Effects of cell division on the stigma of wild-type and an “eyeless” mutant of Chlamydomonas

Regeneration of the Eyespot and Flagellum in Euglena gracilis 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 Euglena gracilis 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 E. gracilis 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 E. gracilis 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 E. gracilis.

Independent localization of plasma membrane and chloroplast components during eyespot assembly

Like many algae, Chlamydomonas reinhardtii is phototactic, using two anterior flagella to swim toward light optimal for photosynthesis. The flagella are responsive to signals initiated at the photosensory eyespot, which comprises photoreceptors in the plasma membrane and layers of pigment granules in the chloroplast. Phototaxis depends on placement of the eyespot at a specific asymmetric location relative to the flagella, basal bodies, and bundles of two or four highly acetylated microtubules, termed rootlets, which extend from the basal bodies toward the posterior of the cell. Previous work has shown that the eyespot is disassembled prior to cell division, and new eyespots are assembled in daughter cells adjacent to the nascent four-membered rootlet associated with the daughter basal body (D4), but the chronology of these assembly events has not been determined. Here we use immunofluorescence microscopy to follow assembly and acetylation of the D4 rootlet, localization of individual eyespot components in the plasma membrane or chloroplast envelope, and flagellar emergence during and immediately following cell division. We find that the D4 rootlet is assembled before the initiation of eyespot assembly, which occurs within the same time frame as rootlet acetylation and flagellar outgrowth. Photoreceptors in the plasma membrane are correctly localized in eyespot mutant cells lacking pigment granule layers, and chloroplast components of the eyespot assemble in mutant cells in which photoreceptor localization is retarded. The data suggest that plasma membrane and chloroplast components of the eyespot are independently responsive to a cytoskeletal positioning cue.

Eyespot-assembly mutants in Chlamydomonas reinhardtii

Chlamydomonas reinhardtii is a single-celled green alga that phototaxes toward light by means of a light-sensitive organelle, the eyespot. The eyespot is composed of photoreceptor and Ca(++)-channel signal transduction components in the plasma membrane of the cell and reflective carotenoid pigment layers in an underlying region of the large chloroplast. To identify components important for the positioning and assembly of a functional eyespot, a large collection of nonphototactic mutants was screened for those with aberrant pigment spots. Four loci were identified. eye2 and eye3 mutants have no pigmented eyespots. min1 mutants have smaller than wild-type eyespots. mlt1(ptx4) mutants have multiple eyespots. The MIN1, MLT1(PTX4), and EYE2 loci are closely linked to each other; EYE3 is unlinked to the other three loci. The eye2 and eye3 mutants are epistatic to min1 and mlt1 mutations; all double mutants are eyeless. min1 mlt1 double mutants have a synthetic phenotype; they are eyeless or have very small, misplaced eyespots. Ultrastructural studies revealed that the min1 mutants are defective in the physical connection between the plasma membrane and the chloroplast envelope membranes in the region of the pigment granules. Characterization of these four loci will provide a beginning for the understanding of eyespot assembly and localization in the cell.

Characterization of the eyespot regions of "blind" Chlamydomonas mutants after restoration of photophobic responses

Chlamydomonas reinhardtii exhibits photophobic and positive and negative phototactic responses that can be defined for cell populations using computerized cell tracking and motion analysis. Mutants CC-2359 and FN68 are pigment deficient mutants that are blocked in carotenoid synthesis and lack these photo responses. In particular, neither mutant exhibits flash-induced photophobic responses to visible light stimuli to which wild-type gametic cells exhibit a strong response, with several behavioral stages. Upon addition of all-trans retinal to these mutants, the photophobic responses are restored with minor quantitative differences from wild-type populations. Using both light and electron microscopy, we have compared the ultrastructural characteristics of wild-type C. reinhardtii to those of both mutants. As previously described, wild-type cells contain an eyespot consisting of 2-4 layers of pigmented granules encased within thylakoid membranes, located between the distal extremities of the flagellar root. This structure is also visible as an orange-red spot in light microscopy. The photoreceptor is thought to be concentrated in the plasma membrane above the eyespot. The mutant, CC-2359, lacks this eyespot as seen by both light and electron microscopy, even when the photophobic response has been restored. FN68-like mutants studied earlier by Morel-Laurens and Feinlieb and others contain an eyespot which can be seen only by electron microscopy. In FN-68, the eyespot generally has the same dimensions as in wt cells, differing mainly in pigment granule appearance.(ABSTRACT TRUNCATED AT 250 WORDS)