Spindle function in the green alga Mougeotia: Absence of anaphase A correlates with postmitotic nuclear migration

Abstract In the filamentous green alga Mougeotia, each daughter nucleus formed by mitosis is then rapidly moved along the recently divided daughter cell to the central cleavage developing in the chloroplast. This movement is brought about by a cone‐shaped array of microtubules (MTs) that ensheath the daughter nucleus and are focused upon a small region, presumably a microtubule‐organising center (MTOC). Movement is completed when the MTOC locates and then resides in the chloroplast cleavage, drawing the nucleus into this position. The mitotic spindle is open with initially broad, ill‐defined poles. Anaphase A contributes minimally, if at all, to chromosome separation since the half spindles remain about the same length during anaphase and telophase. Thus, anaphase is accommodated and probably achieved by spindle elongation; the interzonal MTs also generate a rudimentary phragmoplast at the ingrowing cleavage furrow. The persistent polar MTs become directly transformed into the cone‐shaped array and initiate nuclear movement during early telophase. Various closely or distantly related green algae show this trait of persistent polar MTs. We conclude that this trait has allowed some species to evolve a motility system based directly on the capabilities of astral MTs, for generating the postmitotic nuclear movement essential for the restoration of the interphase cell organization.