Preferential Segregation of Structurally Modified Chromosomes in Maize.

N Zea mays there are two different types of chromosome 10, distinguishable I not only on the basis of their morphology but also by the fact that one segregates preferentially at megasporogenesis and the other does not. The present investigation is concerned with the behavior of two modified chromosomes derived from an abnormal 10 which exhibits the phenomenon of preferential segregation. LONGLEY (1937, 1938), working with strains of Zea mays from southwestern United States and Mexico, found an exceptional form of chromosome 10, known as the abnormal 10. According to RHOADES ( 1942,1952), this chromosome differs from the normal 10 in the chromomere pattern of the distal region of the long arm and by the presence of an extra segment attached to the distal end of the long arm. The extra segment of chromatin consists of (1) a proximal euchromatic region, (2) a large heterochromatic segment, and (3) a distal euchromatic region. Figures 1-3 illustrate pachytene configurations of the knobbed and knobless homologues. It will be seen in Figures 1 and 2 that the abnormal 10 possesses three prominent chromomeres in the distal one sixth of the long arm which are not present in the normal strain (Figure 3) . It should be also noticed that there is another less conspicuous chromomere near the base of the knob (Figure 2). The normal chromosome is shorter in length than the abnormal I O . In plants heterozygous for normal and abnormal 10, pairing of the normal strand ends in the region between the most distal of the three prominent chromomeres and the chromomere close to the knob. In 1942 RHOADES discovered that this abnormal chromosome 10 segregated preferentially at megasporogenesis in plants heterozygous for a normal and an abnormal 10. He found, among the progeny of backcrosses in which the abnormal 10 carried the r locus (one unit from knob) and normal 10 the R locus, approximately 70 percent of the abnormal type (I) instead of the expected 50 percent. Evidence was also obtained by RHOADES ( 1952) that preferential segregation of the knobbed chromosome occurred only when the two homologues were heterozygous for the knob. In plants homozygous for the abnormal 10 and heterozygous for the marker gene R , he found random segregation of the knobbed chromosomes 10. From these tests RHOADES concluded that the causative factor or factors for preferential segregation are located either in the region of the