Ultrastructural analysis of female pachynema and the relationship between synaptonemal complex length and crossing-over in zea mays

Abstract

The synaptonemal complexes at pachynema of megasporocytes and microsporocytes in a single genotype of maize were reconstructed from electron micrographs of serial sections. The structure of the synaptonemal complexes, the structure of centromeres and knobs, the continuity of the synaptonemal complex through centromeres and interstitial knobs, centromere fusion, knob fusion and the mode and amount of telomere attachment to the nuclear envelope are consistent features of female and male meiocytes. The absence of large vacuoles within the nucleoli of megasporocytes is the most conspicuous difference between the two types of meiocytes. The mean total synaptonemal complex length of three female nuclei was 413 μm and not significantly different from total bivalent length in reconstructed male nuclei. The size of female and male pachytene nuclei was likewise similar. The plant studied carried a large terminal knob on the short arm of bivalent 9. According to RHOADES (42), the presence of this knob causes a sex difference in crossing-over in the short arm of bivalent 9, reducing the female crossing-over value between the genes C and Wx. The female synaptonemal complex length of the short arm of chromosome 9 was found to be 66% of that of the male. No significant sex difference in synaptonemal complex length was detectable for the long arm of chromosome 9 or for chromosome 6. It is proposed that the sex difference in crossing-over frequency is causally related to the difference in synaptonemal complex length.