Extensive pairing of the XY bivalent in mouse spermatocytes as visualized by whole-mount electron microscopy

Autosomes and sex chromosomes of mouse spermatocytes were examined during zygotene, pachytene, and diplotene by a whole-mount electron-microscope technique after cell dispersion in a detergent solution (Nonidet-P40). Zygotene, pachytene, and diplotene stages can be adequately identified in the preparations. Thus, asynchronous side-by-side pairing of homologous autosomes, some of them displaying attached nucleoli, defines zygotene. Pachytene is identified by complete pairing of homologues. Diplotene is characterized by disjunction of bivalents (autosomes and sex chromosomes), lack of autosomal-attached nucleoli, divergent expansions observed at lateral element endings of disassembled synaptonemal complexes, end-to-end association of the XY pair and well defined outward deformations (‘bulges’) along sex chromosomal axial cores. X and Y chromosomes display at pachytene an extensive side-by-side pairing segment which decreases in length as meiotic prophase advances. Each sex chromosomal axial core appears double and is formed by close apposition of 2 nearly parallel elements displayed separately along the entire length of the chromosomal core. This double structural feature suggests that each sex chromosomal axial core is presumably composed of 2 chromatid axial cores, each of which, in turn, constitutes the respective lateral elements of short synaptonemal complexes observed at the unpaired segment.

Mitotic recombination in the paba 1 cistron of Aspergillus nidulans

The results of the analysis of 393 mitotic recombinants within the paba1 region of Aspergillus nidulans can be summarized as follows:1. Reciprocal, non-reciprocal and probably inexactly reciprocal exchanges occur in intra-cistron mitotic recombination.2. Negative interference observed in intra-cistron mitotic recombination in the paba1 region of Aspergillus nidulans is more intense than that observed in meiotic recombination in the same cistron.3. In intragenic mitotic recombination multiple exchanges can involve three or even four chromatids within one effective pairing segment.