Genetic movement of Soldanella

If the descendant has parents from different species, then we can talk about hybridization or crossbreeding. The result is the transfer of genes that can be a win for the new generation but often also a loss and can gradually lead to their extinction. However, if the genes are advantageous for a given plant in a certain area, it can lead to the origination of a new species - i.e. speciation.

Since there are hypotheses that hybridisation and the origin of new hybrids also occurred in the past in a plant group called Soldanella, biological experts have focused on this plant. The genus Soldanella is a member of the Primulaceae family and is found in the mountains and foothills of Europe. It comprises a total of 18 different species with distribution across the European alpine system including the Alps, Apennines, Balkans, Carpathians, Cantabrian Mountains, and Pyrenees, and can also be found in the Bohemian Massif. In the Czech Republic, the Soldanella montana is found mainly in the Šumava and Novohradské hory mountains. Most of the species are endemic to one mountain range. However, some are more widespread, such as the S. alpina.

Soldanella montana is one of the most endangered plants and is on the Red List of vascular plants. Photo: K. Fraindová.

Scientists first went to the mountains to collect samples from all known taxa (species and subspecies) of the Soldanella. They thoroughly conserved the samples in silica gel for further analysis. They then isolated the DNA nucleic acids in the laboratory and used the latest DNA sequencing method (a process for determining the order of nucleotides in a DNA molecule) called RAD-seq (sequencing of genomic sites selected on the basis of length after enzymatic restriction digestion of DNA) to identify individual differences and variants in each of the Soldanella samples. To reconstruct the relationships between the Soldanella with respect to historical gene flow, the researchers used a combination of several methods, including a method for inferring patterns of splitting and mixing in the history of a set of populations-called TreeMix. They also looked at the time at which the genetic information of each species of Soldanella was separated, in order to construct a so-called 'genetic tree of life' for the plant.

An interesting result was that the separation of species inhabiting the alpine zone and the western part of the range occurred somewhat earlier. On the other hand, the relatively distant species inhabiting the Apennines and some species of the Pyrenean Soldanella species turned out to share a common ancestor with the other species. In general, it was possible to divide the species into two basic groups according to common ancestry - the Alpine group and the group comprising the species of the Carpathian, Balkan, and southern Apennines (denoted as the CBA clade). The Balkan Peninsula species of Soldanella pindicola had a relatively equal representation of related genes from both groups.

When analysing the age of origin of the genus of the Soldanella, scientists found that the separation of the genus occurred during the Middle Tertiary, with all current species being of Pleistocene (Quaternary) origin, mostly less than a million years old. Some species have been clearly shown to be hybrids (S. pindicola) but others could not be confirmed unequivocally, such as the Soldanella alpina. On the other hand, all 3 species found in the Balkan Peninsula can be classified as hybrids.

Soldanella montana occurs mainly in waterlogged spruce forest stands.
Photo: Zdeněk Kliment.

The results of the genus Soldanella research have yielded interesting findings. It turned out that the extent of introgression (the process whereby the transferred genes are stably incorporated into the genomes of interbreeding individuals) and the origin of new species was unusually extensive in the genus Soldanella compared to previously studied mountain plants of the European Alpine system. While the observed introgression has potentially promoted the formation of new species of these Primulaceae, in other cases it has probably promoted reintegration of previously isolated gene pools and slowed down speciation in the young Carpathian taxa. One of the reasons for the wide ecological range of the taxon may be the diversity of habitats ranging from foothill forests to mountain areas with snow cover, which have been affected by past repeated elevational shifts caused by glacial-interglacial cycles.

Kateřina Fraindová

Slovák M, Melichárková A, Gbúrová Štubňová E, Kučera J, Mandáková T, Smyčka J, Lavergne S, Passalacqua NG, Vďačný P, Paun O. Pervasive Introgression during Rapid Diversification of the European Mountain Genus Soldanella (L.) (Primulaceae). Syst Biol. 2022 Nov 4:syac071. doi: 10.1093/sysbio/syac071.