Sunflower speciation highlights roles for transposable elements in evolution

The findings are reported by Mark Ungerer and colleagues at Kansas State University and appear in the October 24th issue of the journal Current Biology, published by Cell Press.

Theory predicts that for diploid species–that is, those possessing two sets of chromosomes, like most animals and plants–the origin of new species through inter-species hybridization may be facilitated by rapid reorganization of genomes. Previous work on three independently derived hybrid sunflower species has validated this mode of speciation by documenting novel structural rearrangements in their chromosomes, as well as large-scale increases in nuclear DNA content. The nuclear-genome size differences between the hybrids and their parental taxa occur in spite of the fact that all species possess the same number of chromosomes and are diploids.

In the new work, the researchers have determined that the genome size differences between the hybrid and parental sunflower species are associated with a massive proliferation of transposable genetic elements that has occurred independently in the genome of each hybrid species. Transposable elements, made famous by Barbara McClintock in her study of their behavior in maize, are related to infectious retroviruses and are capable of multiplying and inserting themselves at different points throughout a host genome. They are found in virtually all eukaryotic genomes.

The new findings not only add an interesting twist to the origin of new sunflower species through hybridization, but also suggest that the sunflower system may emerge as an excellent model group for studying the natural forces influencing the activation and proliferation of transposable elements in plants. This is because in addition to their hybrid origins, each of the three hybrid species is adapted to, and evolved in, a so-called abiotically extreme environment–two of the species are found in desert environments, while the third is adapted to salt marshes. Both hybridization and abiotic stress have been implicated as natural agents of activation and proliferation of transposable elements.