Chromosome pairing and chiasma formation have been analyzed at the light and electron microscopic levels in wheat plants triisosomic for the long arm of chromosome 5B. Complete reconstruction of the chromosome complement from one zygotene nucleus, as well as analyses of several hundred spread and silver stained nuclei, showed that pairing was arrested at the beginning of zygotene, the mean degree of pairing in five nuclei being 25 percent (range 8-37). At this stage, multiple associations of chromosomes were present, signifying pairing beth between partly-homologous and nonhomologous chromosome segments. A few multivalents were present at metaphase I together with bivalents and univalents. The chiasma frequency (a mean of 18 per cell) was only half of that seen in normal wheat. In spread nuclei at late diplotene, a mean of 41 segments ofsynaptonemal complex was counted, 20 of which had thickened lateral components. The results are discussed in relation to the possible dosage effect of the Ph gene located on the long arm of chromosome 5B.

An ATP-independent strand-transfer activity has been identified in nuclear extracts prepared from Drosophila tissue culture cells and isolated nuclei from Bombyx testes. Extraction of the activity from testes at larval stages where the majority of the cells were in meiotic prophase was only possible when the chromosome scaffold/synaptonemal complex was dissolved by addition of high concentrations of DTT (80 mM). No cross reaction was detected when partly purified extracts were assayed with antibodies against E. coli RecA protein.