Year after year, the brown desert landscape of Namaqualand in South Africa is transformed into one of the most beautiful floral displays in the world. From the end of the South African winter season into spring, thousands of daisies and flowers of all colors transform the arid region into a blooming carpet. Among other plants, species of the genus Heliophila (>100 species) from the mustard family contribute to the annual flowering miracle.
CEITEC researchers from Martin Lysak's research group, in collaboration with scientists from the Institute of Vegetables and Flowers in Beijing, China, sequenced and assembled the ~330-Mb genome of the ephmeral Heliophila variabilis. The work was now published in Plant Journal. Two pairs of differently fractionated subgenomes suggest an allo-octoploid origin of Heliophila genomes – the condition in which eight complete sets of chromosomes occur in a single cell. The ancestral octoploid genome (2n = 8x = ~60) likely arose by hybridization between two allotetraploids (2n = 4x = ~30) at least 12 million years ago. Rediploidization of the ancestral genome was marked by extensive reorganization of the parental subgenomes, resulting in a reduction in chromosome number to only 11 pairs in H. variabilis. The long-lasting diploidization process camouflaged the octoploid nature of Heliophila genomes, which has now been discovered by whole-genome sequencing. The sequenced H. variabilis represents the first chromosome-scale genome assembly of a meso-octoploid representative of the mustard family.
Analysis of the H. variabilis genome has shown that post-polyploid diploidization plays an important role in ecomorphological divergence and adaptation. Evidence has been found for loss-of-function changes in genes associated with leaf development and early flowering, as well as for over-retention and sub/neofunctionalization of genes involved in pathogen response and chemical defense. The genomic resources of H. variabilis will help elucidate the role of polyploidization and genome diploidization in plant adaptation to hot arid environments and origin of the Cape flora.
Although the South African Cape flora is one of the most remarkable biodiversity hotspots, its high diversity has not been associated with whole-genome duplications. The sequenced genome of H. variabilis challenges the traditional picture of the limited contribution of polyploidy to the diversity of the South African flora and suggests that polyploidization–diploidization cycles may have been more widespread in the flora than previously thought.