Swimming zygote formation mediated by symmetrically arranged cell fusion sites in the gametes of Chlamydomonas eugametos (Chlorophyceae, Chlorophyta)

SUMMARY The spatial arrangement of cell fusion sites and eyespots of the gametes in vis‐à‐vis mating pairs of Chlamydomonas eugametos , corresponding to the swimming zygote of Chlamydomonas reinhardtii , were studied using field emission scanning electron microscopy and light microscopy, respectively. After the adhesion of mt + and mt − gametes, each of which has one eyespot and two flagella elongated from the cell anterior, gamete–gamete fusion occurred at the anterior papilla on the cell apex of each gamete, leading to the formation of a vis‐à‐vis mating pair, indicating that the cell fusion site is arranged symmetrically relative to the flagellar beat plane. In vis‐à‐vis mating pairs, two eyespots could be seen on the same or on opposite sides of the cell. As only the mt + flagella and eyespot are functional for swimming and phototaxis in the vis‐à‐vis mating pair of C. eugametos , it is suggested that coordinated alignment of the parental gamete flagella and eyespots in the swimming zygote and asymmetric positioning of the gamete mating structure/cell fusion sites, which is prevalent in C. reinhardtii and other chlorophytes, is not necessary for C. eugametos .