Altered distribution of MLH1 foci is associated with changes in cohesins and chromosome axis compaction in an asynaptic mutant of tomato.

In most multicellular eukaryotes, synapsis [synaptonemal complex (SC) formation] between pairs of homologous chromosomes during prophase I of meiosis is closely linked with crossing over. Asynaptic mutants in plants have reduced synapsis and increased univalent frequency, often resulting in genetically unbalanced gametes and reduced fertility. Surprisingly, some asynaptic mutants (like as1 in tomato) have wild-type or increased levels of crossing over. To investigate, we examined SC spreads from as1/as1 microsporocytes using both light and electron microscopic immunolocalization. We observed increased numbers of MLH1 foci (a crossover marker) per unit length of SC in as1 mutants compared to wild-type. These changes are associated with reduced levels of detectable cohesin proteins in the axial and lateral elements (AE/LEs) of SCs, and the AE/LEs of as1 mutants are also significantly longer than those of wild-type or another asynaptic mutant. These results indicate that chromosome axis structure, synapsis, and crossover control are all closely linked in plants.

Cytological analysis of MRE11 protein during early meiotic prophase I in Arabidopsis and tomato.

Early recombination nodules (ENs) are multiprotein complexes that are thought to be involved in synapsis and recombination, but little is known about their components or how they may be involved in these events. In this study, we describe the cytological behavior of a possible EN component, MRE11, a protein that is important for the repair of the numerous, programmed deoxyribonucleic acid double-strand breaks (DSBs) that occur early in the meiotic prophase. By immunofluorescence, many MRE11 foci were associated with chromosomal axes during early prophase I in both wild-type Arabidopsis and tomato primary microsporocytes. Similar patterns of MRE11 foci were observed in two Arabidopsis mutants (Atspo11-1 and Atprd1) that are defective in DSB formation and synapsis. In tomato chromosomes, MRE11 foci were more common in distal euchromatin than in proximal heterochromatin, consistent with known EN patterns. However, electron microscopic immunogold localization demonstrated that only about 10% of ENs were labeled, and most MRE11 label was associated with synaptonemal complex components. Thus, in plants, MRE11 foci are not dependent on DSB formation, and most MRE11 foci do not correspond to ENs. More generally, our results show that the simple presence of large numbers of fluorescent foci associated with synapsing chromosomes is insufficient evidence to equate these foci with ENs.

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 approximately 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.