A proposal for developing chromosome arm-specific Mutator stocks for transposon tagging
--Kevin D. Simcox

The goal of this proposal is to develop a method of selecting for active Mutator (Mu) lines which contain Mu elements on a specific chromosome arm. The working hypothesis is that the development of selected chromosome arm stocks would enhance the frequency of recovery of Mu tagged alleles in a specific genomic region. The method for selecting chromosome arm-specific Mutator stocks will involve crossing Mu stocks by the series of B-A translocations. Translocation heterozygotes of each B-A stock will be crossed onto a diverse set of Mu lines to uncover recessive mutations present on a particular chromosome arm. In turn, pollen from the Mu plants will be outcrossed onto a standard inbred line. The Mu x B-A crosses will be screened either in the field or the sandbench for recessive mutations. Mu x inbred crosses involving Mu plants found to carry recessive mutations on a particular chromosome arm will then be used in a targeted approach to tag known loci located on that chromosome arm. Mu plants selected using B-A translocations (directed selection) will be compared to randomly selected Mu plants (random selection) to determine if directed selection enhances the recovery of mutations at defined loci.

Ears harvested from the Mu x B-A testcrosses will initially be screened for recessive kernel traits and then planted in a sandbench to screen for seedling traits. If the Mu parent plant was heterozygous for a recessive mutation, pollination by a translocation heterozygote will uncover recessive phenotypes in frequencies less than 15%. The frequency of hypoploids recovered will depend upon the B-A translocation. Seedlings expressing the recessive phenotype should be hypoploid. Heterozygous mutations in the Mu parent plant can be recovered either in the Mu x inbred outcross, or by crossing pollen from the hypoploid onto B73 to recover diploid progeny, heterozygous for the mutation.

Novel mutations will appear as single events uncovered by the hypoploid chromosome. Recovery of interesting seedling mutations will depend, of course, on the pleiotropic effect of the mutation on seedling viability. Novel mutations which survive can be recovered from hypoploid plants by outcrossing to B73. RFLP or PCR analysis could be used to verify that novel mutations are hypoploid individuals, not due to Mu-induced deletions. Another type of mutation that will be encountered is a forward somatic mutation. Forward somatic mutations are typically manifested as colorless or yellow longitudinal stripes in the leaves. Since somatic mutations occur during cell division in the leaf meristem region, these mutations can not be recovered unless the sector includes the tassel. However, somatic sectors may indicate the presence of cis-linked Mu elements.

If selection of Mu lines using B-A translocations enhances the recovery of chromosome arm-specific mutations then the next step would be to develop "enhanced" Mu lines for each chromosome arm. Two different routes can be taken at this point: 1) Mu lines could be developed from a single selected Mu plant, or 2) a population could be developed by sib crosses between different selected Mu plants.

A limited set of Mu x B-A testcrosses were made in the summer 1994 nursery to get an idea of the types and frequency of phenotypes that might be recovered in the sandbench. A number of single mutation events were identified in the sandbench. These were mainly mutable albino or luteus phenotypes. Several cases of somatic sectors were found which could either represent a hypoploid or a forward somatic mutation of a heterozygote. 

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