Dosage analysis of the Vestigial glume1-R phenotype
--Jesaitis, L, Freeling, M

TheVestigial glume1-R (Vg1-R) dominant mutant exhibits several reduced structures including the ligule, sheath margin, and tassel and ear glumes. Our previous studies (unpublished) suggest that the affected structures develop initially normally and that aberrant cell death, occurring late in development, is involved in tissue diminution. To gain insight into the mechanism by which cells die in Vg1-R plants, we examined the effect of wild type gene dosage (vg1+) on the mutant phenotype. We generated a wild type gene dosage series by crossing pollen containing the B-A translocation TB-1La to Vg1-R/+ ears. TB-1La carried vg1+ as well as the enzymatic marker Adh1-C (generously provided by Dr. James Birchler). Vg1-R females carried Adh1-F. Progeny genotypes were determined by starch gel analysis of ADH activity from scutellum. Hypoploids were identified by a single Adh1-F band, euploids and hyperploids by three bands, Adh-F and C homodimers and an F/C heterodimer. The relative ratios of homo to heterodimer bands allowed us to distinguish euploids from hyperploids.

No difference in mutant phenotype severity was observed between the three classes. Ligule, sheath margin, and male and female glumes were reduced to the same extent in hypoploids, euploids, and hyperploids. If the Vg1-R phenotype resulted from a loss of function in a gene for which two functional copies are required, we would expect to observe a normal phenotype in plants containing two copies of vg1+ in addition to the mutant Vg1-R allele. This was not the case, suggesting that cell death in Vg1-R plants results from a gain in gene function. Further support for this conclusion comes from the observation that wild type hypoploid siblings containing only one functional vg1+ gene did not display the Vg1-R phenotype. Cell death in the mutant is not attributable to overproduction of the wild type gene product since we did not detect an increase in mutant phenotype severity with increasing wild type gene dosage. Increasing vg1+ dose also didn't mitigate the mutant phenotype, suggesting that Vg1-R does not interfere with normal wild type gene function. The lack of difference observed between the three ploidy classes suggests that the Vg1-R phenotype is insensitive to the dose of the wild type allele and thus results from either altered gene function or expression pattern.

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