Gametogenesis in the green algaOedogonium cardiacum

Summary

The mitoses and cytokineses leading to the formation of male and female gametes inOedogonium cardiacum are described at the ultrastructural level. Preceeding spermatogenesis, the nucleus and spindle are apically polarized for the first division, abutting the end wall near a ring which is much smaller than that characteristic of vegetative division. Cell division then cuts off a small, discoidal antheridial cell, a process repeated several times for each vegetative cell. The antheridial cell then divides once or twice more, generally transversely and/or longitudinally, but wall rupture and ring expansion play no role in these latter divisions. Fusion of vesicles lead to cross wall formation during the first two divisions, but membrane furrowing through the phycoplast partitions the daughter spermatids at the third and last cytokinesis. Discrete but amorphous bodies inside the poles of spermatogenous spindles are interpreted as Microtubule-Organizing-Centers, responsible for organizing the spindle microtubules.

The nucleus and spindle are also highly polarized for the division that forms an oogonium and its complementary suffultory cell. However, polarization is now strongly basal, and the asymmetric cytokinesis partitions the cell so that most of the cytoplasm moves during expansion into the future oogonium.