To measure the morphological variation, the researchers first recorded the characteristic traits of the 4,000 Brassicaceae species in a checklist describing the identity and correlations of the species. They then constructed a family tree on genus level from next generation sequencing DNA data to visualise and test the underlying evolutionary dynamics. The tree facilitates the study of complex traits and their development over the course of evolution and places them in the context of other processes and events such as genome duplications or major changes in speciation rates. Genome duplications, that is the multiplication of the whole genome in a cell, describe an exceptional process in land plants to make available additional genetic variability.
“One surprising result of our study is that there is no key innovation with respect to the morphological characteristics studied. The character traits constantly change and appear to be arbitrarily assembled over and over. The old evolutionary lines make use of the morphological potential in a different way but do not differ from one another in terms of their disparity. In this way, evolution can proceed quickly and divergently,” states Marcus Koch.
These patterns are associated with genome duplications, which reflect the genetic components, as well as a rapid increase in speciation rates as an expression of selection pressure of past and changing environments. Accordingly, present-day Brassicaceae exhibit more than 40 percent polyploid species, which underwent genome duplications and carry a multiple set of chromosomes. “That means that a species like thale cress, Arabidopsis thaliana, has gone through at least three genome duplications over the course of evolution of the flowering plants in the last 160 million years. Yet this species still has only ten chromosomes because the genomes have to be subsequently stabilised and usually scaled back down over the long term,” explains Prof. Koch.
The research was conducted mainly in the framework of the DFG priority program “Evolutionary Plant Solutions to Ecological Challenges” (SPP 1529). The data are available in a public access database.