Establishing the genetic basis of symbiosis in an insect host

点击翻译按钮获取中文摘要

An animal body is habitat for billions of bacteria, which live in the gut and on the skin. These bacteria were long regard as passengers we called commensals - using the animal as a host, but not strongly affecting the biology of the animal. In contrast, we now recognise that these microbes are an important and active component of the individual - how an organism develops, its immunity and resistance to infection all misbehave when the microbiome is absent or depleted. In insects, symbiosis ('living together') is commonly even better established - microbial partners exist within the body of the insect, not just within the gut and upon skin surfaces. Further, they may be heritable- transmitted from a female to her progeny. These microbial partners define very important properties of the insect -the ability to utilize a plant as a pest; whether the insect can transmit pathogens onward to plants and animals; whether the individual is susceptible to viral/parasitic infection. These properties are exploitable - in Cairns, Australia, mosquitoes carrying a symbiotic microbe that prevents the transmission of dengue are released en masse to protect the residents from infection by dengue. We currently know little about how these symbionts work. One observation is that they commonly have degraded genomes - which predicts increased reliance on the genes remaining. In addition, the microbes have to deploy an array of genes to establish persistent symbiosis - a long life within the host. The first part of this project will examine how much of the genome is required for these functions. We will also ask if symbiosis is a 'redeployment' of pathogen systems, or whether novel mechanisms are involved. Beyond this, we will establish how symbiosis genes enable the microbe to complete their life cycle, and how they modify the biology of their host - in our case, showing male-limited pathogenesis (male-killing).The reason we have not been able to answer this question before is simple: adaptation to life within a host makes these bacteria very hard to study outside of the host. In this project, we will exploit a symbiosis where the microbe can be grown in culture, where we can alter the genetic constitution of the microbe and can re-introduce strains easily to the insect. We will use this system to test which aspects of the microbe's genome determine its ability to be symbiotic. We have created 10,000 strains of the bacterium Arsenophonus nasoniae, each with a different gene 'knocked out'. We will reintroduce these into the host insect (the tiny parasitic jewel wasp). We are interested in the strains that fail to establish a symbiosis - these will be ones where the gene in question is necessary for the bacteria to live in symbiosis. We will then explore how these genes aid in establishing a symbiotic life style. Further to this, we will identify the genes responsible for male-killing.In completing this analysis, we will achieve the first examination of the genes and systems that microbes require to live within an insect and modify its biology. In addition to the intrinsic scientific interest of the research, our findings will allow us to better exploit symbionts to improve human health and food supply. Our focal microbe is closely related to insect vectored plant pathogens that damage fruit plants, and other bacteria that are associated with poor honeybee health. More generally, the knowledge gained will allow us to better engineer novel host-symbiont combinations for pest and vector control, with the aim of improving agricultural yields and human health.