Candidate gene polymorphisms associated with salt tolerance in wild sunflower hybrids: implications for the origin of Helianthus paradoxus, a diploid hybrid species

We have studied the origin of salt adaptation in wild sunflower hybrids (Helianthus annuus x H. petiolaris), the precursors of the diploid hybrid species H. paradoxus, at the level of phenotypic traits and quantitative trait loci (QTLs). Here, we review this work and present new results on candidate gene polymorphisms. Salt tolerance candidate genes were identified in expressed sequence tag (EST) libraries of sunflower, based on homology to genes with known function, and on previous QTL results. EST polymorphisms were assayed by denaturing HPLC and genetically mapped in an interspecific BC2 for which fitness estimates in the wild were available. Out of 11 genes studied, one mapped to a salt tolerance QTL. This EST codes for a Ca-dependent protein kinase (CDPK) and stems from stress-induced root tissue of Helianthus annuus. Two additional stress-induced genes exhibited a significant fitness effect in the wild: an ER-type calcium ATPase, and a transcriptional regulator. Our results suggest a possible adaptive role for Ca-dependent salt tolerance genes in wild sunflower hybrids. Also, transgressive segregation appears to be sufficient to explain the origin of adaptive genetic variation in hybrids.