High-resolution crossover maps for each bivalent of Zea mays using recombination nodules

Recombination nodules (RNs) mark sites of crossing over along pachytene synaptonemal complexes (SCs). Thus, RNs provide the highest resolution cytological marker currently available for defining the frequency and distribution of crossovers along the length of chromosomes because they are observed by electron microscopy. Using the maize inbred line KYS, we have prepared an SC karyotype in which each SC was identified by relative length and arm ratio and related to the proper linkage group using inversion heterozygotes. We mapped 4272 RNs on 2080 identified SCs to produce high-resolution maps of RN frequency and distribution on each bivalent. Average RN frequency per bivalent is closely correlated with SC length. The total length of the RN map is about two-fold shorter than most linkage maps, but there is good correspondence between the relative lengths of the different maps when individual bivalents are considered. Each bivalent has a unique distribution of crossing over, but all bivalen...