Anaphase chromosome separation in dividing generative cells ofTradescantia

Summary

In dividing generative cells ofTradescantia, kinetochore pairs do not line up on a typical metaphase plate, but instead are distributed along the length and depth of the cell prior to anaphase onset. Kinetochore (K) fibers are linked to each other and to a system of axial microtubule (Mt) bundles in an arrangement that makes discrete half spindles, if present, not immediately obvious. Because such arrangements may have important implications for the conduct of the remainder of division, anaphase events were closely scrutinized using a combination of tubulin and kinetochore immunocytochemistry (the latter with CREST serum). Anaphase appears to consist of three principal processes. Around the time of anaphase onset, K-fibers and surrounding Mts become reorganized into two large superbundles. To each superbundle is attached a set of nonfilial kinetochores bound for one end of the cell. The K-fibers then appear to shorten to varying degrees; in many cases, kinetochores become linked directly to the superbundles. The superbundles then separate in an anaphase B-like process, further moving the kinetochores toward opposite ends of the cell. The superbundles themselves shorten, and distances within the bundles also decrease, such that the kinetochores cluster closer together. The results indicate that reorganization of Mts into superbundles (and the consequential manifestation of bipolarity) is important for orderly chromosome separation.