探討草履蟲與綠藻的內共生建立

Abstract

草履蟲Paramecium bursaria 與其內共生的綠藻是常被用來探討內共生關係的模式生物，然而，P. bursaria 及其內共生藻建立的機制尚不清楚。因此，本計畫將探討影響P. bursaria 與內共生藻Chlorella variabilis建立內共生的因素。透過感染實驗發現無綠藻共生的P. bursaria 相較於長期實驗室環境培養的綠藻C. variabilis ，其能夠更有效地與從與新鮮分離的內共生藻狀態的C. variabilis建立內共生關係。此外，從草履蟲剛分離的C. variabilis 在宿主外進行短期培養後，綠藻與P. bursaria 內共生建立的效率會逐漸降低，並在短期培養4天後降低到與長期實驗室環境培養的綠藻相近的內共生建立效率。此現象推測為分離的內共生藻C. variabilis 於生理或形態上有所改變，使其與長期實驗室環境培養的綠藻特徵相似。因此，我們進行連續時間點的轉錄體分析，以進一步瞭解從宿主分離後的內共生藻變化，結果顯示與光合作用、抗氧化能力和細胞壁合成相關的基因有所改變，這些變化與建立能力有潛在的相關性。此外，我們也會將綠藻以不同的條件處理，進而操作感染實驗，以探討各種因素如何影響內共生的建立以及誘發這些因素的宿主環境。我們的研究結果發現，較低的葉綠素含量和內共生藻分泌的因子可能影響內共生的建立，而內共生藻的變化與宿主衍生因子的誘導有關鍵的影響。此研究將有助於更深入地瞭解內共生建立的過程。

The ciliate Paramecium bursaria engages in a facultative mutualistic interaction with hundreds of green algal endosymbionts in freshwater, providing an excellent model for studying endosymbiotic relationships. However, the mechanisms underlying the establishment of P. bursaria and its algal endosymbionts remain unclear. This study aims to elucidate the factors influencing the establishment of P. bursaria and its algal endosymbionts Chlorella variabilis. Through re-infection experiments, we demonstrated that P. bursaria establishes endosymbiosis more efficiently with freshly isolated endosymbiotic C. variabilis from symbiotic P. bursaria than with long-term free-living algal cultures of the same strain. Moreover, when isolated endosymbiotic C. variabilis were cultured outside the host, their establishment efficiency gradually diminished, reaching the level of free-living algal cultures within a few cell divisions. This phenomenon is primarily attributed to non-genetic alterations in the isolated endosymbionts, aligning them with the characteristics of free-living algal cultures. To investigate these alterations, we performed time-course transcriptome analysis, revealing changes in gene expression associated with photosynthesis, oxidative stress response, and cell wall synthesis, which correlated with the establishment ability. Additionally, we conducted physiological measurements and re-infection experiments with algae subjected to different treatments to explore how various factors influence the establishment ability and the host environments that induce these factors. Our findings suggest that lower chlorophyll content and factors secreted from freshly isolated C. variabilis potentially influence the establishment of endosymbiosis, with specific host-derived factors playing a crucial role in inducing algal changes. This comprehensive investigation contributes to a deeper understanding of endosymbiotic establishment mechanisms.