Regeneration of the Eyespot and Flagellum in <i>Euglena gracilis</i> during Cell Division

Cell division of unicellular microalgae is a fascinating process of proliferation, at which whole organelles are regenerated and distributed to two new lives. We performed dynamic live cell imaging of <i>Euglena gracilis</i> using optical microscopy to elucidate the mechanisms involved in the regulation of the eyespot and flagellum during cell division and distribution of the organelles into the two daughter cells. Single cells of the wild type (WT) and colorless SM-ZK cells were confined in a microfluidic device, and the appearance of the eyespot (stigma) and emergent flagellum was tracked in sequential video-recorded images obtained by automatic cell tracking and focusing. We examined 12 SM-ZK and 10 WT cells and deduced that the eyespot diminished in size and disappeared at an early stage of cell division and remained undetected for 26–97 min (62 min on average, 22 min in deviation). Subsequently, two small eyespots appeared and were distributed into the two daughter cells. Additionally, the emergent flagellum gradually shortened to zero-length, and two flagella emerged from the anterior ends of the daughter cells. Our observation revealed that the eyespot and flagellum of <i>E. gracilis</i> are degraded once in the cell division, and the carotenoids in the eyespot are also decomposed. Subsequently, the two eyespots/flagella are regenerated for distribution into daughter cells. As a logical conclusion, the two daughter cells generated from a single cell division possess the equivalent organelles and each <i>E. gracilis</i> cell has eternal or non-finite life span. The two newly regenerated eyespot and flagellum grow at different rates and mature at different timings in the two daughter cells, resulting in diverse cell characteristics in <i>E. gracilis</i>.