Electron Microscopy of Developing Aphanomyces Oogonia and Oospores

SUMMARY

Distinct cytological changes characterize the development of Aphanomyces oogonia and oospores in alfalfa roots. Evidence for gametangial meiosis and development of dense-body vacuoles into fingerprint vacuoles is reported. The contents of the entire oogonium develop into a single oosphere. Fertilization and karyogamy trigger rapid changes in the structure and distribution of cytoplasmic components in the developing oospore. Periplasm is not formed. Fingerprint vacuoles fuse to form membrane-bounded reserve globules in the center of the oospore while lipid bodies congregate toward the oospore periphery. A thick oospore wall develops as a bipartite structure that becomes irregular in outline as the oospore cytoplasm condenses.