Dominant dwarfs
--Rodney G. Winkler and Tim Helentjaris

D8-1 is a dwarf mutant proposed to be involved in gibberellin (GA) reception. In contrast to dwarfs deficient in the biosynthesis of GA, D8-1 is dominant and GA-nonresponsive. To facilitate the identification of putative D8 clones isolated by transposon tagging, 224 backcross progeny were analyzed to resolve the map location of D8-Mpl. The following cross was utilized: zb7 kn1-wt d8-wt bz2 X (zb7 Kn1-0 D8-Mpl Bz2 /zb7 kn1-wt d8-wt bz2). 121 bz2 kernels and 103 Bz2 kernels from this cross were planted and seedlings were treated with GA3 at 7 and 14 and 21 days. The zb7 phenotype did not express well under the summer greenhouse growth conditions used (temperature sensitive?), however Mpl and Kn were easily scored. Recombinants were selected (11 kn1-wt d8-wt Bz2; 14 Kn1-0 D8-Mpl bz2; 2 Kn1-0 d8-wt bz2) and analyzed with molecular markers (umc106, phyA1, umc107, bnl15.18, Mu4, umc140) in the region to define a precise location for the D8 locus. The suggested map is shown below [ed. note - the map should be read from bottom to top]:

Estimated map distances (cM) are shown in parentheses. D8-Mpl was not resolved from umc107 and bnl15.18 in this population but was approximately 1.8cM distal from an endogenous Mu4 element linked to D8-Mpl. Two results are slightly different than expectations. The map position of D8 suggested by this study is more proximal than in the "working" maize genetic map (MNL, 1992). Secondly the region between Kn1 and Bz2 is more condensed than expected from previous reports (MNL63:2, BNL and UMC maps 1992). However, the predicted map order of the RFLP markers in this region is the same as that observed in the BNL and UMC maize RFLP maps. A larger population has been planted in the greenhouse to more precisely define the position of D8-Mpl relative to the closest markers and zb7.

A second dominant anther-eared dwarf, D9 (D*-2319 = D9, isolated by G. Neuffer, MG and mapped by him to chromosome 5S (MNL65:51), was RFLP mapped by the "bulked segregant" method (PNAS 88:9828, 1991). D9 mapped to an interval of 5S that contains duplicate loci with chromosome 1L (region of 1L containing D8). Thus D9 is probably a duplicate of D8. Similar to the six known dominant alleles of D8 (MNL66:21), D9 is dominant and GA-nonresponsive (does not grow in response to GA). D9 and D8 alleles set a maximum limit on plant growth by uncoupling the normal relationship between GA and growth. 

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