Genetic basis of unstable expression of high gamma-tocopherol content in sunflower seeds

Male sterile plants of nuclear male sterile line nmsT2100, with stable high gamma-tocopherol content, were crossed with plants of line IAST-1, with stable high gamma-tocopherol content but derived from a population that exhibited unstable expression of the trait. F2 seeds showed continuous segregation for gamma-tocopherol content from 1.0 to 99.7%. Gamma-tocopherol content in F2 plants (average of 24 individual F3 seeds) segregated from 59.4 to 99.4%. A genetic linkage map comprising 17 linkage groups (LGs) was constructed from this population using 109 SSR and 20 INDEL marker loci, including INDEL markers for tocopherol biosynthesis genes. QTL analysis revealed a major QTL on LG 8 that corresponded to the gamma-TMT Tph2 locus, which suggested that high gamma-tocopherol lines nmsT2100 and IAST-1 possess different alleles at this locus. Modifying genes were identified at LGs 1, 9, 14 and 16, corresponding in most cases with gamma-TMT duplicated loci.Unstable expression of high gamma-tocopherol content is produced by the effect of modifying genes on tph2a allele at the gamma-TMT Tph2 gene. This allele is present in line IAST-1 and is different to allele tph2 present in line nmsT2100, which is not affected by modifying genes. No sequence differences at the gamma-TMT gene were found associated to allelic unstability. Our results suggested that modifying genes are mostly epistatically interacting gamma-TMT duplicated loci.Tocopherols are the main antioxidants present in seed oils. They form a family of four fat-soluble compounds with vitamin E activity named alpha-, beta-, gamma-, and delta tocopherol. Tocopherols contain a 6-chromanol ring structure methylated to varying degrees at positions 5, 7, and 8, and an isoprenoid-derived C16 saturated side chain at position 2. The four tocopherols differ by the number and positions of the methyl groups on the 6-chromanol ring. Alpha-tocopherol is trimethylated, beta- and gamma-tocopherol are dimethylated, and delta-tocopherol i