Bringing homologs together: Sex- and species-specific differences in synapsis. J. Gruhn1, C. Rubio2, P. A. Hunt1, T. Hassold1 1) School of Molecular Biosciences, Washington State University, Pullman, WA; 2) Instituto Valenciano de Infertilidad, Valencia, Spain.
Over the past decade, considerable attention has focused on how homologous chromosomes synapse in meiosis and whether this process is linked to crossover site designation. However, very little information is available on mammals and it is not known whether synapsis varies among species or between sexes. Accordingly, we initiated cytological analyses of synapsis in human spermatocytes and oocytes and, for comparison, in male and female mice. Our analyses of humans indicate surprising sex-specific differences. In the human male, there is typically a single distal synaptic initiation site (SIS) per chromosome arm and initiation at the centromere is never observed. In contrast, human females typically have multiple, interstitially located SISs that typically include the centromere; indeed, in many instances the centromere appears to be the first region to synapse. Interestingly, in mice, we observed few male:female differences, but some similarities with humans: e.g, like the human female, both male and female mice typically have multiple SISs per chromosome but, as in human males, centromeres appear to be refractory to synapsis. Our results provide evidence for at least three different strategies to synapsis among mammals - one involving the union of sequences in distal chromosome segments (human males); one involving centromeric interactions (human females), and one involving multiple points of synaptic initiation in non-centromeric locations (both sexes in mice). After identifying such diverse mechanisms for homolog synapsis, we were interested in determining if these differences had an impact on sex- and species-specific variation in crossovers (COs). Thus, we asked whether there was a correlation between SICs and COs, as has been reported for other non-mammalian species. Comparative mapping studies of SISs and COs failed to demonstrate an exact match in males or females of either humans or mice; therefore, the way in which COs are chosen appears to be different in mammalian species than in non-mammalian model organisms. Taken together, these studies demonstrate remarkable sex and species-specific differences in synaptic initiation, but these differences do not directly control variation in downstream recombination events in either humans or mice.
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