Chromosomal Interchanges in Maize.

Chromosomal interchanges, or reciprocal translocations, in maize are of primary interest because of the opportunities they offer for the study of chromosome behavior and crossing over. The great volume of work which has been done on crossing over and on translocations in Drosophila has yielded an abundance of statistical data on crossing over and chromosome distribution. For this type of work Drosophila is admirably adapted, and great advances in the statistical analysis of crossing over not only have been made but continue still to be made. But for the cytological study of the chromosomal basis of crossing over, Drosophila is less well adapted. Maize, on the other hand, while not so well fitted for crossing over statistics, has the exceptional merit of being excellent material for the cytological study of the earlier stages of meiosis. This presents the opportunity of making studies on the chromosome mechanism in which crossing over statistics and cytological observations are intimately correlated. A beginning has already been made as for example in the studies on the behavior of ring fragments (MCCLINTOCK 1932b) and on crossing over in the ninth chromosome (CREIGHTON and MCCLINTOCK 1931), in interchange T8-9 (BURNHAM 1934c), in maize-teosinte hybrids (BEADLE 1932a) and in trisomics (RHOADES 1933a). But in order to take full advantage of the opportunities presented, it is first necessary to build up a fair knowledge of such items as the location of given genes on the observed chromosome thread, and the linkage relationships of conspicuous markers on the chromosomes; also to make available means of marking or of controlling desired portions of chromosomes. For building up the required groundwork chromosomal interchanges are of especial value. The synaptic pairing of the interchanged chromosomes with their normal homologs may be observed at mid-prophase when the chromosomes are spun out as long thin threads with their finer structures most clearly visible and with the homologous threads closely conjugated. This makes it possible to determine rather accurately the correspondence of loci on the linkage maps with visible points on the chromosomes themselves, and prepares the way for direct observation of the stages immediately following crossing over. Interchanges are also of great value as means of marking or controlling chromosomes or parts of chromosomes which are being studied.