Abstract 2014

キネトプラスチダ類における寄生性鞭毛虫の系統的位置と寄生性形質獲得プロセスの解明 （日本進化学会第16回大会）

真核単細胞鞭毛虫キネトプラスチダ類は、土壌や海洋に主に生息する自由生活種との他、人体に重大な病状を引き起こすトリパノソーマ原虫をはじめ、多数の寄生虫から構成される。キネトプラスチダ類の寄生虫はヒト以外にも魚介類・アメーバなど多様な生物を宿主とするため、この系統群の進化においては自由生活から寄生への生活様式の移行が複数回独立に生じたと考えられている。一方、ゲノムデータの豊富なトリパノソーマ原虫に比べ、ヒト以外を宿主とする寄生虫に関しては分子データが乏しく、キネトプラスチダ類寄生虫の詳細な系統関係は不明瞭なままである。そこで本研究では、魚類住血虫であるTrypanoplasma borreli、ホヤ被嚢寄生虫であるAzuminobodo hoyamushi、およびアメーバ寄生虫であるIchthyobodo-related organismより網羅的発現遺伝子データを取得しした。さらに計57遺伝子に基づくphylogenomics解析を行い、解析した3種がキネトプラスチダ類中での系統的位置を頑健に推測した。本発表では上記phylogenomics解析結果に加え、寄生虫それぞれの網羅的発現遺伝子データを比較することで、キネトプラスチダ類における寄生性形質獲得プロセスの多様性について議論する。

A snap shot of the gene replacement after endosymbiotic gene transfer: plastid GAPDH genes in the dinoflagellete Karenia brevis as a case study. (Protist 2014)

Unlike the vast majority of photosynthetic dinoflagellates containing ‘peridinin-type’ plastids, members of the genus Karenia possess atypical ‘haptophyte-derived’ plastids. The ancestral (peridinin-type) plastid was most likely replaced by the endosymbiotic haptophyte, of which cellular structures were degraded except plastid. The plastid replacement likely associated with transfer of ‘haptophyte’ genes to the host (dinoflagellate) genome, and a part of those replaced their endogenous orthologues in the host genome. Karenia brevis are reported to possess haptophyte-type and original (perinin-type) ‘plastid’ GAPDH genes -- the former was transferred from the endosymbiont, while the latter has resided in the host genome prior to the plastid replacement. We here investigated the structures and quantities of the two gene transcripts, and expressed the two GAPDHs in the model apicomplexan parasite Toxoplasma gondii to predict their cellular localizations. Both ‘plastid’ GAPDH gene transcripts carried the GAPDH-coding region with the N-terminal extension, which potentially acts as plastid-targeting signal. Indeed, the green fluorescent proteins fused to the two N-terminal extensions were found to be localized in apicoplast in T. gondii, suggesting that both GAPDHs can be targeted to the K. brevis plastid. Nevertheless, the peridinin-type gene appeared to be transcriptionally suppressed, comparing to the haptophyte-type gene. These results strongly suggest that the two ‘plastid’ GAPDH genes in K. brevis represent an intermediate step of the gene replacement after endosymbiotic gene transfer, in which the exogenous (haptophyte-type) gene is about to replace the endogenous (peridinin-type) gene.

Cellular localization of evolutionarily distinctive 'plastid-targeted' GAPDHs in Karenia brevis and K. mikimotoi. (CIFAR)

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