Anna-Sophie Hawranek (Vienna University)
Left: Analyses of genetic population structure as well as estimations of migration rates show stronger gene flow from the diploid parental species to their tetraploid daughter species than vice versa. Following our data, but also previous research (e.g. Balao et al. 2016 Heredity 116 p. 351-361), the exchange of genetic material between the three tetraploids is taking place until today.
Right: The evolutionary history of three allotetraploid marsh orchids features three independent origins during warmer periods within the Quaternary. As previously shown (Brandrud et al. 2020 Systematic Biology 69 p. 91-109), our results identify Dactylorhiza majalis as the oldest species, followed by D. traunsteineri and the recently-formed species D. purpurella. The effective population size Ne (that fraction of a population contributing to reproduction) reflects the age, as well as the different dispersal patterns of the three species. The older D. majalis and D. traunsteineri have a significantly higher Ne as compared to D. purpurella. However, following our results, Ne for D. traunsteineri exceeds that of the oldest D. majalis – an observation which can be explained by the higher degree of continuity of D. majalis’ distribution area. In contrast, D. traunsteineri grows in three disjunct areas. Graphs: Anna-Sophie Hawranek
Anna-Sophie Hawranek's Master thesis was awarded with the Prize for the Best Plant Science Master Thesis, which was carried out at Vienna University in the year 2021
Title: The impact of recurrent origins and gene flow on the genetic structure of allopolyploid marsh orchids (Dactylorhiza, Orchidaceae)
Using genomic data, advanced bioinformatic analyses and coalescent approaches, this thesis elucidates the evolutionary history of three sibling allotetraploid marsh orchids.
Whole genome doubling, resulting in polyploidy, has been important for the evolution of plants and other organisms. Marsh orchids are Eurasian species that are often threatened due to the scarcity of their preferred habitats – five of these are the subject of this study: the three sibling allotetraploids Dactylorhiza majalis, D. purpurella and D. traunsteineri together with their parental species, the diploids D. fuchsii and D. incarnata.
The presented study relies on a reference genome of Dactylorhiza incarnata (the paternal species) to analyse genome-wide sequences of over 200 individuals of the three tetraploid species. Specific polyploid-adapted bioinformatic programs enabled addressing population genetic questions by considering the tetraploid set of chromosomes.
As seen in population genetic cluster analyses, the genetic structure of allopolyploid marsh orchids is influenced by geographical isolation on the one hand, and overlapping distribution areas on the other. Further, significantly more gene flow goes from the diploid parental species to the allopolyploids than vice versa. Also, within the allopolyploids a large amount of genetic material is exchanged. Therefore, the three focal polyploid species are not yet fully reproductively isolated from each other. Nevertheless, at least three different gene pools are found, broadly assignable to the three described species.
Despite their frequency across (macro)evolution, the actual formation of polyploids are rare events. Polyploid populations initially have small sizes and little genetic variation. The narrow gene pool can get expanded by hybridisation with relatives and this seems to be also the case in marsh orchids.
A look further back into the past discloses for the investigated polyploids three independent origins from the same parental species within interglacials (i.e., warmer periods) during the Quaternary. Demographic modelling confirms previous results regarding the succession of allopolyploids: with approx. 594,000 years D. majalis is found to be the oldest of the three species. It is a vigorous, large orchid, distributed in continental Europe. Dactylorhiza traunsteineri is estimated to be approx. 430,000 years old. This species shows a disjunct distribution area with an allopatric zone (i.e., it occurs without the other two allopolyploids) in Scandinavia, a sympatric area with D. majalis in the Alps and a second sympatric area with D. purpurella on the British Isles. This type of distribution can promote high genetic variability and could be reflected in the higher effective population size as compared to D. majalis. The youngest of the three analysed species is D. purpurella with approx. 3,100 years. This species occurs on the British Isles and parts of Western Scandinavia.
The chosen methodology is based on a Bayesian approach of assigning the polyploid genomic sequences to their respective diploid origins. This enabled analyses of each of the subgenomes apart from each other as diploid data sets. The developed bioinformatic scripts for data processing are also provided in the thesis’ appendix.
___
Anna-Sophie Hawranek conducted this work at the Department of Botany and Biodiversity Research in the working group of Assoc.-Prof. Dr. Ovidiu Paun.
Read more in Hawranek's thesis: https://plantgenomics.univie.ac.at/fileadmin/user_upload/p_plantgenomics/News/Hawranek_DactThesis.pdf