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Abstract
Ultrastructural details of zygosporogenesis in Zygorhynchus heterogamus were observed with scanning and transmission electron microscopy, using both conventional chemical fixation and the cryopreservation techniques. Detailed observations of origin and development of walls of the entire sexual apparatus are reported. Development of both gametangial septa was observed to be identical and plasmodesmata were observed to penetrate septa beginning with their initial formation through full maturation of the zygospore. Wart formation was apparently initiated with appearance of vesiculate bodies closely appressed to the zygosporangium wall, external to the plasma membrane. Wart development progressed through deposition of a stacked series of electron-opaque disks. Rupture of the zygosporangium wall by the developing zygospore was concurrent with formation of a contiguous warty layer. For the first time, clear transmission electron microscopic observations of multiple wall layers in the fully mature zygospore are presented. A detailed historical discussion on, and a proposal for standardized structural nomenclature are presented as well as advantages of low temperature scanning electron microscopy in the preservation of delicate reproductive structures, and the benefits and failings of freeze substitution in the preservation of mature zygospores are discussed.