Ultrastructural analysis of maize pachytene karyotypes by three dimensional reconstruction of the synaptonemal complexes

Abstract

Aldehyde fixation followed by staining with phosphotungstic acid produces differential contrast between the synaptonemal complex and the chromatin of maize pachytene bivalents. Centromeres, heterochromatic knobs and large chromomeres are easily recognised. With this and other staining techniques the nucleolus organizer region can be differentiated into two components. — Microsporocyte nuclei at pachytene were serially sectioned and all ten bivalents reconstructed in five nuclei. An idiogram was derived from the mean chromosome (= synaptonemal complex) lengths, the arm ratios, positions of knobs and the nucleolus organizer region. The idiogram agrees well with that published from light microscopic analyses. However, bivalent lengths are only two thirds of those observed by light microscopy of squash preparations. Many telomeres of the bivalents are connected via chromatin to the nuclear envelope, but a varying number of free bivalent ends are observed in all five reconstructed nuclei. — Bivalents heterozygous for inversion 3b were reconstructed. In the presence of abnormal chromosome 10 (K10) the lateral components of the synaptonemal complex of chromosome 3 formed a typical inversion loop, while in one of the nuclei having no K10 the two lateral components of the long arms of chromosome 3 remained unpaired in the region of inversion heterozygosity. The presence of K10, which increases crossing-over frequencies and promotes intimate pairing at the light microscopic level, was thus found to permit formation of complete synaptonemal complexes in the inverted region. The extra terminal portion of the K10 chromosome folded back on itself and formed a morphologically normal synaptonemal complex in this — possibly non-homologously paired — region. The chromatin of centromeres and knobs from different bivalents were sometimes found to fuse, but the synaptonemal complexes transversing the fused centromeres or knobs retained their individuality.