The behaviour of large distal C-heterochromatic blocks in spermatogenesis of grey cockroach Nauphoeta cinerea was investigated by light and electron microscopy. Heterochromatic blocks of some autosomes are involved into nonhomologous association with formation of chromocenter in early meiotic prophase I. Fluorescent in situ hybridization with ribosomal DNA-probe revealed the signal on only two pairs of middle chromosomes, which did not participate in the chromocenter formation, therefore appearance of ectopic contacts was not caused by the nucleolus formation. The analysis showed that heterochromatin in the chromocenter was eliminated spatially and functionally from main meiotic events. Heterochromatin does not participate in the bouquet formation, initiation of homologous pairing, and recombination events. Heterochromatic polymorphism results in asymmetric synaptonemal complexes with different degree of synaptic adjustment. Meiotic axis of sex univalent (XO -- sex male determination) is split in different regions irrespective of heterochromatin localization.

DMC1 is a new meiosis-specific yeast gene. Dmc1 protein is structurally similar to bacterial RecA proteins. dmc1 mutants are defective in reciprocal recombination, accumulate double-strand break (DSB) recombination intermediates, fail to form normal synaptonemal complex (SC), and arrest late in meiotic prophase. dmc1 phenotypes are consistent with a functional relationship between Dmc1 and RecA, and thus eukaryotic and prokaryotic mechanisms for homology recognition and strand exchange may be related. dmc1 phenotypes provide further evidence that recombination and SC formation are interrelated processes and are consistent with a requirement for DNA-DNA interactions during SC formation. dmc1 mutations confer prophase arrest. Additional evidence suggests that arrest occurs at a meiosis-specific cell cycle "checkpoint" in response to a primary defect in prophase chromosome metabolism. DMC1 is homologous to yeast's RAD51 gene, supporting the view that mitotic DSB repair has been recruited for use in meiotic chromosome metabolism.

ZIP1 is a novel meiosis-specific gene required for chromosome synapsis and cell cycle progression in S. cerevisiae. zip1 strains undergo homologous chromosome pairing, but are defective in synaptonemal complex (SC) formation. The zip1 mutation confers a uniform arrest in meiosis prior to the first division. zip1 strains display nearly wild-type levels of commitment to meiotic recombination; however, mature reciprocal recombinants are not formed until cells are released from meiotic arrest by return to growth medium. DNA sequence analysis of ZIP1 reveals structural homology to a number of proteins containing coiled coils. Immunofluorescence experiments using anti-ZIP1 antibodies demonstrate that the ZIP1 protein localizes to synapsed meiotic chromosomes but not to unsynapsed axial elements. Taken together, these data suggest that ZIP1 is a component of the central region of the SC. We propose a model in which ZIP1 acts as a molecular zipper to bring homologous chromosomes in close apposition.

BRCA1 immunostaining reveals discrete, nuclear foci during S phase of the cell cycle. Human Rad51, a homolog of bacterial RecA, behaves similarly. The two proteins were found to colocalize in vivo and to coimmunoprecipitate. BRCA1 residues 758-1064 alone formed Rad51-containing complexes in vitro. Rad51 is also specifically associated with developing synaptonemal complexes in meiotic cells, and BRCA1 and Rad51 were both detected on asynapsed (axial) elements of human synaptonemal complexes. These findings suggest a functional interaction between BRCA1 and Rad51 in the meiotic and mitotic cell cycles, which, in turn, suggests a role for BRCA1 in the control of recombination and of genome integrity.

Recent studies of Saccharomyces cerevisiae have significantly advanced our understanding of the molecular mechanisms of meiotic chromosome behavior. Structural components of the synaptonemal complex have been identified and studies of mutants defective in synapsis have provided insight into the role of the synaptonemal complex in homolog pairing, genetic recombination, crossover interference, and meiotic chromosome segregation. There is compelling evidence that most or all meiotic recombination events initiate with double-strand breaks. Several intermediates in the double-strand break repair pathway have been characterized and mutants blocked at different steps in the pathway have been identified. With the application of genetic, molecular, cytological, and biochemical methods in a single organism, we can expect an increasingly comprehensive and unified view of the meiotic process.

