The organization of microtubules during anaphase and telophase spindle elongation in the rust fungus Puccinia.

The entire framework of microtubules (MTs) in the meiotic spindle of the rust fungus Puccinia has been reconstructed during the later stages of meiosis I, by tracking MTs through transverse serial sections. This spindle is of special interest because it elongates considerably during anaphase spindle elongation, from 5 microns at metaphase to 15 microns at telophase. The spindle is composed mainly of MTs from opposite poles which interdigitate or overlap in the middle of the spindle. In the overlap region, MTs from one pole seek out as near neighbors, MTs from the opposite pole at a preferred spacing of 43 to 55 nm. During anaphase elongation three changes in spindle structure occur: 1) the region of overlap decreases, but this reduction in overlap cannot account for all the increase in spindle length; 2) interdigitated MTs (MTs from one pole that are within 80 nm of a MT from the opposite pole) dramatically increase in length by MT polymerization and; 3) kinetochore MTs, free MTs (those unattached to the poles) and non-interdigitated polar MTs shorten and disappear. The mechanism of anaphase elongation and the control over MT polymerization and depolymerization during anaphase are discussed.