Mobilization of the Tetu1 transposable element of Helianthus annuus: evidence for excision in different developmental stages

The tubular ray flower (turf) mutant of sunflower is characterized by a switch of ray flowers from zygomorphic to near-actinomorphic disc flowers. In sunflower, floral symmetry of ray and disc flowers is specified by the activity of members of a CYCLOIDEA (CYC) gene family. The turf mutant is generated by the insertion of a CACTA-like transposable element (TE), named Transposable element of turf1 (Tetu1), in the coding sequence of the HaCYC2c gene. The TEinsertion changes the reading frame of turf-HaCYC2c for the encoded protein and leads to a premature stop codon. Tetu1 is a non-autonomous version of a CACTA TEcarrying the minimum sequences necessary for transposition in the presence of autonomous elements in the sunflower genome. In the previous analysis, performed in more than 11 000 plants homozygous for the turf-HaCYC2c allele, the absence of chimerism and the segregation rate of derived-progenies from reverted phenotypes suggest that Tetu1 transpositions are restricted to a time shortly before and/or during meiosis. Here, we report the analysis of F5 and F6 progenies, derived from an F4 progeny of the cross turf × Chrysanthemoides2, where plants with a chimeric inflorescence were detected. Tetu1 showed active excision in all progenies taken into consideration and named High Frequency of Tetu1 Transposition (HFTT). Within a total of 449 plants, Tetu1 excision generated a 13.81 % of non-chimeric revertants but also a 5.12 % of plants with somatic sectors of variable size in the outmost whorl of the inflorescence. These unexpected results suggest variations in tissue specificity and time of TEexcision. The excision of Tetu1 was confirmed by DNA molecular screening of non-chimeric and chimeric revertants and transcription analysis of the HaCYC2c gene. In HFTT progenies, sequence analyses excluded significant DNA changes with respect to the original Tetu1 transposon as well as to the adjacent 5’- and 3’-TE regions. Genetic and epigenetic regulatory mechanisms were proposed to explain the time and frequency of Tetu1 transposition in HFTT progenies.