Selection of host specific genes in environmentally transmitted mutualism

描述（由申请人提供）：乌贼（软体动物门：头足目）和它们的弧菌共生体之间的互惠关系是一种实验上易于处理的模型，用于研究动物和细菌在各种相互作用水平（遗传学、生理学、种群生态学）上的关系。由于共生细菌在环境中每一代都会传播到新的宿主，因此它提供了一个解决自由生活共生体的生态如何影响细菌-宿主相互作用结构的机会。细菌遇到潜在的冲突的选择压力，相互竞争殖民和坚持在乌贼光器官，但也在鱿鱼外的环境中争夺稀缺资源。存在一个定殖等级，其中特定的V. fischeri菌株在其原生鱿鱼伴侣的定殖方面胜过其他V. fischeri菌株，并且非生物因素如水和温度可以影响不同菌株的适应性。导致品系内适应性变化的遗传因素尚未阐明，也没有可能导致环境中品系内适应性变化的遗传因素。以下项目将研究仅在海水中高度表达或仅在光器官中表达的基因如何有助于细菌适应性。本提案是我实验室当前工作的延伸，重点关注四个具体目标： i）表征在高表达的可变基因中具有无效突变的菌株的环境相关表型。 ii）确定用来自相同菌株的野生型操纵子补充的突变菌株是否比用来自不同菌株的操纵子补充的相同菌株适合于环境持久性（自由生活）。 iii）检查具有用来自相同菌株的野生型操纵子或基因补充的特定可变基因座的共生菌株是否比用来自不同菌株的操纵子/基因补充的相同菌株适合宿主定殖。 iv）通过全基因组比较测量是否在致病性岛中发现共生基因，并证明与正常管家基因相比这些基因的正选择。

DESCRIPTION (provided by applicant): The mutualistic association between sepiolid squids (Mollusca: Cephalopoda) and their Vibrio symbionts is an experimentally tractable model to study the evolution of animal and bacterial associations at various levels of interaction (genetics, physiology, population ecology). Since symbiotic bacteria are environmentally transmitted to new hosts with every generation, it provides an opportunity to resolve how the ecology of the free-living symbiont affects the architecture of bacterial-host interactions. Bacteria encounter potentially conflicting selective pressures, competing with one another to colonize and persist in the sepiolid light organ, but also competing for scarce resources in the environment outside the squid. A hierarchy of colonization exists, where particular V. fischeri strains out-compete other V. fischeri strains for colonization of their native squid partner, and abiotic factors such as water and temperature can affect the fitness of different strains. Genetic factors that contribute to intra-strain variations in fitness have not been elucidated, and neither have the genetic factors that likely contribute to intra-strain fitness in the environment. The following project will examine how genes that are highly-expressed exclusively in sea water or expressed exclusively in the light organ contribute to bacterial fitness. The present proposal is an extension of the current work in my laboratory, and focuses on four specific goals: i) Characterize environmentally-relevant phenotypes of strains with null mutations in highly-expressed, variable genes. ii) Determine whether a mutant strain complemented with the wild-type operon from the same strain is fit for environmental persistence (free-living) than the same strain complemented with the operon from a different strain. iii) Examine whether symbiotic strains with specific variable loci complemented with the wild-type operon or gene from the same strain are fit for host colonization than the same strain complemented with the operon/gene from a different strain. iv) Measure whether symbiotic genes are found in pathogenicity islands through whole genome comparisons, and demonstrate positive selection of these genes compared to normal housekeeping genes.