Identification of zooxanthellate genes that are involved in symbiosis with a reef coral, and analyses of their function and expression

雌雄同体（或造礁）珊瑚与共生的双鞭毛藻共生（俗称虫黄藻）共生。为了鉴定和表征在共生状态下表达水平比离体条件下提高的共生mRNAs，我们试图建立一个由礁珊瑚Acropora tenuis新变态的珊瑚虫和虫黄藻单克隆群体组成的模型共生系统。为此，研究了几种培养的共生菌细胞系对天然非共生线虫息肉的感染性。两个进化支A菌株PL-TS-1和CCMP2467高密度感染幼体，促进寄主生长。为了鉴定PL-TS-1（或A. tenuis）基因与共生关系的建立或维持有关，我们进行了抑制减法杂交。在减少的文库中分析了大约50个cDNA克隆，但在分析的cDNA群体中没有出现PL-TS-1的共生诱导mrna。然而，可以鉴定出两种mrna，它们的表达水平因共生体的存在而增加了两倍以上。这两个mrna分别被命名为AtSym-01和-02，分别编码一个具有一个跨膜结构域的syndecan家族蛋白和一个不依赖Na^+的SLC26A11家族硫酸盐转运蛋白（这是首次在这类珊瑚mrna上发现）。分析了PL-TS-1中SyAct-A1和-A2两种肌动蛋白mRNA在体内和体外条件下的表达水平，结果强烈表明SyAct-A2 mRNA在与珊瑚共生时的表达高于离体培养。

Hermatypic (or reef-building) corals live in obligate mutualistic symbiosis with the symbiotic dinoflagellates Symbiodinium spp. (generally known as zooxanthellae). With the purpose of identifying and characterizing Symbiodinium mRNAs whose expression levels are enhanced in the symbiotic state than in ex hospite conditions, attempts were made to establish a model symbiosis system consisting of newly metamorphosed polyps of the reef coral Acropora tenuis and a monoclonal population of zooxanthellae. To this end, infectivity of several cultured Symbiodinium cell lines was tested on naturally aposymbiotic polyps of A. tenuis. Two clade A strains (PL-TS-1 and CCMP2467) infected the juveniles at high densities, and promoted growth of the host. To identify PL-TS-1 (or A. tenuis) genes involved in the establishment or maintenance of symbiosis, suppression subtractive hybridization was performed. Approx. 50 cDNA clones in the subtracted library were analyzed, but symbiotically induced mRNAs of PL-TS-1 were not represented in the analyzed cDNA population. However, two mRNAs, the expression levels of which were augmented more than twofold by the presence of the symbionts, could be identified. The mRNAs, named AtSym-01 and -02, encode a syndecan family protein with one trans-membrane domain, and an Na^+-independent sulfate transporter of the SLC26A11 family, respectively (it was the first identification on such coral mRNAs). Two actin mRNAs (SyAct-A1 and -A2) in PL-TS-1 were analyzed for the expression levels in the in and ex hospite conditions, and the result strongly suggested that the expression of the SyAct-A2 mRNA was higher in symbiosis with the coral than in in vitro culture.