. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 252 R. G. STEEN AND L. MUSCATINE X OJ 73 c. 0 468 Time (days) 10 20 FIGURE 2. Mitotic index (%) of zooxanthellae in Aiptasia pulchella vs. time in dark (or light) at 25°C after initial treatment as in Figure 1. Symbols and vertical bars as in Figure 1. expulsion, the loci of algae along the long axis of host endoderm cells, before and after exposure to darkness and low temperature, were determined from Rf measure- ments of 80-200 cells in thick sections of tentacles. Results are shown in Figure 3. Zooxanthellae in cells of c
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 252 R. G. STEEN AND L. MUSCATINE X OJ 73 c. 0 468 Time (days) 10 20 FIGURE 2. Mitotic index (%) of zooxanthellae in Aiptasia pulchella vs. time in dark (or light) at 25°C after initial treatment as in Figure 1. Symbols and vertical bars as in Figure 1. expulsion, the loci of algae along the long axis of host endoderm cells, before and after exposure to darkness and low temperature, were determined from Rf measure- ments of 80-200 cells in thick sections of tentacles. Results are shown in Figure 3. Zooxanthellae in cells of controls maintained in darkness for 4 h at 25°C were evenly distributed along the long axis of host cells. During prolonged maintenance in dark- ness, the population shifted toward the apical end of the host cells. In cold-treated anemones, this shift was detectable even at time zero. Within 24 hours after the cold treatment, the pattern appears reversed, but it is likely that the zooxanthellae near the luminal face of the host cell had already been expelled. The apical shift is restored after two days and considerably exaggerated after four days. These data are consistent with the interpretation that a change in intracellular locus precedes and accompanies the discharge of zooxanthellae. Uhrastructure of zooxanthellae in situ and during expulsion. Zooxanthellae in situ in anemones maintained in light at 25°C are shown in Figure 4a. The cell envelope consists of two to five sets of double membranes between the presumptive animal cell vacuolar and zooxanthellae plasma membranes. The average thickness of the envelope is about 60 nm. The chloroplast is lobed, with a prominent chloroplast membrane and lamellae in the three-thylakoid configuration. The pyrenoid (Fig. 4a, inset) is congruent with the chloroplast stroma but apparently not traversed by thyla- koids. It is capped by a thick sheath, presumably consisting mainly of starch. The nucleus contains condensed chromosomes. Mit
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
