CROSSING-OVER IN REARRANGING CHROMOSOMES IN DROSOPHILA - THE ROLE OF DELAYED PAIRING

A Df(2R)MS2-10 deletion of pericentromeric heterochromatin, an 1s(Y; 2 L)419 insertion of Y material in the region 34A, as well as nondisjunc tion of chromosomes 2 in 2/F(2L); F(2R) females did not directly preve nt chromosome arms in chromosome 2 of Drosophila from pairing. However , these events resulted in (1) two- to four-fold decrease in the rate of crossing-over in chromosome 2; (2) a decreased proportion of exchan ge tetrads two to three times greater for multiple-exchange tetrads th an for single-exchange ones; (3) and a decreased rate of cross-over th roughout the entire chromosome arm enhanced in a proximal direction. A n ln(1)dl-49+B-M1 inversion in the X chromosome cancelled the suppress ion of crossing-over. Crossing-over increased due to an increasing pro portion of single-exchange tetrads. The changes in crossing-over found cannot be explained by asynapsis in the chromosomes with rearrangemen ts. According to the authors, these changes are probably accounted for by a delayed pairing of these chromosomes. The delayed pairing of ind ividual chromosome regions or the whole chromosome is considered the m ost common type of pairing disturbance. Its effects on meiosis are dis cussed.