RecA-like proteins are components of early meiotic nodules in lily

Early meiotic nodules (also called recombination nodules) are proteinaceous structures about 100 nm in diameter that are associated with forming synaptonemal complexes (SCs) during early prophase I of meiosis. Although their function is unknown, early nodules may be involved in searching for DNA homology before synaptic initiation. Two potential components of early nodules are Rad51 and Dmc1 proteins. These proteins are important for meiotic recombination in eukaryotes and are homologous to RecA, the major protein that catalyzes homologous pairing and DNA strand exchange in prokaryotes. In addition, Rad51 has been localized by immunofluorescence in abundant foci that may correspond to early nodules in yeast, lily, and mouse. In yeast and lily, Dmc1 and Lim15, the lily homolog of Dmc1, colocalize with Rad51. Here, using electron microscopic immunogold localization to spreads of zygotene and early pachytene SCs from lily, we confirm that RecA-like proteins are components of early nodules. The antibody used was generated to full-length tomato Rad51 protein and binds to both Rad51 and Lim15 in immunoblots of lily primary microsporocyte proteins. The labeled early nodules are heterogeneous in size and are associated with both axial elements and SCs. There are two classes of early nodules, those that are densely labeled with gold and those that are not labeled at all. This result may be due to technical limitations associated with using spread preparations or to differences in the nodules themselves. The presence of Rad51 and/or Lim15 proteins in early nodules supports the hypothesis that early nodules are involved in recombination-related events during meiosis

The Mismatch Repair Protein MLH1 Marks a Subset of Strongly Interfering Crossovers in Tomato

In most eukaryotes, the prospective chromosomal positions of meiotic crossovers are marked during meiotic prophase by protein complexes called late recombination nodules (LNs). In tomato (Solanum lycopersicum), a cytological recombination map has been constructed based on LN positions. We demonstrate that the mismatch repair protein MLH1 occurs in LNs. We determined the positions of MLH1 foci along the 12 tomato chromosome pairs (bivalents) during meiotic prophase and compared the map of MLH1 focus positions with that of LN positions. On all 12 bivalents, the number of MLH1 foci was ∼70% of the number of LNs. Bivalents with zero MLH1 foci were rare, which argues against random failure of detecting MLH1 in the LNs. We inferred that there are two types of LNs, MLH1-positive and MLH1-negative LNs, and that each bivalent gets an obligate MLH1-positive LN. The two LN types are differently distributed along the bivalents. Furthermore, cytological interference among MLH1 foci was much stronger than interference among LNs, implying that MLH1 marks the positions of a subset of strongly interfering crossovers. Based on the distances between MLH1 foci or LNs, we propose that MLH1-positive and MLH1-negative LNs stem from the same population of weakly interfering precursors