Eyespot behavior during the fertilization of gametes in Ulva arasakii Chihara (Ulvophyceae, Chlorophyta)

Behavior of the eyespots during the fertilization of Ulva arasakii Chihara was studied using field emission scanning electron microscopy (FE‐SEM). FE‐SEM enabled the visualization of the eyespot of biflagellate male and female gametes. The smaller male gamete has one protruded smaller (1.3 ± 0.15 μm× 1.0 ± 0.29 μm) eyespot and the larger female gamete has a larger (1.6 ± 0.2 μm× 1.1 ± 0.13 μm) one on a posterior position of the cell. The cell membrane over the eyespot region is relatively smooth compared to other parts of the cell body and exhibits hexagonal arranged lipid globules. Because the size of the cell and the morphology of the eyespot are different between male and female gametes, we could follow the fate of the eyespots during the fertilization. The initial cytoplasmic contact and fusion of the gametes takes place at their anterior end, slightly posterior to the flagellar base. The morphology of the fusing gametes followed two clearly distinguishable patterns. About half the gamete pairs lie side‐by‐side with their longitudinal axes nearly parallel, while the rest are oriented anti‐parallel to each other. In all cases, the larger female gamete fused along the same side as the eyespot, while the smaller male gamete fused along the side away from its eyespot. As fusion proceeds, the gamete pair is transformed into the quadriflagellate planozygote, in which the eyespots are positioned side‐by‐side on the region of cell fusion. These observations indicated that the opposite positioning of the eyespot relative to the cell fusion site in male and female gametes is important for the proper arrangement of the eyespots in the planozygote. The significance of this feature in advanced green algae is briefly discussed.