FERTILIZATION IN OEDOGONIUM. III. KARYOGAMY

Karyogamy is described in Oedogonium cardiacum from ultrastructural studies. Close proximity of the two gamete nuclei in the fusion cell is established by plasmogamy, whereas karyogamy appears to be initiated by multiple contacts formed between the outer membranes of the adjoining nuclear envelopes. Blebs of endoplasmic reticulum (ER) originate from the outer membrane of each nuclear envelope; these ER blebs presumably contact and fuse with the outer membrane of the nuclear envelope of the opposing nucleus. This is followed by the fusion of the inner membranes of the opposing nuclear envelopes, thereby resulting in a series of small connective bridges between the two gamete nuclei. It is estimated that in this manner 30–50 bridges are formed, perhaps many more. Several of these bridges enlarge relative to the others; one presumably becomes the major connection between the fusing nuclei. As it continues to enlarge, any organelles positioned between the fusing nuclei are pushed aside. There is also evidence, particularly in later stages of karyogamy, that the smaller connective bridges fuse to form larger ones. Temporary cytoplasmic channels often result at the juncture of fusion. In other instances, isolated inclusions of cytoplasm may be delimited by remnants of nuclear envelope deep within the developing zygote nucleus; these inclusions disappear with subsequent development. Throughout karyogamy the contribution of the male gamete nucleus is readily recognized by the characteristic appearance of its highly condensed chromatin. Ultimately, however, this distinction is lost and the content of the mature zygote nucleus assumes a more uniform appearance very similar to that of an egg nucleus. The complete process of fertilization in Oedogonium may occur within 15 min of mixing the spermatozoids with eggs.