ultrastructure of meiosis in Woronina pythii (Plasmodiophoromycetes)

During cleavage of cystogenous plasmodia, nuclei of Woronina pythii underwent one meiosis I-like division. The division was centric, closed with polar fenestrations, and characterized by the formation of synaptonemal complexes. Pachytene nuclei exhibited nuclear projections which were heretofore undocumented structural modifications of the nuclear membrane. Hypothetical functions of these structures are proposed. During diakinesis, fragmentation of the nuclear envelope occurred at the poles through the apparent growth of microtubules. Intranuclear membranous vesicles appeared to bud from the inner membrane of the original nuclear envelope during metaphase and anaphase, and coalesced on the surfaces of the separating chromatin masses, thereby forming new daughter nuclear envelopes within the intact original nuclear envelope. Similar descriptions of the spindle apparatus and events relating to nuclear envelope breakdown and reformation have been reported for the mitotic divisions of W. pythii. Nuclei of some mature cysts contained intranuclear cisternae which were continuous with the inner membrane of the nuclear envelope, and these cisternae frequently partitioned the nuclei into two or three separate compartments. The formation of intranuclear cisternae was presumed to result in the eventual fission of nuclei, which may represent functionally the second meiotic division.