Cytokinesis occurs at boundaries of domains delimited by nuclear‐based microtubules in sporocytes of Conocephalum conicum (Bryophyta)

Studies of the microtubular cytoskeleton during sporogenesis in the hepatic Conocephalum conicum (Bryophyta) have revealed several unusual phenomena that contribute to understanding the cytokinetic apparatus in plant cell division. Although a typical phragmoplast forms in the interzonal microtubules of the first division spindle and expands to the cell periphery, no cell plate develops. There is no evidence of predetermined division sites and the orientation of both first and second meiotic spindles is imprecise. Simultaneous division of the cytoplasm follows second nuclear division. Equal apportionment of the cytoplasm appears to be a function of the establishment of cytoplasmic domains in the coenocyte, the boundaries of which are delimited by interaction of postmeiotic microtubule systems radiating from the four nuclei. Primary phragmoplasts are initiated in phragmoplasts that are initiated between nonsister nuclei. Depending upon the arrangement of nuclei in the nonpolar sporocyte, from one to three secondary phragmoplasts develop in the zones of contact between opposing sets of microtubules. Except for the site and time of initiation, the two types of phragmoplasts are identical. Eventually the phragmoplasts become confluent and cell plates form in all second division phragmoplasts. It is clear that typical functional phragmoplasts can form in sites determined by interaction of postmeiotic microtubule systems as well as in interzonal spindles as is common in plant cell division.