Ligulate inflorescence of Helianthus × multiflorus , cv. Soleil d'Or, correlates with a mis‐regulation of a CYCLOIDEA gene characterised by insertion of a transposable element

Members of CYCLOIDEA (CYC)/TEOSINTE BRANCHED1 (TB1) transcription factor family are essential to control flower symmetry and inflorescence architecture. In the Helianthus annuus genome, ten CYC/TB1 genes have been identified. Studies performed on mutants recognised HaCYC2c as one of the key players controlling zygomorphism in sunflower. We identified CYC2c genes in the diploid Helianthus decapetalus ( HdCYC2c ) and in the interspecific hybrid Helianthus  ×  multiflorus ( H   ×   mCYC2cA and H   ×   mCYC2cB ), a triploid (2n = 3× = 51), originated from unreduced eggs of H. decapetalus fertilised by reduced H. annuus male gametes. Phylogenetic analysis showed that HdCYC2c and H   ×   mCYC2c were placed within a CYC2 subclade together with HaCYC2c but distinct from it. The present data showed that in H . ×  multiflorus the allele derived from H. annuus is deleted or highly modified. The H . ×  multiflorus taxon exists as radiate and ligulate inflorescence types. We analysed CYC2c expression in H. decapetalus and in the cultivar ‘Soleil d'Or’ of H . ×  multiflorus , a ligulate inflorescence type with actinomorphic corolla of disk flowers transformed into a zygomorphic ray‐like corolla. In H. decapetalus , the HdCYC2c gene showed differential expression between developing flower types, being up‐regulated in the corolla of ray flowers in comparison to the disk flower corolla. In H . ×  multiflorus , an insertion of 865 bp, which is part of a CACTA transposable element, was found in the 5′‐untranslated region (5′‐UTR) of H   ×   mCYC2cB . This insertion could promote, even with epigenetic mechanisms, ectopic expression of the gene throughout the inflorescence, resulting in the observed loss of actinomorphy and originating a ligulate head.