Distribution of crossing over on mouse synaptonemal complexes using immunofluorescent localization of MLH1 protein

We have used immunofluorescent localization to examine the distribution of MLH1 (MutL homolog) foci on synaptonemal complexes (SCs) from juvenile male mice. MLH1 is a mismatch repair protein necessary for meiotic recombinatio n in mice, and MLH1 foci have been proposed to mark crossover sites. We pre sent evidence that the number and distribution of MLH1 foci on SCs closely correspond to the number and distribution of chiasmata on diplotene-metapha se I chromosomes. MLH1 foci were typically excluded from SC in centromeric heterochromatin. For SCs with one MLH1 focus, most foci were located near t he middle of long SCs, but near the distal end of short SCs. For SCs with t wo MLH1 foci, the distribution of foci was bimodal regardless of SC length, with most foci located near the proximal and distal ends. The distribution of MLH1 foci indicated interference between foci. We observed a consistent relative distance (percent of SC length in euchromatin) between two foci o n SCs of different lengths, suggesting that positive interference between M LH1 foci is a function of relative SC length. The extended length of pachyt ene SCs, as compared to more condensed diplotene-metaphase I bivalents, mak es mapping crossover events and interference distances using MLH1 fuci more accurate than using chiasmata.