Evidence bearing on the coincidence of exchange and DNA replication in the oöcyte of Drosophila melanogaster.

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Abstract
The times of DNA replication and of crossingover in Drosophila melanogaster oocytes were investigated. Incorporation of H3-thymidine was used as a cytological "marker" for the 1st event, and the increase in crossing-over induced by elevated temperature as a genetic "marker" for the 2nd. The results show that the interval between the occurrence of each of the 2 processes and the mature oQcyte is a minimum of 6 and an average of 8-9 days. The similarity in time intervals indicates that the two processes are roughly coincident. The evidence strongly suggests that both events occur in the early 16-cell cysts, which are located predominantly in the middle germarium, perhaps in a stage corresponding to the final premeiotic interphase.