Tom Walker

Genome Evolution of Wolbachia Strain wPip from the Culex pipiens Group

The obligate intracellular bacterium Wolbachia pipientis strain wPip induces cytoplasmic incompatibility (CI), patterns of crossing sterility, in the Culex pipiens group of mosquitoes. The complete sequence is presented of the 1.48-Mbp genome of wPip which encodes 1386 coding sequences (CDSs), representing the first genome sequence of a B-supergroup Wolbachia. Comparisons were made with the smaller genomes of Wolbachia strains wMel of Drosophila melanogaster, an A-supergroup Wolbachia that is also a CI inducer, and wBm, a mutualist of Brugia malayi nematodes that belongs to the D-supergroup of Wolbachia. Despite extensive gene order rearrangement, a core set of Wolbachia genes shared between the 3 genomes can be identified and contrasts with a flexible gene pool where rapid evolution has taken place. There are much more extensive prophage and ankyrin repeat encoding (ANK) gene components of the wPip genome compared with wMel and wBm, and both are likely to be of considerable importance in wPip biology. Five WO-B–like prophage regions are present and contain some genes that are identical or highly similar in multiple prophage copies, whereas other genes are unique, and it is likely that extensive recombination, duplication, and insertion have occurred between copies. A much larger number of genes encode ankyrin repeat (ANK) proteins in wPip, with 60 present compared with 23 in wMel, many of which are within or close to the prophage regions. It is likely that this pattern is partly a result of expansions in the wPip lineage, due for example to gene duplication, but their presence is in some cases more ancient. The wPip genome underlines the considerable evolutionary flexibility of Wolbachia, providing clear evidence for the rapid evolution of ANK-encoding genes and of prophage regions. This host–Wolbachia system, with its complex patterns of sterility induced between populations, now provides an excellent model for unraveling the molecular systems underlying host reproductive manipulation.

Horizontal gene transfer between Wolbachia and the mosquito Aedes aegypti

BackgroundThe evolutionary importance of horizontal gene transfer (HGT) from Wolbachia endosymbiotic bacteria to their eukaryotic hosts is a topic of considerable interest and debate. Recent transfers of genome fragments from Wolbachia into insect chromosomes have been reported, but it has been argued that these fragments may be on an evolutionary trajectory to degradation and loss.ResultsWe have discovered a case of HGT, involving two adjacent genes, between the genomes of Wolbachia and the currently Wolbachia-uninfected mosquito Aedes aegypti, an important human disease vector. The lower level of sequence identity between Wolbachia and insect, the transcription of all the genes involved, and the fact that we have identified homologs of the two genes in another Aedes species (Ae. mascarensis), suggest that these genes are being expressed after an extended evolutionary period since horizontal transfer, and therefore that the transfer has functional significance. The association of these genes with Wolbachia prophage regions also provides a mechanism for the transfer.ConclusionThe data support the argument that HGT between Wolbachia endosymbiotic bacteria and their hosts has produced evolutionary innovation.

Dynamics of the “Popcorn” Wolbachia Infection in Outbred Aedes aegypti Informs Prospects for Mosquito Vector Control

Forty percent of the worlds population is at risk of contracting dengue virus, which produces dengue fever with a potentially fatal hemorrhagic form. The wMelPop Wolbachia infection of Drosophila melanogaster reduces life span and interferes with viral transmission when introduced into the mosquito Aedes aegypti, the primary vector of dengue virus. Wolbachia has been proposed as an agent for preventing transmission of dengue virus. Population invasion by Wolbachia depends on levels of cytoplasmic incompatibility, fitness effects, and maternal transmission. Here we characterized these traits in an outbred genetic background of a potential target population of Ae. aegypti using two crossing schemes. Cytoplasmic incompatibility was strong in this background, and the maternal transmission rate of Wolbachia was high. The infection substantially reduced longevity of infected adult females, regardless of whether adults came from larvae cultured under high or low levels of nutrition or density. The infection reduced the viability of diapausing and nondiapausing eggs. Viability was particularly low when eggs were laid by older females and when diapausing eggs had been stored for a few weeks. The infection affected mosquito larval development time and adult body size under different larval nutrition levels and densities. The results were used to assess the potential for wMelPop-CLA to invade natural populations of Ae. aegypti and to develop recommendations for the maintenance of fitness in infected mosquitoes that need to compete against field insects.

