Isolation of a recombinant between id1-CSH and bz2-m2
--Joseph Colasanti and Venkatesan Sundaresan

In MNL 66:30 we reported the isolation of a new allele of the indeterminate (id1) mutation on chromosome 1. Subsequent genetic and molecular data have demonstrated that this mutation was caused by the excision of a Ds2 element located in the nearby bz2-m2 allele, and its re-insertion into the normal Id1 gene (see previous report). The new id1 allele, id1-CSH, is therefore linked to the restored Bz2 gene that resulted from the germinal excision of the Ds2 element. Purple kernels from this family, when planted and selfed, usually carried the id1-CSH allele (greater than 95% of the time), suggesting that id1 is closely linked to Bz2. The distance between id1 and Bz2 was suspected of being somewhere between 1 and 10 cM, with id1 proximal to the centromere, although, to our knowledge, precise mapping has not been carried out. The experiments described here to obtain a recombinant between these two loci show that the recombination distance is closer to the smaller value.

Maize plants that are homozygous for the id1 mutation flower very late (or not at all) and they often exhibit aberrant floral development; i.e., the ears and tassels have marked vegetative characteristics. Whereas plants that are homozygous for the id1-R allele (from the Maize Coop) flower uniformly but very late in the season, id1-CSH homozygous plants have a wide range of flowering times, ranging from several weeks after normal sibs (i.e. mid to late August) to showing no tassel emergence even at the end of the season (mid-November). We wanted to see if the variation in severity of the id1-CSH phenotype was correlated with the presence of an Ac element; i.e., does somatic excision of Ds2 from id*-1 restore Id1 function and result in plants with intermediate phenotypes?

To test this idea, a set of crosses was performed to create a maize line in which the id1-CSH allele was linked to the bz2-m2 allele. Plants heterozygous for id1-CSH (id1-CSH Bz2/Id1 def(an1..bz2)-6923 were grown in the autumn greenhouse of 1991 and crossed to Id1 bz2-m2/Id1 bz2-m2 plants (MNL 66:30 describes how the original id1-CSH allele was isolated). Homozygous id1 mutant plants cannot be crossed easily because of their late and unpredictable flowering time, and because they rarely produce viable ears. Because id1-CSH and Bz2 are closely linked, purple kernels from the F1 were presumably id1-CSH Bz2/Id1 bz2-m2; this was confirmed by selfing. These kernels were planted in the winter greenhouse of 1992 and crossed to Id1 bz2/Id1 bz2 plants; the bz2 allele of these latter plants is the standard allele available from the Maize Coop (127A- bz2 zb7 bm2). To screen for crossovers, 400 spotted kernels from this cross were selected and planted in the 1992 summer field. Most of these kernels were Id1 bz2-m2/Id1 bz2, however, some of the progeny should be id1-CSH bz2-m2/Id1 bz2 when a crossover occurs between the id1-CSH and Bz2 loci. Of the 400 plants, 346 were selfed and 20 spotted kernels from each ear were planted in the 1993 summer field to screen for segregation of id mutants. Four families out of the 346 screened segregated id plants, suggesting that the genetic distance between id1 and Bz2 is approximately 1 cM (1.2 cM from these data).

In a parallel experiment, a crossover between the standard id1-R allele and the bz2 allele was isolated (the seeds for this experiment were provided by John Larkin, via Scott Poethig.) Basically a similar scheme to the one described above was used; i.e., the id1 allele linked to Bz2 was crossed to bz2-carrying plants and recombinations between id1 and bz2 were screened by selfing. In the 1994 summer field, 139 families (20 bronze kernels from each ear) were screened for the segregation of id plants. Two out of 139 id-segregating families were found, which suggests that the distance between id1 and bz2 is approximately 1.4 cM. This is in agreement with the recombination data obtained for the id1-CSH and bz2-m2. alleles. Further, this suggests that the Ds2 elements located in id1-CSH and bz2-m2 do not have a major effect on recombination between these two loci. 


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