropods. One such manipulation is male-killing, where male progeny of infected females die during embryogenesis. We here provide evidence for a male-killing bacterium in the coccinellid beetle, Adonia variegata. We then address 3 questions. First, is this male-killing bacterium one that is found in other hosts, or does it represent a new transition to male-killing within the eubacteria? Using the sequence of the 16S rDNA of the bacterium, we found that the male-killing bacterium is a member of the Flavobacteria--Bacteroides group, most closely related to the male-killing bacterium in another ladybird beetle, Coleomegilla maculata. Secondly, is there any evidence that this bacterium affects female host physiology? In a paired test under nutritional stress, we found no evidence for a physiological benefit to infection, and weak evidence of a physiological cost, in terms of reduced fecundity. Thirdly, is there any evidence of host involvement in the transmission of the bacterium to the germ line? We found no evidence of host involvement. Rather, bacteria migrated to the ovariole independently of host cells. We conclude that the bacterium is a parasite, and discuss how 2 different species of ladybird come to be infected with 1 lineage of bacterium, and why case studies of male-killing bacteria have generally found little evidence of any symbiont contribution to host physiological functioning.

Animals↗
PubMed1999

Invasion of one insect species, Adalia bipunctata, by two different male-killing bacteria.

Male-killing bacteria, which are inherited through the female line and kill male progeny only, are known from five different orders of insect. Our knowledge of the incidence of these elements has stemmed from discovery of their phenotype in different species. Our estimate of the frequency with which insects have been invaded by these elements therefore depends on each observation of the male-killing phenotype within a species being associated with a single microorganism. We here record an example of a single insect species being infected with two taxonomically distinct male-killing bacteria. Western European populations of the two-spot ladybird, Adalia bipunctata, have previously been shown to bear a male-killing Rickettsia. However, we here show that the majority of the male-killing lines tested from Central and Eastern Europe do not bear this bacterium. Rather, 16S rDNA sequence analysis suggests male-killing is associated with a member of the genus Spiroplasma. We discuss this conclusion in relation to the evolutionary genetics of male-killing bacteria, and the evolution of male-killing behaviour in the eubacteria.

Animals↗
PubMed1999

Wolbachia pipientis: microbial manipulator of arthropod reproduction.

The alpha-proteobacterium Wolbachia pipientis is a very common cytoplasmic symbiont of insects, crustaceans, mites, and filarial nematodes. To enhance its transmission, W. pipientis has evolved a large scale of host manipulations: parthenogenesis induction, feminization, and male killing. W. pipientis's most common effect is a crossing incompatibility between infected males and uninfected females. Little is known about the genetics and biochemistry of these symbionts because of their fastidious requirements. The affinity of W. pipientis for the microtubules associated with the early divisions in eggs may explain some of their effects. Such inherited microorganisms are thought to have been major factors in the evolution of sex determination, eusociality, and speciation. W. pipientis isolates are also of interest as vectors for the modification of wild insect populations, in the improvement of parasitoid wasps in biological pest control, and as a new method for interfering with diseases caused by filarial nematodes.

Animals↗