Variation in gene expression across species is widespread, but linking this variation to its phenotypic consequences requires a trait that is well characterized genetically and morphologically — one example of this is the dorsoventral asymmetry of the Antirrhinum flower. The authors examined expression variation in two of four transcription-factor genes that control this conserved trait. They then correlated the variation in the expression of these two genes (CYCLOIDEA (CYC) and RADIALIS (RAD)) with the degree of flower asymmetry in F1 hybrids.
First, the phenotypes and expression levels of Antirrhinum majus CYC and RAD mutants were used to generate a GEM (gene expression–morphology) map. Here, genotypes are quantitatively related, through their expression levels, to variation in flower ventralization. When wild-type RAD and CYC alleles of various other Antirrhinum species were characterized for their expression levels and positioned on the GEM, they mapped to a plateau, which means that the variation in gene expression that exists within each species did not affect the phenotype. However, crosses between species would be expected to tip the genotypes off this plateau: indeed, when double-heterozygote hybrids were generated by crossing Antirrhinum species that have divergent CYC or RAD expression levels, lower expressing hybrids yielded more ventralized flowers (the figure shows a fully ventralized flower).
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