Insect guts present distinctive environments for microbial colonization, and bacteria in the gut potentially provide many beneficial services to their hosts. Insects display a wide range in degree of dependence on gut bacteria for basic functions. Most insect guts contain relatively few microbial species as compared to mammalian guts, but some insects harbor large gut communities of specialized bacteria. Others are colonized only opportunistically and sparsely by bacteria common in other environments. Insect digestive tracts vary extensively in morphology and physicochemical properties, factors that greatly influence microbial community structure. One obstacle to the evolution of intimate associations with gut microorganisms is the lack of dependable transmission routes between host individuals. Here, social insects, such as termites, ants, and bees, are exceptions: social interactions provide opportunities for transfer of gut bacteria, and some of the most distinctive and consistent gut communities, with specialized beneficial functions in nutrition and protection, have been found in social insect species. Still, gut bacteria of other insects have also been shown to contribute to nutrition, protection from parasites and pathogens, modulation of immune responses, and communication. The extent of these roles is still unclear and awaits further studies.

The gut microbiota of insects - diversity in structure and function.

The Zika outbreak, spread by the Aedes aegypti mosquito, highlights the need to create high-quality assemblies of large genomes in a rapid and cost-effective way. Here we combine Hi-C data with existing draft assemblies to generate chromosome-length scaffolds. We validate this method by assembling a human genome, de novo, from short reads alone (67× coverage). We then combine our method with draft sequences to create genome assemblies of the mosquito disease vectors Ae. aegypti and Culex quinquefasciatus, each consisting of three scaffolds corresponding to the three chromosomes in each species. These assemblies indicate that almost all genomic rearrangements among these species occur within, rather than between, chromosome arms. The genome assembly procedure we describe is fast, inexpensive, and accurate, and can be applied to many species.

https://science.sciencemag.org/content/sci/early/2017/03/22/science.aal3327.full.pdf

De novo assembly of the Aedes aegypti genome using Hi-C yields chromosome-length scaffolds

We present an end-to-end genome assembly of a female Aedes aegypti mosquito, which spreads viral diseases such as yellow fever, dengue, chikungunya, and Zika to humans. The assembly is based on an earlier genome published in 2007 and improved in 2013. The new assembly has a scaffold N50 of 419Mb, with 96.9% of the ungapped sequence anchored to chromosomes. We used the new assembly to examine the conservation of A. aegypti chromosomes. Our results suggest that synteny is strongly conserved between Ae. aegypti and An. gambiae. Comparison to D. melanogaster highlights the extent to which the identity of entire chromosome arms is preserved across dipterans.

An end-to-end assembly of the Aedes aegypti genome

Female Aedes aegypti mosquitoes infect more than 400 million people each year with dangerous viral pathogens including dengue, yellow fever, Zika and chikungunya. Progress in understanding the biology of mosquitoes and developing the tools to fight them has been slowed by the lack of a high-quality genome assembly. Here we combine diverse technologies to produce the markedly improved, fully re-annotated AaegL5 genome assembly, and demonstrate how it accelerates mosquito science. We anchored physical and cytogenetic maps, doubled the number of known chemosensory ionotropic receptors that guide mosquitoes to human hosts and egg-laying sites, provided further insight into the size and composition of the sex-determining M locus, and revealed copy-number variation among glutathione S-transferase genes that are important for insecticide resistance. Using high-resolution quantitative trait locus and population genomic analyses, we mapped new candidates for dengue vector competence and insecticide resistance. AaegL5 will catalyse new biological insights and intervention strategies to fight this deadly disease vector.

/pdf/improved-reference-genome-of-aedes-aegypti-informs-arbovirus-4twjg0mxs8.pdf

Improved reference genome of Aedes aegypti informs arbovirus vector control

Field studies indicate adult mosquitoes (Culicidae) host low diversity communities of bacteria that vary greatly among individuals and species. In contrast, it remains unclear how adult mosquitoes acquire their microbiome, what influences community structure, and whether the microbiome is important for survival. Here, we used pyrosequencing of 16S rRNA to characterize the bacterial communities of three mosquito species reared under identical conditions. Two of these species, Aedes aegypti and Anopheles gambiae, are anautogenous and must blood-feed to produce eggs, while one, Georgecraigius atropalpus, is autogenous and produces eggs without blood feeding. Each mosquito species contained a low diversity community comprised primarily of aerobic bacteria acquired from the aquatic habitat in which larvae developed. Our results suggested that the communities in Ae. aegypti and An. gambiae larvae share more similarities with one another than with G. atropalpus. Studies with Ae. aegypti also strongly suggested that adults transstadially acquired several members of the larval bacterial community, but only four genera of bacteria present in blood fed females were detected on eggs. Functional assays showed that axenic larvae of each species failed to develop beyond the first instar. Experiments with Ae. aegypti indicated several members of the microbial community and Escherichia coli successfully colonized axenic larvae and rescued development. Overall, our results provide new insights about the acquisition and structure of bacterial communities in mosquitoes. They also indicate that three mosquito species spanning the breadth of the Culicidae depend on their gut microbiome for development.

Mosquitoes rely on their gut microbiota for development.

