Ty1/copia- and Ty3/gypsy-like DNA sequences in Helianthus species

Abstract.

Two repeated DNA sequences isolated from a partial genomic DNA library of Helianthus annuus, pHaS13 and pHaS211, were shown to represent portions of the int gene of a Ty3/gypsy retroelement and of the RNase-H gene of a Ty1/copia retroelement, respectively. Southern blotting patterns obtained by hybridizing the two probes to BglII- or DraI-digested genomic DNA from different Helianthus species showed pHaS13 and pHaS211 were parts of dispersed repeats at least 8 and 7 kb in length, respectively, that were conserved in all species studied. Comparable hybridization patterns were obtained in all species with pHaS13. By contrast, the patterns obtained by hybridizing pHaS211 clearly differentiated annual species from perennials. The frequencies of pHaS13- and pHaS211-related sequences in different species were 4.3×10⁴–1.3×10⁵ copies and 9.9×10²–8.1×10³ copies per picogram of DNA, respectively. The frequency of pHaS13-related sequences varied widely within annual species, while no significant difference was observed among perennial species. Conversely, the frequency variation of pHaS211-related sequences was as large within annual species as within perennials. Sequences of both families were found to be dispersed along the length of all chromosomes in all species studied. However, Ty3/gypsy-like sequences were localized preferentially at the centromeric regions, whereas Ty1/copia-like sequences were less represented or absent around the centromeres and plentiful at the chromosome ends. These findings suggest that the two sequence families played a role in Helianthus genome evolution and species divergence, evolved independently in the same genomic backgrounds and in annual or perennial species, and acquired different possible functions in the host genomes.