Crossing-over and interference in the centromere region of linkage group I of Neurospora.

qWO previous studies have provided extensive tetrad data on crossing-over in the centromere region of linkage group I. LINDEGREN and LINDEGREN (1942) found an excess of multiple exchanges across the centromere. These multiple exchanges included a proportion of %strand pairs in excess of the 0.25 expected if all four chromatids participated in exchanges at random. Such a deviation might be attributed in part to centromere misassortment during meiosis, but a significant excess of 2-strand pairs of exchanges was still found in intervals that were not subject to this objection because they did not involve using the centromere as a marker. The experiments of HOWE (1956) employed not only linked group-I markers but also another independent gene close to the centromere of linkage group VI. This enabled him to detect nuclear passing. There appeared to be an excess of 2-strand double exchanges in HOWE’S data when nuclear passing was not taken into account, but chromatid interference disappeared when the spurious multiple exchanges caused by nuclear passing were reclassified as nonexchanges. Similar observations were reported by STADLER (195613) for regions in linkage group VI. There was no chiasma interference across the centromere in STADLER’S experiments, and HOWE’S data, though limited on this point, were in agreement. In analyzing chromatid interference it has seemed preferable to use close markers so that undetected multiple exchanges would be kept to a minimum. STRICKLAND (1961) used four markers within an arm of linkage group V that met this requirement. Double exchanges between the short intervals were very infrequent. however, because chiasma interference was strongly positive. The present experiments were designed to take advantage of the absence of chiasma interference across a centromere. Five genes were used that were spaced from three to ten units apart in opposite arms of linkage group I, and a sixth gene was present 23 units away. Three of the intervals were located in each arm, and the centromere itself was marked with a closely linked gene, thus avoiding depend-