Wolbachia in the Culex pipiens Group Mosquitoes: Introgression and Superinfection

Wolbachia bacteria in mosquitoes induce cytoplasmic incompatibility (CI), where sperm from Wolbachia-infected males can produce inviable progeny. The wPip strain in the Culex pipiens group of mosquitoes produces a complexity of CI crossing types. Several factors are thought to be capable of influencing the expression of CI including Wolbachia strain type and host genotype. In this study, the unidirectional CI that occurs between 2 C. pipiens complex laboratory strains, Col and Mol, was further investigated by nuclear genotype introgression. The unidirectional CI between Col and Mol was not found to be influenced by host genetic background, in contrast to a previous introgression study carried out using bidirectionally incompatible C. pipiens group strains. A line containing both wPip strain variants superinfection was also generated by embryonic cytoplasmic transfer. The same crossing type as the parental Col strain was observed in the superinfected line. Quantitative polymerase chain reaction demonstrated a low density of the injected wPipMol variant in the superinfected line after 18 generations, which was considered likely to be responsible for the crossing patterns observed. The Wolbachia density was also shown to be lower in the parental Mol strain males compared with Col strain males, and no inverse relationship between WO phage and Wolbachia density could be detected.

Comparative susceptibility of mosquito populations in North Queensland, Australia to oral infection with dengue virus

Dengue is the most prevalent arthropod-borne virus, with at least 40% of the worlds population at risk of infection each year. In Australia, dengue is not endemic, but viremic travelers trigger outbreaks involving hundreds of cases. We compared the susceptibility of Aedes aegypti mosquitoes from two geographically isolated populations to two strains of dengue virus serotype 2. We found, interestingly, that mosquitoes from a city with no history of dengue were more susceptible to virus than mosquitoes from an outbreak-prone region, particularly with respect to one dengue strain. These findings suggest recent evolution of population-based differences in vector competence or different historical origins. Future genomic comparisons of these populations could reveal the genetic basis of vector competence and the relative role of selection and stochastic processes in shaping their differences. Lastly, we show the novel finding of a correlation between midgut dengue titer and titer in tissues colonized after dissemination.

Comparison of Methods for Xenomonitoring in Vectors of Lymphatic Filariasis in Northeastern Tanzania

Monitoring Wuchereria bancrofti infection in mosquitoes (xenomonitoring) can play an important role in determining when lymphatic filariasis has been eliminated, or in focusing control efforts. As mosquito infection rates can be low, a method for collecting large numbers of mosquitoes is necessary. Gravid traps collected large numbers of Culex quinquefasciatus in Tanzania, and a collection method that targets mosquitoes that have already fed could result in increased sensitivity in detecting W. bancrofti-infected mosquitoes. The aim of this experiment was to test this hypothesis by comparing U.S. Centers for Disease Control and Prevention (CDC) light traps with CDC gravid traps in northeastern Tanzania, where Cx. quinquefasciatus is a vector of lymphatic filariasis. After an initial study where small numbers of mosquitoes were collected, a second study collected 16,316 Cx. quinquefasciatus in 60 gravid trap-nights and 240 light trap-nights. Mosquitoes were pooled and tested for presence of W. bancrofti DNA. Light and gravid traps collected similar numbers of mosquitoes per trap-night, but the physiological status of the mosquitoes was different. The estimated infection rate in mosquitoes collected in light traps was considerably higher than in mosquitoes collected in gravid traps, so light traps can be a useful tool for xenomonitoring work in Tanzania.

