Abstract
During meiosis, the number of crossovers vary in correlation to the length of prophase chromosome axes at the synaptonemal complex stage. It has been proposed that the regular spacing of the DNA loops, along with the close relationship of the recombination complexes and the meiotic axes are at the basis of this covariation. Here, we use a cytogenomic approach to investigate the relationship between the synaptonemal complex length and the DNA content in chicken oocytes during the pachytene stage of the first meiotic prophase. The synaptonemal complex to DNA ratios of specific chromosomes and chromosome segments were compared against the recombination rates obtained by MLH1 focus mapping. The present results show variations in the DNA packing ratios of macro- and microbivalents and also between regions within the same bivalent. Chromosome or chromosome regions with higher crossover rates form comparatively longer synaptonemal complexes than expected based on their DNA content. These observations are compatible with the formation of higher number of shorter DNA loops along meiotic axes in regions with higher recombination levels.
Highlights
The synaptonemal complex (SC) is an evolutionarily conserved structure that has been found in most sexually reproducing organisms
The present results show the existence of differences in the DNA packing ratio in chicken pachytene chromosomes, both at intra- and interchromosomal levels
From the comparison of mitotic vs. meiotic chromosome lengths that the SC is under-represented in highly compact chromatin, that is, the DNA density per μm of SC is higher in heterochromatin versus euchromatin
Summary
The synaptonemal complex (SC) is an evolutionarily conserved structure that has been found in most sexually reproducing organisms. Lateral elements of the SC develop from the axial elements or chromosomal axes that appear during leptotene and contain coiled-coil domain proteins, cohesin complexes, and the bases of large loops of genomic DNA [3,4]. In addition to serve as anchoring sites for loops of chromosomal DNA, meiotic axes provide support for the protein complexes that initiate recombination [3,5]. Since the SC structure has limited variation, it was proposed that this relationship depends on the spacing of DNA/chromatin loops along the chromosome axes and that the loop density is a conserved feature across species. Considering the role of axis length in determining the frequency of CO events, longer/shorter loops imply shorter/longer axes, which in turn imply fewer/more COs [11]. A great deal of this argument is based on estimates of DNA loop length in micrographs of pachytene nuclei
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