Colorado State University

University of Missouri

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

--Anderson, LK, Doyle, GG, Brigham, B, Carter, J, Hooker, KD, Lai, A, Rice, M, Stack, SM

Recombination nodules (RNs) are closely correlated with crossing over, and, because they are observed by electron microscopy of synaptonemal complexes (SCs) in extended pachytene chromosomes, RNs provide the highest resolution cytological marker that is currently available for defining the frequency and distribution of crossovers along the length of chromosomes. Using the maize inbred line KYS, we prepared an SC idiogram in which each SC was identified by relative length and arm ratio. This idiogram is very similar to published idiograms of maize pachytene chromosomes based on squashes. We further confirmed identification of each SC with its linkage group by examining inversion heterozygotes. We mapped 4267 RNs on 2080 identified SCs to produce high resolution maps of RN frequency and distribution on each bivalent. RN frequencies are closely correlated with both chiasma frequencies and SC length. The total length of the RN recombination map is about two-fold shorter than most maize 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 bivalents share a high frequency of distal RNs and a severe reduction of RNs at and near kinetochores. The frequency of RNs at knobs is either similar to or higher than the average frequency of RNs along the SCs. We suggest a model for knob structure that may explain this observation. These RN maps represent a measure of crossing over along maize bivalents that is independent from and complementary to linkage maps. RN maps can be used to evaluate patterns of crossing over and genetic interference.

This work is reported fully in: Anderson, L.K., G.G. Doyle, B. Brigham, J. Carter, K.D. Hooker, A. Lai, M. Rice, and S.M. Stack, 2003. High resolution crossover maps for each bivalent of Zea mays using recombination nodules. Genetics 165:849–865.

Acknowledgements. We thank Ben Burr and Ed Coe for providing KYS seeds. This work was supported by a grant from the National Science Foundation (MCB-9728673). K.D.H., A.L. and B.B. were supported by grants from the National Science Foundation for Research Experience for Undergraduates.