Regulation of meiotic recombination levels in mammalian males

Brian Scott Baier

Formation of gametes relies heavily on a carefully choreographed sequence of events during meiotic prophase. Homologs must pair and synapse in order to form crossovers, physical exchanges that help the homologs align and segregate during meiosis. In most organisms, bivalents are joined by at least one crossover per chromosome arm and display interference resulting in evenly spaced exchanges along the chromosomes. Although the placement of crossovers is influenced by these restrictions, variation in crossover frequencies is not uncommon among mammals. For example, three strains of mice, C57BL/6J, C3H/HeJ, and CAST/EiJ, display "high", "medium", and "low" rates of recombination, respectively, as measured by the number of MLH1 foci per cell. Since errors in segregation are associated with altered recombination, we have been interested in defining factors responsible for this variation. Our approach utilizes immunofluorescence to compare the number and distribution of proteins that function at various stages of recombination with the crossover resolution protein, MLH1: RAD51 and DMC1, involved in strand invasion shortly after double-strand break (DSB) formation, MSH4, part of the complex stabilizing double Holliday junctions, and BLM, a helicase responsible for non-crossovers. We detected consistent strain-specific variation in the average number of foci at each stage of the recombination pathway: i.e., for RAD51, DMC1, MSH4, and BLM, the average numbers of foci were highest in C57BL/6J males and lowest in CAST/EiJ males, with C3H/H3J males having intermediate value. This indicates that differences established at, or prior to, DSB formation translate into similar step-wise differences throughout the stages of recombination and ultimately dictate crossover frequency. Aspects of chromatin configuration, such as synaptonemal complex length and chromatin loop size may also affect recombination; i.e., strains with higher MLH1 levels displayed longer SCs and, consequently, shorter chromatin loops. Interestingly, strains heterozygous for Spo11, responsible for DSB formation, displayed decreased DSB sites yet MLH1 levels and chromatin configuration remain unaffected, suggesting that DSBs themselves are merely the result of upstream variation. Taken together, this indicates a relationship established prior to DSB formation between recombination and chromatin conformation in which CO frequency is related to SC length and inversely associated with chromatin loop size.

Regulation of meiotic recombination levels in mammalian males

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