Gamete dimorphism of the isogamous colonial green alga <scp><i>Gonium pectorale</i></scp> (<scp>V</scp>olvocales, <scp>C</scp>hlorophyta)

SUMMARY Transmission electron microscopy and field emission scanning electron microscopy were used to investigate the relationships between the spatial arrangement of the mating structure and the presence or absence of the mating type minus‐determining gene MID in the isogamous volvocine green alga Gonium pectorale , using strains NIES‐1712 and NIES‐1713, which belong to opposite mating types. The MID gene was present in NIES‐1712 and absent in NIES‐1713, representing mating types mt − and mt + , respectively. The gametes of both mating types had two flagella extending from the cell apex and an eyespot on the lateral side of the cell. The mating structure of the mt + gamete (NIES‐1713) was located at the cell anterior near the flagellar base on the opposite side of the flagellar beat plane to the eyespot (type α), whereas the mating structure in the mt − gamete (NIES‐1712) was located on the same side as the eyespot (type β), indicating that type α and type β gametes were produced in the MID ‐deficient and MID ‐containing mating types, respectively. After the gametes of the two strains were mixed, gamete fusion began between tubular mating structures extending from both gametic protoplasts and proceeded from the anterior to the posterior regions of the cell. This resulted in the formation of a quadriflagellate planozygote with two pairs of two flagella aligned in parallel and two eyespots located on the same side of the cell. Such mating type‐specific positioning of the mating structure in the gametes and the coordinated arrangement of flagella and eyespots in the planozygote, produced as a result of gamete fusion at the asymmetrically arranged mating structures, are the same as in the closely related species Chlamydomonas reinhardtii , suggesting the conserved subcellular architecture during the gametogenesis of each orthologous mating type in volvocine algae.