Combined Fluorescent and Electron Microscopic Imaging Unveils the Specific Properties of Two Classes of Meiotic Crossovers - Tomato

Meiotic crossovers (COs) shuffle genetic information and allow balanced homolog segregation. In several organisms, mutants demonstrate that two molecularly distinct pathways produce COs. One pathway produces class I COs that exhibit interference (lowered probability of nearby COs), and the other pathway produces class II COs with little or no interference. However, the relative contributions, genomic distributions, and interactions of these two pathways are essentially unknown in nonmutant organisms because marker segregation only indicates that a CO has occurred, not its class type. Here we combine the efficiency of light microscopy (LM) for revealing cellular functions via fluorescent probes with the high resolution of electron microscopy (EM) to localize and characterize COs in the same sample of pachytene meiotic chromosomes. Our technique allows for the first time each CO along each chromosome to be identified by class, unveiling specific characteristics of each pathway in wild-type tomato. We observe enrichment of class II COs in pericentric heterochromatin, a feature potentially exploitable in plant breeding. Furthermore, our results demonstrate that the two pathways are not independent because there is interference between class I and II COs.