SUMMARY In the planozygotes of chlorophytes, parental flagella and eyespot(s) are generally arranged in a coordinated manner in the cell following the fertilization of biflagellate gametes. This phenomenon can be attributed to the process of gamete‐gamete fusion occurring at a specific location on the cell surface, which is designated as the mating structure/cell fusion site. Notably, this structure/site occupies distinct positions in the gametes of opposite mating types or sexes. To ascertain whether analogous cell fusion processes and planozygote formation are observed in species lacking eyespots, the fertilization of the markedly anisogamous marine green macroalga Codium fragile was observed using fluorescence microscopy, field emission‐scanning electron microscopy (FE‐SEM) and transmission electron microscopy (TEM). Both the female and male gametes were biflagellate and, presumably, lacked the mating structure. Following the mixing of female and male gametes, the male gametes immediately gathered around the female gamete and adhered to its surface within 10 s. Subsequently, one male gamete entered the female gamete at the vicinity of the flagellar base, where the female nucleus was located. In the mating gametes, gamete‐gamete adhesion and fusion occurred between the lateral side of the male gamete, which was parallel to the flagellar beat plane, and the cell anterior of the female gamete, which was close to the flagellar base. This suggests that cell fusion occurred at the limited region of the gamete cell surface. Consequently, female and male flagella became a pair and showed a Y‐shaped arrangement in the planozygote when viewed from the cell anterior.
Miyamura et al. (Tue,) studied this question.