Elucidation of the enigma of the “metaphase band” ofChlamydomonas reinhardtii

Summary

Conventional, computer-aided, and morphometric analysis of ultrathin serial sections through cells at consecutive stages of mitosis has clarified several aspects of the morphogenesis of the socalled “metaphase-band” (=MB) inChlamydomonas reinhardtii. In contrast to the original interpretation, the MB is not a single set of 4 microtubules (=MTs). This erroneous impression is created by the transient spatial association of the two parental 4-membered microtubular flagellar roots. A long section of the roots, which are constituents of the flagellar basal apparatus and the cortical cytoskeleton at interphase, persists during mitosis. Each parental 4-membered root is tightly coupled to half of the bisected basal body complex, and each daughter cell receives one root. During migration of the bisected basal body-root complexes towards the nuclear poles during prophase, the two opposite roots are bent in the middle. Starting at these bent regions, the central thirds become parallelly aligned during metaphase. The roots shorten during anaphase elongation of the nucleus, which occurs without any further change in distance between the daughter basal body-root complexes.

At anaphase de novo assembly of cytoplasmic MTs was observed. Two intracellular MT arrays, focussed at the roots like the cortical MTs, penetrate deeply into the cell. They are interpreted as the incipient cleavage MTs of the phycoplast, or radicoplast.