Near-neighbor analysis of spindle microtubules in the alga Ochromonas.

The near-neighbor spacing of microtubules (MTs) in the spindle of the alga Ochromonas is analyzed. The technique of near-neighbor analysis of MTs (as developed by McDonald et al. [9]) in the mid-region of the Ochromonas spindle (overlap) shows that MTs from one pole preferentially associate with MTs from the opposite pole at a center-to-center distance of 35 to 43 nm. However, in the half spindle between the chromosomes and the poles, kinetochore MTs (kMTs) do not preferentially associate with other MTs in the half spindle but instead are arranged essentially at random. Individual polar MTs (MTs attached to one pole), kMTs and free MTs (MTs unattached to the poles) were selected for near-neighbor analysis over their entire lengths. The spacing of MTs in the overlap is compatible with those models for mitosis which propose that separation of the poles is accomplished by sliding between closely spaced MTs of opposite polarity. In contrast to the overlap, the arrangement of MTs in the half spindle is not compatible with MT2MT sliding theories that propose that chromosome movement is accomplished by sliding between kMTs and polar MTs.