USDA Plant Gene Expression Center

Mapping of the abphyl locus which regulates phyllotaxy in maize
--Jackson, D and Hake, S

In last year's newsletter we described a new heritable abphyl mutation which causes an increase in the size of the shoot apical meristem in the coleoptilar stage embryo and a subsequent change in phyllotaxy from distichous to decussate in a large proportion of mutant individuals (MNL 69:2). We have continued to introgress this mutation into different genetic backgrounds, and it acts as a single recessive locus. We used bulked segregant analysis (Michelmore et al., PNAS 88:9828-9832) to map the locus, with much appreciated advice from Mike McMullen (USDA-ARS, University of Missouri, Columbia). Plants showing the abphyl phenotype were outcrossed to B73 (all of the F1 were normal) then backcrossed to abphyl, so the F2 segregated 1:1 mutant: normal (heterozygotes). Two pools of DNA from approximately 30 mutant and 30 normal individuals, respectively, as well as B73 DNA, were digested with SstI, EcoRI, EcoRV or BamHI and subjected to Southern analysis using core RFLP probes. No linkage was found with probes from chromosomes 1, 3, 6, 9 or 2L, however two probes from 2S, umc6 and umc131, showed very clear differences in hybridization patterns between the mutant and normal pools with all of the enzymes used.

We prepared DNA from 30 individual mutant and 30 normal plants and used these to get a more accurate map position. Probes umc6 and umc131 detected 10 and 8 recombinants, respectively, from the 60 individuals tested, and since the current UMC maize RFLP map shows these RFLP loci to be 35 cM apart this is highly suggestive that the locus (symbol abph1 ) lies between them. To confirm this we used a probe, umc34, which lies between umc6 and umc131, and failed to observe any recombinants in our population, suggesting that abph1 probably lies within a couple of cM of umc34. We also used a probe from b1 (kindly provided by Vicki Chandler, University of Oregon) and estimate that abph1 is in the order of 8 cM from b1. We are in the process of refining these data by mapping relative to other mutants on 2S, as well as initiating tagging strategies using stocks carrying transposable elements at b1 (kindly provided by Vicki Chandler), and a new Ac transposition onto 2S, from the Maize Genetics Coop Stock Center (originally from Hugo Dooner, MNL 69:115). 

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