Functional and geographical differentiation of candidate balanced polymorphisms in Arabidopsis thaliana

Molecular population genetic analysis of three chromosomal regions in Arabidopsis thaliana suggested that balancing selection might operate to maintain variation at three novel candidate adaptive trait genes, including SOLUBLE STARCH SYNTHASE I (SSI), PLASTID TRANSCRIPTIONALLY ACTIVE 7(PTAC7), and BELL-LIKE HOMEODOMAIN 10 (BLH10). If balanced polymorphisms are indeed maintained at these loci, then we would expect to observe functional variation underlying the previously detected signatures of selection. We observe multiple replacement polymorphisms within and in the 32 amino acids just upstream of the protein-protein interacting BELL domain at the BLH10 locus. While no clear protein sequence differences are found between allele types in SSI and PTAC7, these two genes show evidence for allele-specific variation in expression levels. Geographical patterns of allelic differentiation seem consistent with population stratification in this species and a significant longitudinal cline was observed at all three candidate loci. These data support a hypothesis of balancing selection at all three candidate loci and provide a basis for more detailed functional work by identifying possible functional differences that might be selectively maintained.