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

Abstract

Brood pattern analysis of the effect of a heat shock (35 degrees C 24h) given to the parental females in presence and absence of In (2L + 2R) Cy in heterozygous condition in the second chromosome on recombination and interference 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 intervals increased in the daily broods, which represent oocytes in which premeiotic DNA synthesis was occurring during the heat shock treatment. (ii) In the presence of the inversion recombination frequencies did not increase in any of the daily broods. (iii) The frequencies of double crossovers did not increase in any broods either in the presence or in the absence of inversion. (iv) The coefficient of coincidence changed in virtually all broods both 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 similar during premeiotic DNA synthesis both in the absence and presence of the inversion, but different in the broods representing oocytes which were undergoing the first meiotic division, suggesting that crossing over occurs during this division, probably during pachytene.