SPONTANEOUS X-CHROMOSOME MI AND MII NONDISJUNCTION EVENTS IN DROSOPHILA-MELANOGASTER OOCYTES HAVE DIFFERENT RECOMBINATIONAL HISTORIES

Recent studies of human oocytes have demonstrated an enrichment for di stal exchanges among meiosis I (MI) nondisjunction events and for prox imal exchanges among meiosis II (MII) events. Our characterization of 103 cases of spontaneous X chromosome nondisjunction in Drosophila ooc ytes strongly parallels these observations. The recombinational histor ies of MI (97/103) and MII (6/103) nondisjunctional ova were strikingl y different. MI nondisjunction occurred primarily in oocytes with non- exchange X chromosomes; of the few nondisjoining exchange bivalents, m ost carried distal crossovers. Thus, spontaneous MI nondisjunction ref lects the failure of the achiasmate segregation systems. MII nondisjun ction occurred only in oocytes with proximal exchanges. We propose sev eral models to explain how very proximal exchanges might impair proper segregation.