Mapping of icr genes reveals that tt6 should be placed on Chromosome 5

James J. Campanella(1), Weiqin Dong and Christopher D. Town
Biology Department, Case Western Reserve University, Cleveland, OH 44106.
(1) Current address: Department of Biological Sciences, Lehigh University, Bethlehem, PA 18015.

We recently described the identification of two genes, icr1 and icr2, mutations which confer increased resistance to growth inhibition by amino acid conjugates of indole-3-acetic acid (IAA) (Campanella et al., 1996). In a cross between icr1 and MSU25 (er, gl1, hy2, tt6), we found that icr1 was linked to tt6, but not to hy2 or gl1. We then used RFLP, CAPS (Konieczny and Ausubel, 1993) and SSLP (Bell and Ecker, 1994) markers to more precisely locate icr1. Surpringly, we found that neither icr1 nor tt6 were linked to either pCIT1210 or nga6, both located close to the stated position of tt6 on the unified map (Table 1).

Table 1. Linkage between icr1, tt6 and markers on Chromosome 3
Loci Examined r ± S.E.1
tt6/icr1 0.12 ± 0.07
pCIT1210*/gl1 0.12 ± 0.08
gl1/nga6 0.19 ± 0.01
pCIT1210/nga6 0.02 ± 0.02
icr1/nga6 0.33 ± 0.10
icr1/pCIT1210 0.35 ± 0.10
tt6/pCIT1210 0.50 ± 0.13
tt6/nga6 0.46 ± 0.19

(*Using XbaI, pCIT1210 detects a polymorphism between Ws-2 and Landsberg erecta. Using a set of SSLP markers covering the remaining four chromosomes, both icr1 and tt6 were found to be linked to nga 151, which is located near the top of chromosome 5. (Table 2). Correspondence with Dr. Maarten Koornneef revealed that the original placement of tt6 on chromsome 3 was based on loose linkage to gl1 (38cM), and is thus not inconsistent with our current observations. We therefore believe that tt6 and icr1 should be placed above nga 151 on chromsome 5.

Table 2 Linkage between icr1, tt6 and markers on Chromosome 5
Loci Examined r ± S.E.
nga225/icr1 0.31 ± 0.05
nga225/tt6 0.24 ± 0.05
nga158/icr1 0.30 ± 0.05
nga158/tt6 0.23 ± 0.05
nga158/nga151 0.19 ± 0.05
icr1/tt6 0.18 ± 0.05
icr1/nga151 0.42 ± 0.08
tt6/nga151 0.44 ± 0.10
nga225/nga158 0.09 ± 0.02
nga225/nga151 0.19 ± 0.05

Similar mapping of icr2 places this marker towards the bottom of chromosome 5 (Table 3).

Table 3 Linkage between icr2 and markers on Chromosome 5
Loci Examined r ± S.E.
icr2/DFR 39 ± 13
icr2/LFY3 29 ± 8
icr2/ILL1/2 14 ± 6
icr2/g2368 15.7 ± 6.5
icr2/CATHANKA 26 ± 8
cathanka-g2368 12±4
cathanka-ill1/2 25 ± 6
g2368-ill1/2 12 ± 4

We would like to point out that these map locations are not precise, since due to the incomplete dominance of both icr1 and icr2, it is sometimes difficult to distinguish heterozygotes from homozyotes. We report here the re-location of tt6 since it may have bearing on the research of other groups (e.g. Pelletier et al., 1997). We have included our current data on icr1 and icr2 since we are not pursuing this at present and the information may be of interest to others.

Literature Cited

Bell, C.J. and Ecker, J.R. (1994) Assignment of 30 Microsatellite Loci to the Linkage Map of Arabidopsis . Genomics 19137-144.

Campanella, J.J., Ludwig-Mueller, J and Town, C.D. (1996). Isolation and Characterization of Mutants of Arabidopsis thaliana with Increased Resistance to Growth Inhibition by IAA-amino acid Conjugates. Plant Physiol. 112 735-745.

Konieczny A. and Ausubel, F.M. (1993). A Procedure for Mapping Arabidopsis Mutations Using Co-dominant Ecotype-specific PCR-based Markers. Plant J. 4 403-410.

Pelletier, M.K., Murrell, J.R., Shirley, B.W. (1997). Characterization of flavonol synthase and lecuoanthocyanidin dioxygenase genes in Arabidopsis. Plant Physiol. (in press).