Distribution of Crossing Over on Mouse Synaptonemal Complexes Using Immunofluorescent Localization of MLH1 Protein

Abstract

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 recombination in mice, and MLH1 foci have been proposed to mark crossover sites. We present evidence that the number and distribution of MLH1 foci on SCs closely correspond to the number and distribution of chiasmata on diplotene-metaphase I chromosomes. MLH1 foci were typically excluded from SC in centromeric heterochromatin. For SCs with one MLH1 focus, most foci were located near the middle of long SCs, but near the distal end of short SCs. For SCs with two 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 on SCs of different lengths, suggesting that positive interference between MLH1 foci is a function of relative SC length. The extended length of pachytene SCs, as compared to more condensed diplotene-metaphase I bivalents, makes mapping crossover events and interference distances using MLH1 foci more accurate than using chiasmata.

Therapeutic Dose of Hydroxyurea-Induced Synaptic Abnormalities on the Mouse Spermatocyte

“…To determine whether the formation of CO was affected by HU treatment, we evaluated the number of MLH1 foci in the pachytene of mice treated with HU. In control pachytene nuclei, the average number of MLH1 foci was 22.34 per cell on autosomal SCs in agreement with the published results (Anderson et al, 1999). However, in both the high-and low-dose groups, we observed that the number decreased to 17.74 and 17.62, respectively.…”

“…In this study, a common feature of recombination foci in pachytene spermatocytes of mice treated with HU was demonstrated that a large proportion of MLH1 foci located near the distal ends of SC lengths, rarely close to centromeres, suggesting this distribution of MLH1 foci relied on the SC length (Anderson et al, 1999). Moreover, the distribution of MLH1 foci in infertile men was found near the telomere and sub-telomeric regions, suggesting these regions as the hot spots for the alteration of meiotic recombination (Ferguson et al, 2009).…”

“…The absolute/average relative SC lengths for the experimental and control groups were measured per cell and classified into five groups based on similar SC lengths as follows: SCs 1-2, 3-5, 6-11, 12-16, and 17-19 (Supplementary Table 1). This criterion is similar to the classification of a continuous gradation in relative SC lengths (Anderson et al, 1999). MLH1 foci were considered to locate on pachytene SCs when the overlap between two fluorescent signals appeared, but not on zygotene or diplotene SCs, as previously described (Anderson et al, 1999).…”

“…This criterion is similar to the classification of a continuous gradation in relative SC lengths (Anderson et al, 1999). MLH1 foci were considered to locate on pachytene SCs when the overlap between two fluorescent signals appeared, but not on zygotene or diplotene SCs, as previously described (Anderson et al, 1999).…”

Crossover patterning through kinase-regulated condensation and coarsening of recombination nodules

“…Members of a widely conserved family of RING finger proteins are also associated with ERNs and LRNs and are required for their appearance. The MutLɣ complex (Mlh1/Mlh3) is specifically associated with LRNs in many plants, fungi, and mammals, and plays an essential role in resolution of intermediates to form crossovers (Anderson et al, 1999;Rogacheva et al, 2014). C. elegans lacks a Mlh3 homolog, as do many other nematodes, and mlh-1 mutants are viable and fertile, indicating that the protein is dispensable for meiosis (Meier et al, 2018); it is not yet clear how resolvases are recruited to execute crossover resolution.…”