MECHANISM OF INTERCHROMOSOMAL EFFECT ON C ROSSING-OVER IN DROSOPHILA-MELANOGASTER - DELAYED CROSSING-OVER

Interchromosomal effect on crossing-over (IEC) in autosome 2 has been studied in 2/F(2L); F(2R) females heterozygous for free arms (acrocent rics) and in Is(Y;2)419/+ females with an insertion of Y-material into the region 34A. IEC was induced by In(1)dl-49 + B(M1) inversion. Mani festations of IEC included increased recombinational length of chromos ome 2 and decreased interference. IEC was not observed in Df(2L)TW161/ + females with 38A-40 deletion. The patterns of IEC in three types of gametes of the 2/F(2L); F(2R) female depended on the pairing relations of the affected chromosome (chromosome-responder). In the case of nor mal pairing between the metacentric autosome 2 (the metacentric) and t he F(2R) acrocentric, the increment in 2R length was minimal (20%), an d the increment in the proportion of multiple-exchange (high-rank) tet rads (E2 + E3), maximal (8 to 10%). In the case of disturbed pairing 2 -F(2R) nondisjunction, 2R length was increased by 77%, paralleled by a minimal increase in the proportion of high-rank tetrads (4%). Similar ly, in females with the insertion, a pronounced increase in 2L length (74%) was associated with a moderate level of high-rank tetrads. When pairing in the chromosome-responder was normal, the increment in cross ing-over was maximal in the pericentromeric region. In the case of dis turbed pairing, this maximum either shifted toward the middle of the a rm 2-F(2R) nondisjunction, or occupied a distal position (in females w ith the insertion). It is concluded that IEC pattern depends on the or der of pairing in the chromosome-responder. The mechanism of IEC appea rs to be related to pairing ''defects'' within the responder. It is te mpting to speculate that the onset of crossing-over is a whole-cell ev ent, which is regulated by the overall level of chromosome pairing wit hin the meiotic cell. Chromosomal aberrations increase the time requir ed for attaining this level, and the start of crossing-over is delayed . As a result, (1) exchanges are observed in the regions of late synap sis, which are usually not involved in crossing-over; (2) overabundanc e of recombination enzymes, caused by delayed start of crossing-over, creates the conditions for decreased interference in paired regions.