Occurrence of double strand DNA breaks and pairing of homologous chromosomes are preconditions of crossing over in Drosophila melanogaster, and its timing

Brood pattern analysis of the effect of a heat shock (35 degreesC 24 h) giv en to the parental females in presence and absence of In (2L + 2R) Cy in he terozygous condition in the second chromosome on recombination and interfer ence in the cv - v - f region of the X chromosome showed the following: (i) In the absence of the inversion recombination frequencies in both gene int ervals increased in the daily broods, which represent oocytes in which prem eiotic DNA synthesis was occurring during the heat shock treatment. (ii) In the presence of the inversion recombination frequencies did not increase i n any of the daily broods. (iii) The frequencies of double crossovers did n ot increase in any broods either in the presence or in the absence of inver sion. (iv) The coefficient of coincidence changed in virtually all broods b oth in the presence and absence of the inversion, indicating that the heat shock affects the distribution of single crossovers. (v) The results showed that the effect of the heat shock on the coefficient of coincidence was si milar during premeiotic DNA synthesis both in the absence and presence of t he inversion, but different in the broods representing oocytes which were u ndergoing the first meiotic division, suggesting that crossing over occurs during this division, probably during pachytene.