The C. reinhardtii eyespot. (a) A diagram illustrating asymmetric localization of the eyespot relative to the cytoskeleton. Two flagella and four microtubule rootlets extend from a pair of basal bodies at the anterior end of the cell; both the mother basal body (small black oval) and the daughter basal body (small gray oval) are associated with a four-membered rootlet (M4 or D4) and a two-membered rootlet (M2 or D2). The single eyespot (large oval) is associated with the four-membered daughter rootlet (D4), and the flagellum assembled from the daughter basal body is termed the cis-flagellum, whereas the trans-flagellum is assembled from the mother basal body. (b) A light micrograph of a wild-type C. reinhardtii cell showing the single large equatorial eyespot. (c) A diagram illustrating the components of the eyespot. Rhodopsin photoreceptors, including ChR1 (light gray ovals), in the plasma membrane (PM) directly apposed to the chloroplast envelopes (CE) and layers of carotenoid pigment granules (dark gray circles) subtended by thylakoid membrane (TM). The microtubules of the D4 rootlet are arranged in a three-over-one pattern. (d) A diagram of a C. reinhardtii cell (as in panel a) illustrating measurements discussed throughout the text and listed in Table I. L, cell length; R1, distance from the basal bodies to the tip of the most extensively acetylated rootlet; R2, distance from the basal bodies to the tip of the second most extensively acetylated rootlet; E1, distance from the basal bodies to the posterior edge of ChR1 photoreceptor–specific fluorescence.