Visualizing the Spatial Arrangement of Flagella-Eyespots-Cell Fusion Sites in Gametes and Planozygotes of Chlamydomonas reinhardtii (Chlorophyceae, Chlorophyta) with High-Resolution FE-SEM

Abstract

Cell fusion sites of the gametes relative to the flagella and eyespot occupy different positions between the opposite mating types in several green algae. This phenomenon has been found by light microscopy in Chlamydomonas reinhardtii and later confirmed by a computer-aided 3D-reconstruction of serial sections viewed with transmission electron microscopy (TEM). In this study, we determined this spatial relationship in the gametes and planozygotes of C. reinhardtii using field emission scanning electron microscopy (FE-SEM). This technique is very easy, straightforward, and enables the observation of numerous cells with high resolution comparable to TEM. In addition to flagella and fertilization tubules, FE-SEM visualized the eyespot and their lipid globules that has not been detected in C. reinhardtii gametes using conventional SEM. Using the eyespot as a positional marker, it was shown that mt− gamete fusion site is located on the same side of the plane bisecting the flagellar beat as the eyespot whereas the mt+ site is located in the opposite position. After cell fusion, 2 eyespots aligned on the same side of the quadriflagellate planozygote. The 2 no. 1 and 2 no. 2 flagella each from the mt+ and mt− gamete became a pair and pointed to the same direction. These results are consistent with the previous observations. Therefore, it is suggested that FE-SEM is an ideal technique for the visualization of topographical images of the flagella-eyespots-cell fusion sites in the mating gametes and planozygotes in C. reinhardtii.