Light and Electron Microscopic Studies of Ascobolus Stercorarius. I. Nuclear Divisions in the Ascus

SUMMARY

A study of nuclear divisions in the asci of Ascobolus stercorarius using acetocarmine for light microscopy and glutaraldehyde-OsO₄ fixation for electron microscopy has revealed that the associated achromatic structures are essentially as described in other nucleated organisms. Chromosomal fibers were observed at several stages of meiosis, and the continuous spindle fibers were demonstrated at the first post-meiotic mitotic division. Astral rays were evident at all stages of the three divisions that occur in the ascus. The microtubules composing the astral rays, the chromosomal fibers, and the continuous spindle fibers were of the same diameter and morphology as the microtubules associated with nuclear division in other fungi and in other organisms. A centriolar plaque was detected at all stages of the nuclear cycle in the ascogenous hyphae, croziers, and asci. Usually the centriolar plaque was closely associated with the outer membrane of the nuclear envelope. A tubular structure of the centriolar plaque was evident in some electron micrographs. It is suggested that the centriolar plaque is a modified centriole, since it has several characteristics of a true centriole. The nuclear envelope seems to remain intact, at least, until early anaphase in meiosis and late anaphase in mitosis.