Timing of meiotic crossing-over in Drosophila melanogaster.

Abstract

Investigations on the effect of temperature shock on crossing-over frequency and interference in the X chromosome of Drosophila melanogaster revealed that a decrease in recombination frequency due to response of oocytes in an early meiotic stage which is known to be coincidental with DNA synthesis was accompanied by a decrease in interference. It is therefore suggested that crossing-over and DNA synthesis are not coincidental events, and that the target of temperature treatment is some precondition of exchange. Later in meiosis a minor effect on crossing-over frequency was observed which was not accompanied by any alteration in interference. This later effect may thus be coincidental with the event of exchange itself. The question concerning the timing of meiotic crossing-over in relation to DNA synthesis, cytological stages of meiosis, and other events lacks an unambiguous solution. Many contradictory interpretations are found in the literature. Some authors have presented evidence for the traditional hypothesis, i.e. that crossing-over occurs in zygo-pachytene after the main bulk of DNA synthesis. Others favor the view that crossing-over is coincidental with premeiotic DNA synthesis which is known to occur during interphase. The most convincing evidence for the traditional view comes from the works of ROSSEN and WESTERGAARD, PEACOCK, and STERN and his coworkers. ROSSEN and WESTERGAARD (1966) observed that premeiotic DNA synthesis in Neottiella rutilans occurs before caryogamy, i.e. unequivocally before crossing-over. PEACOCK (1970) observed that the temperature-sensitive period of chiasma formation in Coniaeu australasiae is coincidental with “early pachytene”. STERN’S group has convincingly shown that a certain amount (0.3 yo) of total DNA in Liliaceous plants has a delayed replication which does not occur until zygo-pachytene (HOTTA, ITO and STERN 1966). There is DNA synthesis both in zygo- and pachytene. The synthesis in zygotene is apparently responsible for chromo