A joint seminar of the section Systematics, Biodiversity & Evolution of Plants, the Botanische Staatssammlung München and the Botanical Garden München-Nymphenburg
When? Wednesdays at 1:15pm (during the semester, unless specified otherwise).
Where? Room 109 Botanical Institute, Menzinger Str. 67, 80638 München.
Prof. Dr. Christian Parisod, Department of Biology, University of Fribourg, Switzerland.
Whole-genome duplication and ecological radiation in the Alps
Host: Anna Pasinato
Whole-genome duplication (WGD), in which an organism gains an extra complete set of chromosomes, is an important force in plant evolution. However, it is still unclear whether WGD itself directly promotes adaptation to the environment. Our work examines how WGD contributed to plant adaptive diversification using the Biscutella laevigata species complex, a textbook example of diploid and tetraploid taxa whose distribution is associated with climate changes of the Pleistocene.
Genome analyses from across the genus revealed that an ancient WGD event occurred about 11 million years ago. After this duplication, genes responding to environmental stress were preferentially retained, with one of the duplicated copies becoming continuously expressed. Over time, changes in chromosome number led to the evolution of the ancestral diploid forms of B. laevigata that spread into central Europe and became geographically isolated from the onset of the last ice age some 100,000 years ago. About 25,000 years ago, just before the last glacial maximum, a diploid population living at high elevation in a non-glaciated part of the southern Alps underwent another WGD event. This produced autotetraploid plants, which present tetrasomic inheritance and have spread throughout the Alps after the glaciers melted.
Ecological modelling indicated that the tetraploids originated from diploids without shift into a new ecological niche. Both diploid and tetraploid plants indeed occur at high elevations, although they both included populations at low elevations. Using this opportunity to study how differences in the environment have influenced adaptation in both ploidal levels, we have combined whole-genome sequencing with transplant experiments across diploids and tetraploids from low and high elevations in a factorial design. Although tetraploids showed more non-synonymous genetic variation consistent with relaxed purifying selection than the diploids, they also presented a higher proportion of loci involved in adaptation to elevation. This supports the hypothesis that WGD allows genetic variation to accumulate and increases the potential for adaptation. The experiments also showed important differences between the ploidal levels, with tetraploids showing genetic differences in fitness, whereas diploids contrastingly showed substantial adaptive phenotypic plasticity.
The extant to which such differences were caused directly by WGD itself vs subsequent evolution having occurred after WGD was further addressed by comparing natural diploids, artificially created tetraploids, and naturally established tetraploids under both stable and changing temperatures. Although WGD caused immediate changes in phenotypes, changes in gene expression were mainly associated with post-WGD evolution, which also reverted some of the traits toward values typical of diploids. Most of the novel traits and patterns of gene expression in tetraploids however evolved after WGD itself. Importantly, WGD initially reduced survival and fitness, which were compensated by post-WGD evolution and resulted in tetraploids eventually showing higher fitness under stressing environmental conditions.
Overall, this work shows that WGD has distinct roles in plant evolution. WGD itself immediately creates reproductive isolation, helping tetraploids become genetically independent from their diploid ancestors. Promoting the accumulation of genetic variation, it further improves adaptation to challenging environments and increases stress tolerance within a few thousand generations following WGD.