The HIS4LEU2 meiotic recombination hot spot specifies two double-strand break (DSB) sites, I and II. Results presented demonstrate that DSBs at site I occur at many positions throughout a region of approximately 150 bp; we infer that breaks occur in a sequence non-specific fashion. Single-strand nicks at sites I and II are not detectable. Analysis of the effects of a 36 bp linker insertion at site I reveals the existence of communication along and between homologs prior to DSB formation. In cis, the insertion allele causes an increase in DSBs at site I but a decrease in DSBs at site II. In trans, two effects are observed. One effect likely reflects very early pre-DSB interhomolog interactions; the second is suggestive of a later, more intimate interaction in which sites I and II on the two homologs all compete for DSBs. The existence of interhomolog interactions in early meiotic prophase can explain how the sites of crossovers come to lie between the homolog axes at pachytene.

The recessive mutation, hop1-1, was isolated by use of a screen designed to detect mutations defective in homologous chromosomal pairing during meiosis in Saccharomyces cerevisiae. Mutants in HOP1 displayed decreased levels of meiotic crossing over and intragenic recombination between markers on homologous chromosomes. In contrast, assays of the hop1-1 mutation in a spo13-1 haploid disomic for chromosome III demonstrated that intrachromosomal recombination between directly duplicated sequences was unaffected. The spores produced by SPO13 diploids homozygous for hop1 were largely inviable, as expected for a defect in interhomolog recombination that results in high levels of nondisjunction. HOP1 was cloned by complementation of the spore lethality phenotype and the cloned gene was used to map HOP1 to the LYS11-HIS6 interval on the left arm of chromosome IX. Electron microscopy revealed that diploids homozygous for hop1 fail to form synaptonemal complex, which normally provides the structural basis for homolog pairing. We propose that HOP1 acts in meiosis primarily to promote chromosomal pairing, perhaps by encoding a component of the synaptonemal complex.

An examination of synaptic data from a series of X-autosome translocations and crossover data from an extensive series of autosome-autosome translocations and autosomal inversions in mice has lead to the development of a hypothesis which predicts synaptic and recombinational behavior of chromosomal aberrations during meiosis. This hypothesis predicts that in heterozygotes for chromosomal rearrangements that meiotically align G-light chromatin with G-light chromatin lack of homology will be recognized. If homologous synapsis cannot proceed, synaptonemal complex formation will cease and there will be no physical suppression of crossing over in such rearrangements. However, if a chromosomal rearrangement aligns G-light chromatin with G-dark chromatin at the time of synapsis, lack of homology will not be recognized and synaptonemal complex formation will proceed nonhomologously through the G-dark chromatin. Crossing over will be physically suppressed in this region and this suppression of crossing over will be confined to the chromosome in which the G-light chromatin is nonhomologously synapsed with G-dark chromatin. When G-light chromatin is once again aligned with G-light chromatin, lack of homology again will be recognized and either homologous synapsis will be reinitiated (as in an inversion loop), or will cease altogether (as in some translocations). Unlike the previously described "synaptic adjustment", this nonhomologous synapsis of G-light with G-dark chromatin appears to compete with homologous synapsis during early pachynema.

The Saccharomyces cerevisiae red1 mutant fails to assemble synaptonemal complex during meiotic prophase. This mutant displays locus-specific reductions in interchromosomal gene conversion and a moderate reduction in crossing over. The occurrence of a significant amount of meiotically induced recombination in the red1 mutant indicates that the synaptonemal complex is not absolutely required for meiotic exchange. The RED1 gene product is required for intrachromosomal recombination in some assays but not others. Chromosomes that have undergone reciprocal exchange nevertheless nondisjoin in red1 mutants, indicating that crossovers are not sufficient for disjunction. Epistasis studies reveal that HOP1 is epistatic to RED1, and that RED1 acts in an independent pathway from MER1. A model for the function of the RED1 gene product in chromosome synapsis is discussed.

The synaptonemal complexes of oocytes from 16-22 week human fetuses were spread using detergent and silver-stained for examination by light microscopy. Zygotene chromosome synapsis generally begins at the telomeres, without obvious prealignment, and proceeds towards the centromeres. Synapsis is not synchronous and longer bivalents may sometimes be completely paired before shorter ones. At pachytene, when pairing is usually complete, some regions presumed to correspond to the heterochromatic blocks of chromosomes 1.9 and 16 may remain unpaired. Residual univalents are uncommon, and little interlocking is evident at this stage. Desynapsis indicating the beginning of diplotene frequently begins at the telomeres, although there is a general relaxation of pairing throughout the bivalents which become increasingly diffuse as diplotene proceeds. The total synaptonemal complex complement length at pachytene in the female is 519 micron, which is about twice that found in the human male. The implications of these results for genetic mapping are discussed.