Computer-aided 3D-reconstruction of ultrathin serial sections through unactivated gametes of both mating types (mt+, mt-) of Chlamydomonas reinhardtii (strain 137c) and through zygotes at different stages of maturation (15-180 min) revealed the following: The mt+ gamete differs from the mt- gamete not only by the size of but also the location of the mating structure (MS). If the gametes were to be bisected in a median plane to the long axis and to the flagella, the mt+ MS would be found in the half lacking the eyespot and the mt- MS in the half harbouring the eyespot. The remnant(s) of the mt+(mt-) MS(s) in young zygotes almost always helped to identify which half of the zygote originated from the mt+ gamete, and which one from the mt-gamete. The eyespot-mating structure-flagellar apparatus (EMSFA) associations of the gamete pairs largely retained their original identities during plasmogamy. It is evident from the topographical arrangement of the partner EMSFA associations in the zygotes that plasmogamy may proceed at least in three different ways, producing zygotes: (i) in which the eyespot of the mt-gamete becomes located adjacent to the plane of fusion (ii) in which both eyespots are located in the vicinity of the plane of fusion (iii) in which both eyespots are located not only opposite to one another but also opposite to the plane of fusion. Ca. 2.5-3 h after the onset of zygote formation the flagellar roots shorten and the probasal bodies and the remnant MS start to escape detection. The dynamics of gamete fusion are discussed by comparison with known data.
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