[B-PT24_29AM2] 化学合成生態系の進化をめぐって

★ [BPT24-07] 分子系統解析から考える深海性二枚貝シロウリガイ類における化学合成共生細菌の宿主転換の可能性

*小澤 元希¹、金子 隆司²、島村 繁³、高木 善弘³、輿石 武³、加藤 千明³、丸山 正³、吉田 尊雄³ (1.北里大学大学院海洋生命科学研究科、2.独立行政法人海洋研究開発機構、3.東京バイオテクノロジー専門学校)

キーワード：共生, シロウリガイ, 共進化, 宿主転換

Calyptogena clams are living in deep-sea chemosynthetic habitats and globally distributed in seeps and hydrothermal vents. They are nutritionally dependent on chemoautotrophic sulfur oxidizing bacteria, which are harbored within their gill epithelial cells. The Calyptogena symbionts are thought to be vertically transmitted via clam′s egg to the next generation. Both host and symbiont are though to coevolve, because topologies of the phylogenetic trees of them form a mirror image. However, their phylogenetic trees have not been robust enough for analyzing their coevolutional relationship, because of using partial gene sequences of host (mitochondrial cox1 and rrnL genes) and symbiont (16S rRNA gene). The possibility of lateral acquisition of the symbiont has been reported in some Calyptogena lineages. To improve the phylogenetic trees of Calyptogena clams and of symbiont, we sequenced the mitochondrial genomes of Calyptogena clams, and several their symbiont genes, and analyzed the phylogenetic trees by using the concatenated sequences. Mitochondrial genomes of C. phaseoliformis, C. okutanii and C. fossajaponica were sequenced. Based on these mitochondrial genome sequences, primer sets for PCR of mitochondrial genes of other Calyptogena clams were designed. Using them, 11 mitochondrial genes (cox1, cox2, cox3, nad1, nad3, nad4, nad5, cytb, atp6, atp8 and rrnL) of other 8 Calyptogena species (C. fausta, C. kawamurai, C. kilmeri, C. laubieri, C. nautilei, C. pacifica, C. soyoae, C. stearnsii) were amplified by PCR and sequenced. Eight genes (16S rRNA, 23S rRNA, uvrA, uvrD, mfd, groEL, groES and gyrb) of symbionts of these Calyptogena clams were also sequenced. Phylogenetic trees of clams and symbionts were constructed by maximum likelihood and bayesian analysis based on concatenated 11 mitochondrial and 8 symbionts genes, respectively. The reliabilities of phylogenetic trees of the hosts and their symbionts were significantly improved by using the concatenated genes sequences (Fig.1). Bootstrap values and posterior probabilities of inernal nodes were better supported than those of the previous phylogenetic trees using partial gene sequences. Topological congruence of host and symbiont that was supported by bootstrap value (100%) and posterior probabilities (1.0), was shown in C. okutanii, C. soyoae, C. kilmeri, C. pacifica and C. fausta. These results suggested that these symbionts were cospeciated with their host clams (green boxes in Fig.1). Although the topologies of host and symbiont were congruent with C. fossajaponica and C. phaseoliformis, there were the low bootstrap values and low posterior probabilities in the host clade. Topological incongruence between host and symbiont trees was shown in C. kawamurai - C. laubieri clade and C. nautilei - C. stearnsii clades (Fig.1) Congruence of topologies was rejected by approximately unbiased test using sitewise log-likelihoods (red branches in Fig.1). This result suggested that these symbionts have not cospeciated with their host clams. Host switching of the symbionts in the clades of C. kawamurai - C. laubieri and C. nautilei - C. stearnsii were examined by coevolution software, which compared the topologies of host and symbiont. Host switching is the event that symbiont is transferred from a host to a new host in a different lineage during speciation. The host switching of symbiont between C. kawamurai and C. laubieri was suggested by this software. Moreover, both clams are living in different depths of the same area (blue box on Fig.1). However, this software did not suggest the host switching of sym