The ability to vary the dosage of chromosomes and chromosomal segments is useful in a variety of studies. An optimal situation would be one in which the maize geneticist could manipulate the dosage of any region of the genome using but a single genetic marker for all regions and using one which could be reliably classified at the mature kernel stage. Such a situation would allow analyses on regions not presently amenable to such studies and greatly conserve time and space for field operations. The potential for this exists with the TB-10L18 translocation, induced by B-Y. Lin (Genetics 92:931). This chromosome differs from the usual B-A translocation in that the break in the B chromosome is in the short arm of the supernumerary chromosome, leaving the major long arm unaffected. The A chromatin is broken in 10L thus producing a metacentric with the B centromere, the BL and 10L being the respective arms. This leaves the B short arm appended to 10S on the 10 centromere. The 10L portion of the BL-10L chromosome is marked by an R self color (R-sc) allele.
If this chromosome were now used to induce translocations with the various arms of chromosomes 1-9, the respective arms would be linked to the B centromere with the property of nondisjunction at the second microspore division and simultaneously be genetically marked by the R-scm allele which is also linked to the B. Thus regardless of the particular chromosome involved in this hypothetical type of compound B-A translocation, it could be used as a male onto silks of plants of an r-g tester and the dosage of any chromosome arm could be detected on the basis of the R-scm phenotype.
It may be possible to convert the existing B-A translocations to this system. The necessary contingent is that recombination occur in the B chromosome, for which Lin's TB-10L18 will allow a genetic test. If such is the case, recombination in the BL region between TB-10L18 and any other particular TB-A (all with breakpoints in BL) would give a product which now links the B centromere to both 10L and the respective portion of the other chromosome involved in the original TBA. The resultant chromosomes would be A-BL distal, A-BL prox-B10L, and 10-BS, which collectively contain all portions of the B chromosome.
Once isolated, these translocations could be maintained in homozygous condition by self pollination and crossed as males onto an r-g tester to produce the phenotypically (R-scm) marked dosage series as described by Beckett (1978, J. Hered.). Such translocations could also be maintained as hyperploid heterozygotes. In this case, it is necessary to alternate generations of crossing the stocks to R-nj and R-st to be able to completely distinguish duplicate gamete transmission and cases of recombination between centromere 10 and the R locus. The details are complex and are not included here.
In each dosage analysis, the TB-10L18 can serve as a control on any effects produced by this chromosome arm. Finally, it is noted that chromosome arm 10S can not be included in this scheme and some other marker will be required in that case.
James A. Birchler
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