EVIDENCE FOR SISTER-STRAND CROSSING OVER IN MAIZE

Abstract

ROSSING OVER, the exchange of chromatin between homologous chro-C mosomes, is of utmost importance in genetic studies. Although little is known about the mechanism which is responsible for this exchange, a number of facts on this subject have been fairly well established: (1) Crossing over is produced by breakage and reunion of broken end's (even on BELLING'S hypothesis , breakage and reunion have to be postulated to explain 3-strand and 4-strand doubles). (2) The position of the exchange is exaotly between homolo-gous regions of the chromosomes. (3) Crossing over occurs at prophase of the first meiotic division. (4) Only 2 of the 4 chromatids are involved in any one crossover. (5) The occurrence of one crossover decreases the probability of another occurring in its vicinity-the phenomenon of interference. It has generally been assumed that there is little or no sister-strand crossing over. This FIGURE 1.-Diagrammatic representation of the formation of the ring chromosome following breakage in the distal regions of chromosome 6. The nucleolus is represented by the large black circle and the centromere by the clear oval. The shaded area represents the nucleolar organizer. The arrows indicate the probable sites of X-ray-induced breakage. (Reprinted through courtesy of the American Naturalist.) conclusion has been drawn from studies in Drosophila with attached-X chromosomes (BEADLE and EMERSON 1935) and with the Bar locus (STURTEVANT 1925, 1928; MULLER and WEINSTEIN 1933). However this is still an open question. The pioneering work of MCCLINTOCK (1938, 1941b) on dicentric double-sized ring formation and the evidence to be presented in this paper suggest that crossing over does occur between sister chromatids. Such an event can be detected only with ring chromosomes with the possible exception of " unequal " crossing over in sister rod chromatids giving rise to duplicated segments. In a previous paper (SCHWARTZ 1953) the behavior of a large ring in maize involving almost the whole of chromosome 6 was discussed. Gametophytes possessing nine chromosomes plus the ring are viable even though deficient for the terminal regions of chromosome 6 (fig. 1). Crossing over in the hetero-1 Work performed under USAEC Contract No. W-7405-eng-26. GENETICS 38: 251 May 1953.