As the first step in an attempt to identify the breakpoints of B-A translocations, a set of the most proximal simple translocations was chosen for studying the break positions on the B chromosome. The set includes translocations between the B chromosome and eight A chromosomes which cover 13 chromosome arms. It does not include translocations on chromosome 2, since no simple translocation is available. Also not included are translocations involving chromosome 8 and the short arm of chromosome 5, because no favorable cytological material was available this summer. The most proximal translocations on the long arm of chromosome 10 (TB-10L18 and TB-10L19) are not in this set, since they have been published elsewhere (Lin, Genetics 92: 931-945, 1979).
The maize B chromosome at pachytene has a very short arm, a centromeric knob and a long arm. The long arm consists of, from the proximal to the distal end, a proximal euchromatic region, four segments of heterochromatic region and a tiny distal euchromatic tip as documented by Ward (Chromosoma 43:177-186, 1973). The length of the euchromatic region is about half that of the heterochromatic region. The first and the second segments of the heterochromatic region have an equal length, and are closely associated most of the time to become a single long segment. The third segment is the largest of the four segments, about one and one-half times the length of the first segment. The fourth segment is as long as the first one.
No translocation of this group has a break on either the short arm or the distal euchromatic tip of the long arm. There is a translocation (TB-10Sc) that has a break at the centromeric knob. This conclusion is based on two facts: (1) the B-A chromosome of this translocation has a centromeric knob smaller than that of the B or a regular B-A chromosome. (2) The same chromosome does not carry any heterochromatic region of the B chromosome, and the length of its euchromatic region is slightly shorter than that of the short arm of chromosome 10.
Four translocations (TB-1La, TB-4Sa, TB-7Lb and TB-9Lc) have a break in the proximal euchromatic region of the long arm. Their relative position in the region can not be determined at this point.
There are nine translocations with breaks in the heterochromatic region of the B long arm. TB-1Sb, TB-4Lb, TB-6Lc and TB-6Sa have breaks in the second segment, with the breakpoint of TB-6Sa close to the junction between the second and the third segment and the breakpoints of TB-1Sb, TB-4Lb and TB-6Lc in the middle of the segment. TB-3Lc, TB-5La and TB-9Sd have breaks in the third segment, with the breakpoint of TB-9Sd at the medial position of the segment and that of TB-3Lc proximal but close to it. TB-5La has a break position slightly proximal to the junction between the third and the fourth segment. The translocation having the most distal breakpoint of the set is TB-3Sb, at the proximal one-third position of the fourth segment.
The break positions of TB-4Sa and TB-6Sa in this study are consistent with those documented by Ward (1973).
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