Meiotic Recombination in Drosophila Females Depends on Chromosome Continuity Between Genetically Defined Boundaries

Abstract

In the pairing-site model, specialized regions on each chromosome function to establish meiotic homolog pairing. Analysis of these sites could provide insights into the mechanism used by Drosophila females to form a synaptonemal complex (SC) in the absence of meiotic recombination. These specialized sites were first established on the X chromosome by noting that there were barriers to crossover suppression caused by translocation heterozygotes. These sites were genetically mapped and proposed to be pairing sites. By comparing the cytological breakpoints of third chromosome translocations to their patterns of crossover suppression, we have mapped two sites on chromosome 3R. We have performed experiments to determine if these sites have a role in meiotic homolog pairing and the initiation of recombination. Translocation heterozygotes exhibit reduced gene conversion within the crossover-suppressed region, consistent with an effect on the initiation of meiotic recombination. To determine if homolog pairing is disrupted in translocation heterozygotes, we used fluorescent in situ hybridization to measure the extent of homolog pairing. In wild-type oocytes, homologs are paired along their entire lengths prior to accumulation of the SC protein C(3)G. Surprisingly, translocation heterozygotes exhibited homolog pairing similar to wild type within the crossover-suppressed regions. This result contrasted with our observations of c(3)G mutant females, which were found to be defective in pairing. We propose that each Drosophila chromosome is divided into several domains by specialized sites. These sites are not required for homolog pairing. Instead, the initiation of meiotic recombination requires continuity of the meiotic chromosome structure within each of these domains. M EIOTIC recombination usually occurs between sophila have supported the view that SC formation ocsimilar or identical sequences on homologous curs prior to DSB formation (Jang et al. 2003). chromosomes. In most organisms, at least part of the The presence of single-strand tails at DSB sites promeiotic recombination pathway occurs within the convides a mechanism for homology searching and chromotext of the synaptonemal complex (SC), which holds some alignment (Roeder 1997). In organisms like Droaligned homologous chromosomes together along their sophila and C. elegans, however, another mechanism entire lengths (von Wettstein et al. 1984). Surprisaside from recombination must exist to precisely align ingly, organisms can be classified into at least two types homolgous chromosomes during meiosis. Indeed, DSBon the basis of the relationship of double-strand-break independent mechanisms for aligning meiotic chromo-(DSB) formation to the SC. In Saccharomyces cerevisiae, somes appear to be widespread. Similar to Drosophila DSB formation occurs prior to, and is required for, SC and C. elegans, homolog pairing in fission yeast involves a formation (Padmore et al. 1991). A similar course of DSB-independent component (Ding et al. 2004). While events occurs in...

Meiotic Pairing and Disjunction of Mini-X Chromosomes in Drosophila Is Mediated by 240-bp rDNA Repeats and the Homolog Conjunction Proteins SNM and MNM

“…Several of these have been found to affect segregation by acting as local enhancers of chiasmata formation, the products of homologous crossovers that act as stable connectors between homologs (Hawley 1980;McKim et al 1993;Villeneuve 1994;MacQueen et al 2002MacQueen et al , 2005Sherizen et al 2005). In contrast, other cis-acting sites promote homolog segregation independently of recombination.…”

Components of the RNAi Machinery That Mediate Long-Distance Chromosomal Associations Are Dispensable for Meiotic and Early Somatic Homolog Pairing inDrosophila melanogaster

“…Germaria were fixed for 4 min in 3.7% formaldehyde in 13 fix buffer (100 mm potassium cacodylate, 100 mm sucrose, 40 mm sodium acetate, and 10 mm EGTA). After fixation, ovaries were rinsed three times in 23 SSCT and hybridization was performed as previously described (Sherizen et al 2005). Subsequent to hybridization and after the final 23 SSCT rinse, ovaries were rinsed two times in PBST and the immunostaining protocol (above) was resumed.…”

“…In fact, in the somatic tissues of broader Dipterans, homologous chromosomes are paired (Metz 1916;Hiraoka et al 1993;Fung et al 1998). Thus, it has been proposed that somatic and meiotic pairing in Drosophila are achieved by the same mechanism and that meiotic pairing is simply an extension of pairing established in the mitotically dividing germline (Stevens 1908;Brown and Stack 1968;Vazquez et al 2002;Gong et al 2005;Sherizen et al 2005). The mechanism underlying the pairing interaction is thought to be mediated either directly by nucleic acid homology [through either direct DNA-DNA interactions (i.e., Wilson 1979, McGavin 1989 or interactions that may include transcribed RNA] or indirectly through protein-protein interactions (Comings and Riggs 1971;Gemkow et al 1998;Phillips et al 2005;Fritsch et al 2006;Phillips and Dernburg 2006).…”

The diverse roles of transverse filaments of synaptonemal complexes in meiosis

“…Furthermore, synapsis appears to be insensitive to homology: nonhomologous synapsis can occur under a variety of conditions (reviewed by Zickler and Kleckner 1999), and given the ease with which TF proteins self-assemble into regular stacks of SC-like structures (polycomplexes) (Öllinger et al 2005;Sym and Roeder 1994b), there are more reasons to wonder what prevents nonhomologous synapsis than what causes homologous synapsis. However, at least in some species, TFs are important for the maintenance of homolog alignment (Colaiácovo et al 2003;MacQueen et al 2002;Sherizen et al 2005).…”

“…, which is effectively a null allele, still display homolog alignment; however, maintenance of alignment in euchromatic regions and sister chromatid arm cohesion appear to be affected (Gong et al 2005;Sherizen et al 2005). Furthermore, meiotic DSBs are reduced, though not eliminated (Jang et al 2003), and CO formation is abolished (Page and Hawley 2001, and references therein).…”