An ultrastructural study of cytomixis in tobacco pollen mother cells.

Intercellular chromatin migration (cytomixis) in the pollen mother cells of two tobacco (Nicotiana tabacum L.) lines was analyzed by electron microscopy during the first meiotic prophase. The maximal manifestation of cytomixis was observed in the pachytene. As a rule, several cells connected with one another by cytomictic channels wherein the nuclei migrated were observable at this stage. In the majority of cases, nuclei passed from cell to cell concurrently through several closely located cytomictic channels. Chromatin migrated between cells within the nuclear envelope, and its disintegration was unobservable. The nucleus, after passing through cytomictic channels into another cell, can be divided into individual micronuclei or, in the case of a direct contact with another nucleus, can form a nuclear bridge. It has been demonstrated that the chromatin structure after intracellular migration visually matches the chromatin structure before it passed through the cytomictic channel. No signs of pyknosis were observable in the chromatin of the micronuclei formed after cytomixis, and the synaptonemal complex was distinctly seen. The dynamics of changes in the nucleoli during cytomixis was for the first time monitored on an ultrastructural level. Possible mechanisms determining cytomixis are discussed and the significance of this process in plant development is considered.