An alternative method of using B-A translocations to locate duplicate factors
--Philip S. Stinard
A method for using B-A translocations to locate duplicate factors to chromosome arm was presented by Kindiger and Beckett in MNL 60:43, and in modified form by Neuffer and Beckett in MNL 61:49-50. This method involves crossing the B-A translocation set onto plants carrying both duplicate factors, selecting hypoploids, and self-pollinating or testcrossing the hypoploids to detect diagnostic segregation ratios.
An alternative method can be used to locate duplicate factors to chromosome arm directly, in a manner similar to that of using B-A translocations to locate single factors. This method relies on the idea that the second factor of a duplicate factor pair behaves as a single factor when the first factor is homozygous. If a B-A translocation stock carrying the first factor is crossed onto a stock carrying both factors, the possibility arises that progeny kernels homozygous for the first factor, and hyperploid/hypoploid for the chromosome arm that uncovers the second factor will be produced, yielding a mutant genotype in the endosperm or embryo.
The crosses can be set up as follows: a stock homozygous for the mutant allele of the first factor (fac1), but not carrying the mutant allele of the second factor (fac2), can be obtained from the self of a stock homozygous for fac1 and heterozygous for fac2. The homozygous fac1 stock (fac1 fac1 Fac2 Fac2) is crossed onto each of the different B-A translocation stocks. F1 plants heterozygous for fac1 and carrying the B-A translocations are crossed onto plants carrying both fac1 and fac2 (either fac1 fac1 fac2 fac2, fac1 fac1 Fac2 fac2, Fac1 fac1 fac2 fac2, or Fac1 fac1 Fac2 fac2, depending on the availability of stocks, and whether the double homozygote is lethal). If a cross uncovers the mutant phenotype (i.e., produces kernels with hyperploid endosperms and hypoploid mutant germs, or vice versa), then fac2 is located on the A chromosome arm involved in the B-A translocation used in the cross. The same procedure can be done starting with a homozygous fac2 line to locate fac1 to chromosome arm.
This method has the advantage that it is not necessary to grow hypoploids to maturity unless it is for the observation of mature plant traits, or for the purpose of self-pollinating hypoploids to locate mutants that are proximal to the B-A breakpoints.
An accompanying article describes how this method was applied to the lw3 lw4 duplicate factor pair to isolate a homozygous lw4 line and demonstrate that lw3 is uncovered by TB-5La.
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