Linkage mapping of maize and barley myo-inositol 1-phosphate synthase DNA sequences:: correspondence with a low phytic acid mutation

We sequenced and genetically mapped the myo-inositol 1-phosphate synthase (MIPS) genes of maize (Zea mays L.) and barley (Hordeum vulgare L). Our objective was to determine whether the genetic map positions of these MIPS loci correspond with the location of the low phtyic acid 1 (Ipa1) mutations that were previously identified in maize and barley. Seven MIPS-homologous sequences were mapped to positions on maize chromosomes 1S, 4L, 5S, 6S, 8L, 9S and 9L, and a similar number of divergent MIPS sequences were amplified from maize. To the extent that we can compare across different genetic mapping populations, the position of the MIPS gene on maize chromosome 1S is identical to the location of the maize Ipa1 mutation. However, only one MIPS sequence was identified in barley and this gene was mapped to a locus on chromosome 4H that is separate from the barley Ipa1 mutation on chromosome 2H. Although several RFLP markers linked to the barley MIPS gene on chromosome 4H also detect loci near barley Ipa1 on chromosome 2H, our experiments failed to reveal a second MIPS gene that could be associated with the barley Ipa1 mutation. Therefore, genetic mapping results from this study support the MIPS candidate-gene hypothesis for maize Ipa1, but do not support the MIPS candidate-gene-hypothesis for barley Ipa1. These opposing results contradict the hypothesis that maize Ipa1 and barley Ipa1 are mutations of orthologous genes, which is suggested by the similar biochemical phenotypes of these mutants. Yet, comparisons of RFLP mapping studies show loci that are homologous between maize chromosome 1S, barley chromosome 4H and barley chromosome 2H, including regions flanking the respective MIPS and/or Ipa1 loci. This putative relationship, between the regions flanking the Ipa1 mutations on maize 1S and barley 2H, also supports the assertion that these mutations are orthologous despite contradictory results between our maize and barley candidate-gene experiments.