NEOCENTROMERE-MEDIATED CHROMOSOME MOVEMENT IN MAIZE

Neocentromere activity is a classic example of nonkinetochore chromoso me movement. In maize, neocentromeres are induced by a gene or genes o n Abnormal chromosome 10 (Ab10) which causes heterochromatic knobs to move poleward at meiotic anaphase. Here we describe experiments that t est how neocentromere activity affects the function of linked centrome re/kinetochores (kinetochores) and whether neocentromeres and kinetoch ores are mobilized on the spindle by the same mechanism. Using a newly developed system for observing meiotic chromosome congression and seg regation in living maize cells, we show that neocentromeres are active from prometaphase through anaphase. During mid-anaphase, normal chrom osomes move on the spindle at an average rate of 0.79 mu m/min. The pr esence of Ab10 does not affect the rate of normal chromosome movement but propels neocentromeres poleward at rates as high as 1.4 mu m/min. Kinetochore-mediated chromosome movement is only marginally affected b y the activity of a linked neocentromere. Combined in situ hybridizati on/immunocytochemistry is used to demonstrate that unlike kinetochores , neocentromeres associate laterally with microtubules and that neocen tromere movement is correlated with knob size, These data suggest that microtubule depolymerization is not required for neocentromere motili ty. We argue that neocentromeres are mobilized on microtubules by the activity of minus end-directed motor proteins that interact either dir ectly or indirectly with knob DNA sequences.