--Ru-Ying Chang and Peter A. Peterson
Transposable elements randomly insert into a specific locus at a frequency of 10-6 to 10-5. In previous studies the En element has been shown to insert more frequently into closely linked sites than to other chromosome sites (P.A. Peterson, TAG 40:367-377,1970; E.M. Nowick and P.A. Peterson, MGG 183:440-448, 1981). Thus it is appropriate to place an En element onto each of the 20 chromosome arms in maize. This is called chromosome labelling. After an arm has been labeled, genes existing on the arm and targeted for isolation will have a greater chance of becoming the recipient of the element insertion.
Five En-containing alleles [a-m-(papu), arm 3L; a2-m-655064, 5S; c2-m-1, 4L; c2-m-826019, 4L and wx-84-4, 9S] are used to label all the 20 chromosome arms of maize. They are linked to the arms with the assistance of translocations. Homologous pairing, followed by the appropriate cross-over between a translocated segment and its normal counterpart, will enable us to relocate an En allele in the genome (R-Y. Chang, R-Y and P.A. Peterson, Abstracts of Life Sciences Symposium, Iowa State University, 1989; S. Dash and P.A. Peterson, Maydica 34:247-261, 1989). The strategy used is shown in Figure 1.
The a-m(papu) line is first crossed with the translocation line, which has the A allele corresponding to a-m(papu). At meiosis, four types of gametes are formed. Testing those gametes by an a/a tester enables us to select the genotype T a-m/N a (spotted and pollen-semi-sterile). This genotype is selfed. The resulting spotted and pollen-normal offspring are selfed again and tested to prove the homozygous translocation status (see Figure 1).
The labeling results are shown in Table 1. Seven or eight arms (1S, 2L, 6S, 6L or 6S, 7L, 8L and 10L) are labeled and achieved homozygosity [T6-9(5454) has the break point at the centromere of chromosome 6, but it is not known which side of the centromere it is on]. Those lines have been crossed to normal individuals and all give semi-sterile offspring. A few have also been crossed to the original homozygous translocations. In this case normal offspring are achieved (see Figure 1). Four arms (1L, 2S, 4S and 7S) are labeled in heterozygous condition. Achieving homozygosity is needed. Four arms already contain an element (3L, a-m(papu); 4L, c2-m; 5S, a2-m and 9S, wx-844). Four arms are yet to be labeled (3S, 5L, 9L and 10S).
Table 1. Results of chromosome labelling
| Arm | Mutable allele | Translocation | Labelling status |
| 1S | a-m(papu) | T1-3(5597) | homozygous |
| 2L | a-m(papu) | T2-3d | " |
| a2-m665064 | T2-5(032-9) | " | |
| 6S | c2-m-1 | T4-6(033-16) | " |
| 6L or 6S | wx-84-4 | T6-9(5454) | " |
| 7L | a-m(papu) | T3-7(6466) | " |
| wx-84-4 | T7-9(027-9) | " | |
| 8L | a-m(papu) | T3-8(4874) | " |
| 8S | a-m(papu) | T3-8(043-14) | " |
| 10L | a-m(papu) | T3-10(036-15) | " |
| 1L | a-m(papu) | T1-3e | heterozygous |
| 2S | a-m(papu) | T2-3e | " |
| 4S | a2-m655064 | T4-5e | " |
| 7S | a-m(papu) | T3-7e | " |
As an example the labelling of chromosome arm 2L with a-m(papu) and a2-m655064 through T2-3d and T2-5(032-9) is shown in Figure 2.
With T2-3d, the proximal 0.67 portion of 2L is labeled with a-m(papu). With T2-5(032-9), the distal 0.60 portion of 2L is labeled with a2-m655064. With the two translocations, the arm 2L is completely labeled with either a-m(papu) or a2-m. In fact the portion between 0.40 and 0.67 is labeled twice as shown in Figure 2.
Figure 1. Strategy for chromosome labelling.
Figure
2. An example of labeled chromosome arms, labelling of 2L with
am(papu)
and a2m255064 through T2-3 d and T2-5 032-9
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