Concurrent Mapping of MLH1-Positive and MLH1-Negative Recombination Nodules in Tomato

Meiotic crossovers in budding yeast, mammals, and plants occur via several different pathways, but only one pathway (that includes MLH1 protein) appears to be subject to interference – in which one crossover reduces the likelihood of another nearby.  Crossovers from other pathways, including the predominant alternative MUS81 pathway, are thought to show little or no interference.  Because the two types of crossovers cannot be distinguished using linkage mapping, little is known about how different crossover types interact on a chromosomal level in a non-mutant system. Here, we have mapped late recombination nodules (LNs) from spreads of tomato synaptonemal complexes (SCs) after immunolocalization of MLH1 protein.  LNs are protein complexes that mark sites of crossing over, and MLH1 is a component of most LNs.  MLH1-negative LNs are thought to mark crossovers completed through one of the non-interfering pathways.  We have used both light and electron microscopic visualization of the same immunolabeled SC spreads to identify and map MLH1-positive and MLH1-negative LNs on 150 complete sets of 12 SCs plus 92 individual SCs from partial SC sets (total = 1892 SCs, 2966 LNs).   Although our analysis has just begun, we observed an average of 18.7 LNs per complete SC set with 15.3 (82%) MLH1-positive and 3.5 (18%) MLH1-negative LNs.  Among different chromosomes, the frequency of SCs with at least one MLH1-negative LN averages 24% with SC6 (15%) and SC3 (33%) representing the extremes.  Additional information on distributions and interference characteristics of the two LN types will be presented.