Rearrangement of the flagellar apparatuses and eyespots of isogametes during the fertilization of the marine green alga, Monostroma nitidum (Ulvophyceae, Chlorophyta)

Abstract

SummaryBehaviors of the flagellar apparatuses (flagella, basal bodies, microtubular roots, etc.), mating structures and eyespots of gametes during the fertilization of Monostroma nitidum were studied using field emission scanning electron microscopy and transmission electron microscopy. The biflagellate isogamete (mt+ and mt−) mating structure has a position that is converse between mt+ and mt− gametes relative to the flagellar beat plane and the eyespot. After the adhesion of mt+ and mt− gametes, gamete fusion occurred between the two mating structures. The cell fusion plane expanded to the cell surface as circumscribed by 1s–2d roots in mt+ gamete and 1d–2s roots in the mt− gamete. Two sets of flagellar apparatuses lay side by side in the planozygote and soon become mutually close. The no. 1 basal body of mt+ gamete and the no. 2 basal body of mt− gamete rotated in a counterclockwise direction, as viewed from the cell anterior. Then, the no. 2 basal body of mt+ gamete and the no. 1 basal body of mt− gamete slid into a face to face position. Finally, four flagella and basal bodies exhibited a cruciate arrangement. The basal bodies of the opposing pair (no. 1 and no. 2) were offset in a counterclockwise orientation by the basal body diameter. The 1s and 2d roots of the mt+ gamete lay nearly parallel to the 1d and 2s roots of the mt− gamete, respectively, at the cell fusion plane. Because of the asymmetric localization of the mating structure, association, and subsequent rearrangement of basal bodies and microtubular roots, two eyespots lay on the same side of the planozygote. After the settlement of the planozygote, the flagellar apparatus started to disintegrate in the zygote cytoplasm.