[Crossing over in rearranged Drosophila chromosomes: the role of delayed pairing].

A Df(2R)MS2-10 deletion of pericentromeric heterochromatin, an Is(Y;2L)419 insertion of Y material in the region 34A, as well as nondisjunction of chromosomes 2 in 2/F(2L); F(2R) females did not directly prevent 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 exchange tetrads two to three times greater for multiple-exchange tetrads than for single-exchange ones; (3) and a decreased rate of crossing-over throughout the entire chromosome arm enhanced in a proximal direction, An In(l)dl-49+BMl inversion in the X chromosome cancelled the suppression of crossing-over. Crossing-over increased due to an increasing proportion of single-exchange tetrads. The changes in crossing-over found cannot be explained by asynapsis in the chromosomes with rearrangements. According to the authors, these changes are probably accounted for by a delayed pairing of these chromosomes. The delayed pairing of individual chromosome regions or the whole chromosome is considered the most common type of pairing disturbances. Its effect on meiosis are discussed.