第1359回生物科学セミナー
In between photosymbiosis and phototrophy
柏山祐一郎 教授（福井工業大学）
2021年05月19日(水) 17:05-18:35 Zoomによるweb講義
Originating in a single cyanobacterial endosymbiotic event surmisably over 1 Ga, the chloroplast of Archaeplastida has further been descended among various, distinct eukaryotic lineages to be so called secondary, tertiary chloroplasts, or collectively, “complex plastids”. The process of plastid acquisition has generally been attributed to an evolutionary extension of photoendosymbiosis where microalgal cells are nested within another host eukaryotic cell. The critical evidences that suggest endosymbiotic origins of the complex plastids are retentions of remnant nuclei of the symbionts in two discrete lineages: cryptophytes and chlorarachniophytes. The former possesses rhodophyte-derived plastids with largely reduced its nuclear genome in the plastidal nucleomorph; yet, the latter instead possesses chlorophyte-derived ones with respective genome in the analogous nucleomorph. Therefore, they are regarded as independent symbiosis-derived plastid acquisitions, which can be extrapolated to the origins of other complex plastids. Importantly, such a process must have accompanied with massive gene transfer from the symbiont nucleus genome to the host nucleus genome as well as invention of a mechanism retargeting the gene products to the permanently subjugated plastid. Nonetheless, neither evidence of such massive gene transfer nor expression of proteins encoded in the host genome in the symbiont has been reported so far. Lacking any true intermediate, the tangible process of plastid acquisition has been no more than a best guesstimation. Indeed, given the large costs and risks to oxidative stress that photosynthesis inevitably entails, the benefit of integrating them as a bona fide organelle, rather than maintaining them as an exploitable symbiont, is unclear. In light of the above background, this lecture presents the latest progress on an as yet unreported example of kleptoplasty (a process that utilizes temporary chloroplasts derived from prey), which may represent such an intermediate leading to a permanent chloroplast.
参考文献
A. Yamaguchi, et al. (2012) BMC Evolutionary Biology 12: 29.
担当： 東京大学大学院理学系研究科・生物科学専攻・発生細胞生物学研究室