The mosquito midgut is a hostile environment that vector-borne parasites must survive to be transmitted Commensal bacteria in the midgut can reduce the ability of mosquitoes to transmit disease, either by having direct anti-parasite effects or by stimulating basal immune responses of the insect host As different bacteria have different effects on parasite development, the composition of the bacterial community in the mosquito gut is likely to affect the probability of disease transmission We investigated the diversity of mosquito gut bacteria in the field using 454 pyrosequencing of 16S rRNA to build up a comprehensive picture of the diversity of gut bacteria in eight mosquito species in this population We found that mosquito gut typically has a very simple gut microbiota that is dominated by a single bacterial taxon Although different mosquito species share remarkably similar gut bacteria, individuals in a population are extremely variable and can have little overlap in the bacterial taxa present in their guts This may be an important factor in causing differences in disease transmission rates within mosquito populations

http://sitka.gen.cam.ac.uk/research/jiggins/pdfs/OseiPoku2012.pdf

Deep sequencing reveals extensive variation in the gut microbiota of wild mosquitoes from Kenya

The mosquito Aedes aegypti transmits some of the most important human arboviruses, including dengue, yellow fever and chikungunya viruses. It has a large genome containing many repetitive sequences, which has resulted in the genome being poorly assembled — there are 4,758 scaffolds, few of which have been assigned to a chromosome. To allow the mapping of genes affecting disease transmission, we have improved the genome assembly by scoring a large number of SNPs in recombinant progeny from a cross between two strains of Ae. aegypti, and used these to generate a genetic map. This revealed a high rate of misassemblies in the current genome, where, for example, sequences from different chromosomes were found on the same scaffold. Once these were corrected, we were able to assign 60% of the genome sequence to chromosomes and approximately order the scaffolds along the chromosome. We found that there are very large regions of suppressed recombination around the centromeres, which can extend to as much as 47% of the chromosome. To illustrate the utility of this new genome assembly, we mapped a gene that makes Ae. aegypti resistant to the human parasite Brugia malayi, and generated a list of candidate genes that could be affecting the trait.

http://www.jiggins.gen.cam.ac.uk/pdfs/juneja2014.pdf

Assembly of the Genome of the Disease Vector Aedes aegypti onto a Genetic Linkage Map Allows Mapping of Genes Affecting Disease Transmission

Wolbachia bacteria are common endosymbionts of insects, and some strains are known to protect their hosts against RNA viruses and other parasites. This has led to the suggestion that releasing Wolbachia-infected mosquitoes could prevent the transmission of arboviruses and other human parasites. We have identified Wolbachia in Kenyan populations of the yellow fever vector Aedes bromeliae and its relative Aedes metallicus, and in Mansonia uniformis and Mansonia africana, which are vectors of lymphatic filariasis. These Wolbachia strains cluster together on the bacterial phylogeny, and belong to bacterial clades that have recombined with other unrelated strains. These new Wolbachia strains may be affecting disease transmission rates of infected mosquito species, and could be transferred into other mosquito vectors as part of control programs.

http://www.jiggins.gen.cam.ac.uk/pdfs/OseiPoku2012_b.pdf

Identification of Wolbachia strains in mosquito disease vectors.

A diverse range of endosymbionts are found within the cells of animals. As these endosymbionts are normally vertically transmitted, we might expect their evolutionary history to be dominated by host-fidelity and cospeciation with the host. However, studies of bacterial endosymbionts have shown that while this is true for some mutualists, parasites often move horizontally between host lineages over evolutionary timescales. For the first time, to our knowledge, we have investigated whether this is also the case for vertically transmitted viruses. Here, we describe four new sigma viruses, a group of vertically transmitted rhabdoviruses previously known in Drosophila. Using sequence data from these new viruses, and the previously described sigma viruses, we show that they have switched between hosts during their evolutionary history. Our results suggest that sigma virus infections may be short-lived in a given host lineage, so that their long-term persistence relies on rare horizontal transmission events between hosts.

/pdf/host-switching-by-a-vertically-transmitted-rhabdovirus-in-miyxo0ches.pdf

Host-switching by a vertically transmitted rhabdovirus in Drosophila

Some mosquito strains or species are able to lay eggs without taking a blood meal, a trait named autogeny. This may allow populations to persist through times or places where vertebrate hosts are scarce. Autogenous egg production is highly dependent on the environment in some species, but the ideal conditions for its expression in Aedes aegypti mosquitoes are unknown. We found that 3.2% of females in a population of Ae. aegypti from Kenya were autogenous. Autogeny was strongly influenced by temperature, with many more eggs laid at 28°C compared with 22°C. Good nutrition in larval stages and feeding on higher concentrations of sugar solution during the adult stage both result in more autogenous eggs being produced. The trait also has a genetic basis, as not all Ae. aegypti genotypes can lay autogenously. We conclude that Ae. aegypti requires a favorable environment and a suitable genotype to be able to lay eggs without a blood meal.

/pdf/environmental-and-genetic-factors-determine-whether-the-z0buk8t3cr.pdf

Jewelna Osei-Poku

Papers

Deep sequencing reveals extensive variation in the gut microbiota of wild mosquitoes from Kenya

Assembly of the Genome of the Disease Vector Aedes aegypti onto a Genetic Linkage Map Allows Mapping of Genes Affecting Disease Transmission

Identification of Wolbachia strains in mosquito disease vectors.

Host-switching by a vertically transmitted rhabdovirus in Drosophila

Environmental and Genetic Factors Determine Whether the Mosquito Aedes aegypti Lays Eggs Without a Blood Meal