Detection of spindle pushing forces in vivo during anaphase B in the fungus Nectria haematococca

Abstract

AbstractForces that elongate the spindle during anaphase B of mitosis might be generated in the asteis, in the spindle, or in both. In the fungus Nectria haematococca, it has already been shown that the asters pull on the spindle pole bodies (SPBs) through‐out anaphase B. In this study, we used computerized video motion analysis to characterize brief episodes of spindle bending and straightening to find out if such bending is caused by spindle pushing forces. In three episodes there were two distinct components of spindle bending and straightening: one spanning the entire episode and comprising spindle elongation and another, superimposed on the first, involving a shortening of the distance between the SPBs. In a fourth episode, only spindle elongation was involved. All four spindles elongated rapidly while bending and underwent net growth during the overall bending‐straightening episode at an average rate of 4.2 μm/min. The path of one aster of a fifth mitotic apparatus was blocked by a large, occluding vacuole. This obstacle caused the migration of the mitotic apparatus to stop, resulting in a long (25 sec) episode of spindle curving and bending, usually without any substantial reduction in the distance between the SPBs as well as a marked reduction (from 4.7 to 0.65 μm/min) in the rate of spindle elongation. The results provide evidence that spindle pushing forces are active in vivo during anaphase B in N. haematococca and that they, along with astral pulling forces, help to elongate the spindle at a mostly constant rate. This is the first demonstration of both kinds of spindle elongation forces in the same organism.