Tanabe Yuuhiko, Sano Tomoharu, Kasai Fumie, Watanabe Makoto M
Graduate School of Life & Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan.
BMC Evol Biol. 2009 May 22;9:115. doi: 10.1186/1471-2148-9-115.
The water-bloom-forming cyanobacterium Microcystis aeruginosa is a known producer of various kinds of toxic and bioactive chemicals. Of these, hepatotoxic cyclic heptapeptides microcystins have been studied most intensively due to increasing concerns for human health risks and environmental damage. More than 70 variants of microcystins are known, and a single microcystin synthetase (mcy) gene cluster consisting of 10 genes (mcyA to mcyJ) has been identified to be responsible for the production of all known variants of microcystins. Our previous multilocus sequence typing (MLST) analysis of the seven housekeeping genes indicated that microcystin-producing strains of M. aeruginosa are classified into two phylogenetic groups.
To investigate whether the mcy genes are genetically structured similarly as in MLST analysis of the housekeeping genes and to identify the evolutionary forces responsible for the genetic divergence of these genes, we used 118 mcy-positive isolates to perform phylogenetic and population genetic analyses of mcy genes based on three mcy loci within the mcy gene cluster (mcyD, mcyG, and mcyJ), none of which is involved in the production of different microcystin variants. Both individual phylogenetic analysis and multilocus genealogical analysis of the mcy genes divided our isolates into two clades, consistent with the MLST phylogeny based on seven housekeeping loci. No shared characteristics within each clade are known, and microcystin analyses did not identify any compositional trend specific to each clade. Statistical analyses for recombination indicated that recombination among the mcy genes is much more frequent within clades than between, suggesting that recombination has been an important force maintaining the cryptic divergence of mcy genes. On the other hand, a series of statistical tests provided no strong evidence for selection to explain the deep divergence of the mcy genes. Furthermore, analysis of molecular variance (AMOVA) indicated a low level of geographic structuring in the genetic diversity of mcy.
Our phylogenetic analyses suggest that the mcy genes of M. aeruginosa are subdivided into two cryptic clades, consistent with the phylogeny determined by MLST. Population genetic analyses suggest that these two clades have primarily been maintained as a result of homology-dependent recombination and neutral genetic drift.
形成水华的蓝藻铜绿微囊藻是多种有毒和生物活性化学物质的已知生产者。其中，由于对人类健康风险和环境破坏的日益关注，肝毒性环状七肽微囊藻毒素受到了最深入的研究。已知微囊藻毒素有70多种变体，并且已鉴定出一个由10个基因（mcyA至mcyJ）组成的单一微囊藻毒素合成酶（mcy）基因簇负责所有已知变体微囊藻毒素的产生。我们之前对七个管家基因的多位点序列分型（MLST）分析表明，产微囊藻毒素的铜绿微囊藻菌株可分为两个系统发育组。
为了研究mcy基因在遗传结构上是否与管家基因的MLST分析相似，并确定导致这些基因遗传分化的进化力量，我们使用118个mcy阳性分离株，基于mcy基因簇内的三个mcy位点（mcyD、mcyG和mcyJ）对mcy基因进行系统发育和群体遗传分析，这些位点均不参与不同微囊藻毒素变体的产生。mcy基因的个体系统发育分析和多位点谱系分析均将我们的分离株分为两个进化枝，这与基于七个管家位点的MLST系统发育一致。每个进化枝内没有已知的共同特征，微囊藻毒素分析也未发现每个进化枝特有的任何组成趋势。重组的统计分析表明，mcy基因之间的重组在进化枝内比在进化枝之间更为频繁，这表明重组是维持mcy基因隐秘分化的重要力量。另一方面，一系列统计检验没有提供有力证据支持选择来解释mcy基因的深度分化。此外，可以解释分子方差分析（AMOVA）表明，mcy的遗传多样性中地理结构水平较低。
我们的系统发育分析表明，铜绿微囊藻的mcy基因可细分为两个隐秘进化枝，这与MLST确定的系统发育一致。群体遗传分析表明，这两个进化枝主要是由于同源依赖性重组和中性遗传漂变而得以维持。
