DOI: 10.5061/dryad.6hdr7sr69
title: Data from: High-resolution crossover maps for each bivalent of Zea mays using recombination nodules

description: 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
 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.
 These RN maps represent an independent measure of crossing over along
 maize bivalents.
description: SC spreads were prepared on plastic-coated slides as described by
 STACK and ANDERSON (2002). SC spreads were stained with 2% uranyl acetate
 followed by Reynold’s lead citrate (UP) or with 33% silver nitrate
 (SHERMAN et al. 1992). The slides were scanned using phase light
 microscopy, and good SC spreads on plastic were picked up onto 50 or 75
 mesh grids. The grids were examined in an AEI 801 electron microscope, and
 SC spreads without detectable stretching and with kinetochores were
 photographed at a magnification of 1600X or 2500X. In total, 2080
 individual SCs from 290 sets were identified and mapped with regard to
 RNs. Both total RN number and total SC set length (the combined length of
 all SCs in a cell) could be assessed for 206 of these SC sets. In the
 remainder of the sets, certain of the SCs could not be identified, usually
 due to unclear or missing kinetochores. Electron microscope negatives were
 scanned into a computer using a Hewlett-Packard ScanJet 4c and Adobe
 Photoshop (version 5.0) software. Montages of SC spreads were assembled
 using Photoshop. Proper tracing of each SC and the position of
 kinetochores and RNs were determined directly from the negatives using an
 8X magnifying loupe and recorded onto prints of the montages. One lateral
 element from each SC was measured in micrometers using the computer
 program MicroMeasure (REEVES 2001). Total SC length varied from set to
 set, but the relative length of SCs and SC arms, i.e., arm ratios, within
 each set remained consistent. SCs were identified by relative lengths
 (percentages of total SC length for the set) and arm ratios (length of
 long arm divided by length of short arm). RNs were recognized using
 criteria of size, shape, staining intensity, and association with SC as
 described by STACK and ANDERSON (2002). RN positions were measured and
 expressed as a percentage of the arm length from the centromere. Using
 average lengths for each of the 10 SCs and their average arm ratios, each
 of the SCs was divided into 0.2 μm segments, and each observed RN was
 placed into one of these segments based on its original relative position
 from the centromere. After compiling the RN data, the genetic map length
 of each SC was calculated by determining the average number of RNs per SC
 and then multiplying by 50 (one RN = one crossover = 50 cM). 
description: # Data from: High-resolution crossover maps for each bivalent of Zea mays
 using recombination nodules
 [https://doi.org/10.5061/dryad.6hdr7sr69](https://doi.org/10.5061/dryad.6hdr7sr69)   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 currently available for defining the frequency and distribution of crossovers along the length of chromosomes. Using the maize inbred line KYS, we used two-dimensional spreads of SCs to 1) prepare 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 and 2) map 4267 RNs on 2080 identified SCs.  These high-resolution maps of RN frequency and distribution represent an independent measure of crossing over along maize bivalents. ## Description of the data and file structure The data are presented in an Excel spreadsheet with 12 sheets.  Sheet 1 (karyo) defines the characteristics of each maize SC in terms of arm ratio, relative length (% of the total SC set length), and average total length (um).  Sheet 2 (Notes) contains definitions of the headings used for the 10 sheets labeled SC1 through SC10 where RN positions on each SC have been mapped.  An asterisk (\*) in Sheets SC1 – SC10 indicates an absent value in the dataset.  This is because the number and placement (short arm and/or long arm) of RNs varies among SCs.  Other notes explaining the nuances of the data and two references in which this data was used for publication are also presented in the Notes sheet.