Selection of host specific genes in environmentally transmitted mutualism

描述(由申请人提供)：海鞘乌贼(软体动物：头足类)与其弧菌共生体之间的互惠关系是一个实验上易于处理的模型，用于研究动物和细菌在不同相互作用(遗传学、生理学、种群生态学)水平上的进化。由于共生细菌每一代都在环境中传播给新的宿主，这为解决自由生活的共生体的生态如何影响细菌-宿主相互作用的架构提供了机会。细菌面临着潜在的相互冲突的选择压力，它们相互竞争在海泡石发光器官中定居和坚持，但也在争夺鱿鱼以外环境中稀缺的资源。存在着一种定植等级，其中特定的费氏弧菌菌株在与其本土鱿鱼伴侣的定殖竞争中胜过其他菌株，而非生物因素，如水和温度，可以影响不同菌株的适合性。导致菌株内适应性差异的遗传因素尚未阐明，也没有可能有助于菌株内环境适应性的遗传因素。下面的项目将研究在海水中高度表达或仅在发光器官中表达的基因如何有助于细菌的适应性。本提案是我实验室目前工作的延伸，侧重于四个具体目标： I)鉴定高表达、可变基因零突变菌株的与环境相关的表型。 Ii)确定与同一菌株的野生型操纵子互补的突变菌株是否比补充不同菌株的操纵子的同一菌株适合环境持久性(自由生活)。 Iii)检测与野生型操纵子或来自同一菌株的基因互补的具有特定可变位的共生菌是否比具有来自不同菌株的操纵子/基因的相同菌株适合寄主定植。 Iv)通过全基因组比较，衡量在致病岛中是否发现共生基因，并证明与正常的家务基因相比，这些基因具有积极的选择作用。

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.