Genetic consequences of unstable Ds transposition in maize (Zea)

    In the indicated cross, C and c are alternate alleles at a locus that determines seed coat color. C produces anthocyanin-pigmented (purple) seeds, c produces colorless (yellow) seeds. The parents are both double homozygotes: one is C Ds // C Ds and the other c Ds⁺ // c Ds⁺ thus all of the offspring are C Ds // c Ds⁺ trans double heterozygotes. The dominant C allele is closely linked to the Ds (Dissociator) element, and the recessive c allele linked to the wild-type Ds⁺ (that is, no element). The Ac locus is on another chromosome pair. The first parent is also an Ac⁺Ac heterozygote with an Ac (Activator) element on one chromosome. The second parent is an Ac⁺Ac⁺ homozygous, with no Ac element. The Mendelian expectation is that half of the offspring seeds will be Ac⁺Ac⁺ and half Ac⁺Ac. [Instructor: remember that Ds+ and Ac+ are the wildtype absence of Dissociator and Activator elements, respectively. Student: You said that before. Instructor: Yes, but you had forgotten it. Student: Nope. Instructor: Very well, let's continue]

    In the absence of an Ac element [top], the Ds element is stable: the C allele dominates c, and a uniform pigmented seed results.

    In the presence of the Ac element [middle], the Ds element can "jump" and produce a chromosome break and loss of the linked C. In each cell where this occurs, the c allele on the alternate chromosome is now expressed as a colorless patch against a pigmented background. The timing of the break determines the size of the patch: early breaks produce larger patches.

    In one unusual kernel with the same genotype [bottom], Ds has "jumped" into the middle of the C locus very early in the formation of the kernel, converting it to a C^u ("c unstable") allele without breaking the chromosome. The C^uc combination is expressed as a uniformly colorless seed. If Ds subsequently "jumps out" again, the C^u allele reverts to C, and the Cc combination is expressed as a colored patch against the colorless background. Early breaks produce larger patches. Though the genotypes are identical, the phenotypes are the reverse of each other. [See also explanation of panels E & F from Federoff (2012)]