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During meiosis, crossovers (COs), which represent the reciprocal exchange of genetic material between homologous chromosomes, occur with high frequency. Protein complexes named late recombination nodules (LNs) mark the chromosomal positions of COs along synaptonemal complexes that form during meiotic prophase. In most eukaryotes, meiotic COs are more evenly spaced than expected if they were randomly distributed along the chromosomes, a phenomenon called CO interference. Tomato is one of the few species that exhibit favorable meiotic cytology for observing synaptonemal complexes and investigating CO positions in detail by both immunofluorescence and electron microscopy. Lhuissier et al. (pages 862–876) found tomato mismatch repair protein MLH1 in distinct foci along meiotic synaptonemal complexes in this system. The authors show that MLH1 occurs in a subset of LNs, and interference among these MLH1-positive LNs was much stronger than that of all LNs. Two classes of COs have been identified in yeast: class I COs depend on proteins termed ZMMs and on yeast Mlh1, whereas class II COs depend on the Mus81/Mms4 endonuclease but do not require ZMMs or Mlh1. It is generally assumed that only class I COs display interference. Lhuissier et al. propose that MLH1-positive LNs in tomato correspond to yeast class I COs and suggest that MLH1-positive and -negative LNs stem from common CO precursors that display weak interference, implying that CO interference is imposed in two steps. !!...!!

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