Most woody species belong to the flowering plants, which also include many non-woody species. Woodiness is the ancestral condition within flowering plants, meaning that the herbs lost woodiness during evolutionary history. Surprisingly, many of these herbaceous lineages gave rise to new woody species, an evolutionary reversal known as derived woodiness. However, why have so many herbaceous lineages returned to a woody life form? This key question has baffled scientists since Darwin. The group leader has compiled a unique global derived woodiness database for flowering plants, representing some 700 independent transitions towards woodiness, which is much more than the few dozen transitions that had previously been identified. This database shows that most derived woody species inhabit regions with marked drought cycles, which has laid the foundation for our novel hypothesis, proposing drought as the major driver of the evolutionary transitions towards woodiness across most lineages.
The database shows that the mustard family (Brassicaceae, 4000 species in total) provides a new, representative model group to understand the multiple woodiness shifts. This family includes 400 woody Brassicaceae species, which have resulted from some 100 shifts. Using a collaborative project (DFG grant 41845228; PI prof. Klaus Mummenhoff, Osnabruck Univ.), we are currently building a 1000-species phylogeny of the family based on hybridization sequencing to identify all ca. 100 transitions. This phylogenetic framework will allow for solid statistical testing of the drought hypothesis in wild plants and crops.