EVIDENCE FOR POLEWARDS FORCES ON CHROMOSOME ARMS DURING ANAPHASE

We have identified new mitotic forces in crane-fly spermatocytes, sepa rate from forces on the kinetochore, that propel chromosome arms in an aphase towards the spindle pole. In normal spermatocytes, the chromoso me arms in anaphase generally trail the kinetochore to the pole. After ultraviolet-microbeam irradiation of a kinetochore spindle fibre, how ever, chromosome arms moved closer to the pole than the kinetochore. T his poleward arm-movement occurred regardless of whether the irradiati on stopped the movement of the associated chromosomes, and occurred bo th in chromosomes associated with the irradiated fibre and in chromoso mes not associated with the irradiated fibre. Arms that moved ahead of the kinetochore continued to lead the kinetochore To the pole for the duration of anaphase. Ultraviolet-microbeam-irradiation-induced movem ent of arms ahead of the kinetochore is specific for irradiation of sp indle fibres: irradiations of the cytoplasm outside the spindle had no effect, and irradiations of the region between spindle and mitochondr ial sheath (that outlines the spindle) and irradiations of the interzo nal region are much less effective than irradiations of spindle fibres in causing arms to move. We argue that in crane-fly spermatocytes for ces propelling chromosome arms toward the pole are part of normal anap hase. (C) 1996 Wiley-Liss, Inc.