Linkage studies of lesion and necrotic mutants

For the past couple of years I have been mapping a number of dominant and recessive lesion mutants, as well as a number of recessive necrotic mutants. Since several chromosomes are involved in the study, I have divided this summary by chromosome. While much of the data is only preliminary, some may be of value for those interested in particular chromosome arms or in mapping in general. As somewhat of a novice at mapping, I have found it very easy to determine linkage between genes already on the map; however, placement of new genes has been much more difficult. The potential location of new genes distal to the known genes on a chromosome points to the need for mapping of many different types of new genes, as well as perhaps some sort of selection scheme for mapping. Rapid selection of those mutants linked to known, easily scored markers could be done using small progeny sizes. Unlinked mutants, confirmed to arm, could then be scanned with additional TB stocks with breakpoints further out on the arm for potential new distal mutants. In any case, extending the map is not going to be easy.

CHROMOSOME 1S: The two dominant lesion mutants, Les2 and Les*-1449, are both located by T wx linkage studies (Table 1) on chromosome 1S. It is suspected that these two mutants are the first case of allelism between the EMS-induced lesion mutants, since both have a similar phenotype, small whitish lesions. Les2 appears linked to sr1 (Table 2) and approximately 20 to 30 map units from lls1, a recessive lesion mutant. Similar data for Les*-1449 will be available following the summer, 1984 and may provide evidence of allelism with Les2. Several allelism tests were performed with the recessive mutants on 1S. les*-501B, an EMS-induced recessive lesion mutant, is allelic to lls1. nec*-495C, a necrotic mutant characterized by large white patches on the leaves, is not allelic to zb4 or nec2, but is linked to sr1 (Table 2). It is interesting to speculate that nec*-495C is the recessive counterpart to Les2, since Les2 produces small white lesions and nec*-495C, large white patches.

CHROM0SOME 1L: Les*-1461 is located by T wx linkage data on 1L (Table 1); however, backcross linkage data showed no linkage with either br1, an1, bz2, gs1, or bm2. If the T wx data are correct, then Les*-1461 must be distal to bm2. If so, further mapping-of Les*-1461 will require the placement of a new gene distal to bm2.

CHROMOSOME 2: The dominant lesion mutant, Les*-A607, was originally given to Dr. Neuffer by Dr. Kermicle. It is believed to be a spontaneous mutation. T wx linkage data have placed this mutant on chromosome 2 (Table 1). No linkage with any other T wx stock was observed. Since it is linked to both wx T2-9b (2S.18) and wx T2-9d (2L.83), arm placement is not possible. This mutant has a phenotype resembling that of Les1, and allelism is not known at this time. Since Les1 is linked to sk1 and wt1 on 2S, Les*-A607 is also being mapped relative to these two genes, as well as other standard markers on 2S and 2L.

CHROMOSOME 3L: Spc1, a dominant EMS mutant on 3L, appears linked to lg2 (Table 3). I believe that Spc1 is much closer to lg2 than indicated since in the backcross, no Spc1 lg2 plants were observed. The high number of normal plants could be due to contamination, since the Spc1 plant was not used as the female.

CHROMOSOME 4L: nec*-642A has previously been shown to be uncovered by TB-3La in our lab. Backcross linkage data with su1 gl4 c2 showed nec*-642A to be unlinked to any of these genes. Linkage data with dp1 are still to come." This mutant may be located on the distal tip of the chromosome, similar to Les*-1461, and may require additional mutants to be located before it can be placed accurately.

CHROMOSOME 5: Two recessive necrotic mutants have been located on chromosome 5. Both nec*-493 and nec3 have very similar phenotypes. In fact, these mutants were originally presumed to be allelic. Allelism tests this past summer have proved these two to be non-allelic. In addition to the allelism tests, nec*-493 appears linked to a2 and nec3 linked to bm1 bt1 (Tables 3 and 4). Further linkage data are necessary in order to establish the exact order of these genes; however, for nec*-493, the order is either a2 nec*-493 bm1 or nec*-493 a2 bm1. It is interesting that two very similar mutants should be so close together.

CHROMOSOME 7: A new EMS-induced dominant lesion mutant, Les*-F331035142, has been tentatively located on chromosome 7 by T wx linkage data (Table 1). This mutant has not shown linkage with seven other T-wx stocks, eliminating chromosome 9 as a possible location. Testing with wx T7-9a(7L.63) is still in progress. This mutant is characterized by numerous, small, necrotic lesions appearing in mid-season.

Table 1. T wx Linkage Data for Dominant Mutants

Table 2. 2-Point Linkage Data

Table 3. 3-Point Linkage Data

Table 4. 4-Point Linkage Data

Dave Hoisington

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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