CHECKPOINT CONTROL IN CRANE-FLY SPERMATOCYTES - UNATTACHED CHROMOSOMES INDUCED BY CYTOCHALASIN-D OR LATRUNCULIN TREATMENT DO NOT PREVENT ORDELAY THE START OF ANAPHASE

Variable numbers of bivalents and sex chromosomes do nor attach to the spindle when prophase or early prometaphase cranefly spermatocytes (2 n = 8) are treated with cytochalasin D or latrunculin. The unattached bivalents lie in the cytoplasm or at the spindle pole, and they do nor delay onset of autosomal anaphase; sometimes they disjoin at the same rime as the attached bivalents, so they respond to the global signals that initiate anaphase. Unattached sex chromosomes do not delay autos omal anaphase, either. Of various interpretations of these data, we th ink the best explanation is that the checkpoint system responds to phy sical rather than chemical cues; we think that the spindle is a ''tens egral'' structure, that chromosomes need to interact with the spindle in order to be recognised by the anaphase-onset ''checkpoint control'' , and that the physical interaction of chromosomes with spindle acts a s a signalling network. Cytochalasin D and latrunculin treatments dela y onset of sex chromosome anaphase (which normally occurs about lj min after autosomal anaphase) and cause altered patterns of sex-chromosom e segregation.