Louis MacNeice and the Irish poetry of his time

Introduction Yeatss MacNeice Racial and Regional Rhythms Letters Home Irish Characters A Little Solemnity MacNeices Byzantium

Integration of Parallel Opposing Memories Underlies Memory Extinction

Summary Accurately predicting an outcome requires that animals learn supporting and conflicting evidence from sequential experience. In mammals and invertebrates, learned fear responses can be suppressed by experiencing predictive cues without punishment, a process called memory extinction. Here, we show that extinction of aversive memories in Drosophila requires specific dopaminergic neurons, which indicate that omission of punishment is remembered as a positive experience. Functional imaging revealed co-existence of intracellular calcium traces in different places in the mushroom body output neuron network for both the original aversive memory and a new appetitive extinction memory. Light and ultrastructural anatomy are consistent with parallel competing memories being combined within mushroom body output neurons that direct avoidance. Indeed, extinction-evoked plasticity in a pair of these neurons neutralizes the potentiated odor response imposed in the network by aversive learning. Therefore, flies track the accuracy of learned expectations by accumulating and integrating memories of conflicting events.

‘An Inconstant Stay’: Paul Muldoon, Seamus Heaney and the Ends of Shakespeare’s Sonnets

The sonnet has been a much used form in modern Irish poetry. W.B. Yeats, Patrick Kavanagh, Eilean Ni Chuilleanain, Michael Longley, Ciaran Carson, Leontia Flynn and many others have used it to considerable effect, whether in single poems or across longer sequences. The phenomenon of the modern Irish sonnet has been examined by critics in relation to the fraught Anglo-Irish political climate of the 1970s and 1980s, and Irish poetry’s growing internationalist self-confidence. The sonnet has also played a key role in critical discussions of Irish poetry’s seeming adherence to traditional notions of poetic form. However, the Shakespearean dimensions to the profusion of sonnets in modern Irish poetry have yet to be evaluated in either cultural-political or formal terms. Focusing on the spectacular, compulsive engagement of Paul Muldoon with the sonnet, this chapter traces a line back from Muldoon’s recent ‘Sonnet 15: A Graft’ (a direct response to Shakespeare’s Sonnet 15) to Muldoon’s earlier work in the form. It argues that Shakespeare’s Sonnets were one of the means by which Muldoon in the 1970s and 1980s sought out the end of the poem – to draw on the key guiding term of his Oxford Professor of Poetry lectures. Furthermore, the Shakespearean ends offered a means of engaging with the limits of the Irish sonnet, not least as it emerged under Seamus Heaney’s stewardship. In Muldoon’s sonnets, Shakespeare’s sonnets offer a field of ironic, disquieting excess that pushes beyond either the benighted circumstances of Anglo-Irish history or the comforts offered by any investment in the literary and its traditions.

The culture of art in 1880s Ireland and the genealogy of Irish modernism

ABSTRACT The emergence of Irish modernism over recent decades as a distinct sub-field within modernist studies and Irish studies has frequently seen the alignment of advanced aesthetics with various quests for cultural progress. This emphasis on the socio-political origins and ends of many art works and artists from the 1890s through to the mid-twentieth century, however, largely draws on a narrow characterisation of late-nineteenth-century Ireland as a terrain of cultural turmoil and fracture, marked by linguistic decline and agrarian conflict. This constitutes a partial evidential and ideological basis for tracing a genealogy of Irish modernism. Among various occlusions, such a construction elides the ideology of the aesthetic itself, as well as its attendant cultures and institutions, as they operated in Ireland. Focusing on the institutionalisation and mediation of the nineteenth century’s culture of art, this article considers the specific terms in which aesthetics and politics were already imbricated in writing on the visual arts in Ireland during the 1880s. It seeks to illustrate how aesthetic ideologies and desires also constituted, alongside other social and political conditions, the field of cultural production from which Irish modernism emerged.