Mapping a meiotic pairing site in the second chromosome of Drosophila melanogaster males

Homologous chromosome pairing at early meiotic prophase I is very important for key meiotic events such as chromosome segregation and recombination, but little is known about the process. One approach to further understanding of the process of chromosome pairing has been to identify chromosome pairing sites. This has been accomplished by genetic and cytological analysis of pairing behavior of chromosome rearrangements involving mutants such as deletions and translocations with altered pairing capacity and by the use of transgenic methodology. A major focus of such studies has been Drosophila males, in which synaptonemal complex is not formed during early prophase of meiosis I, and where there is no recombination or chiasma formation even though chromosomes are paired properly. These phenomena simplify the analysis of pai ring and of genetic studies using Drosophila males. By using this material, the X-Y chromosome pairing site has already been shown to reside within rDNA promoter region. The purpose of the present study was to refine the mapping of a strong pairing site previously shown to be in the proximal region of the left arm of the second chromosome (delimited by the break points of a 2-Y transposition, Tp (2:Y) G) using several deletions in proximal 2L (TW2,12, 50,65,84,161). Prophase I pairing patterns were analyzed by examination of squashed, orcein-stained adult testes from males carrying Tp(2;Y)G and each of the TW deletions. Anaphase i segregation patterns were analyzed by progeny testing males carrying Dp(2;Y)G element and each of the TW deletions along with a normal chromosome 2. Three TW deletions proved to retain the strong pairing site in the cytological test and to compete effectively with a normal chromosome 2 in the segregation test, whereas two TW deletions both lacked pairing ability in the cytological test and tailed to compete effectively in the segregation test. A sixth TW deletion gave intermediate results in both tests. Thus, the results from those two methods indicate that the chromosome pairing site exists in a narrow interval in the left arm of chromosome #2 (39D3 - F1) and possibly within the histone gene cluster.