Reticulate evolution in Gentianella

Reticulate evolution, which can lead to hybrid speciation in the extreme case is an often observed feature in the plant world. Hybridization events that do not result in new species are obviously common. Such events resulting from secondary contact of previously isolated populations or species in hybrid zones are termed introgression. In those northern latitudes that were involved in many cycles of glaciations secondary contact was especially frequent as the organisms were forced repeatedly to migrate between higher and lower latitudes and/or elevations to find suitable habitats. Traces of introgression may be found in morphological clines or discordance between morphological and genetic data and in discordant patterns found by nuclear DNA and chloroplast DNA markers or in the local or regional sharing of chloroplast DNA haplotypes by two sympatric species. Besides hybridization (gene flow after speciation), incomplete lineage sorting (gene flow before speciation) can generate similar patterns in phylogenetic analysis. Although it is difficult to distinguish between these two processes there are several approaches that allow to identify hybrid zones with some confidence. Besides phylogenetic evidence we will focus on spatial patterns of genetic admixture among several gene pools and paleoclimatic evidence. The genus Gentianella sect. Gentianella provides an ecellent model group of closely related taxa to study a broad spectrum of evolutionary processes including reticulate evolution, cryptic speciation, ancient polymorphism and lineage sorting. A nested grid sampling design and data from cpDNA markers in combination with AFLP and morphological data will be used to study: (a) Plastid haplotype relationships within the fimbriate section Gentianella where a high amount of reticulation and incomplete lineage sorting after relatively recent divergence are assumed. Spatial patterns of genetic admixture along range boundaries of taxa will be used to infer hybrid zones and introgression; (b) What is the nature of the uncertainty zones in the western Alps, where G. germanica and G. rhaetica probably co-occur in secondary contact. Outside their contact zone, which was not investigated genetically so far, cryptic differention was found between these two taxa that are morphologically highly similar although genetically strongly differentiated. (c) Is there one highly polymorphic taxon in eastern and south-eastern Europe or varying introgression from adjacent taxa. The key question here is, whether conserved ancient polymorphism in a highly variable taxon that shows morphological affinities to its descendents or varying introgression from adjacent taxa is the prevailing pattern. The methodological approaches to resolve those questions have proved helpful in revealing reticulate patterns. We combine three independent data sets and a nested grid sampling design to gain insight into processes of micro-evolution on a narrow regional scale as well as macro-evolution on a larger regional and continental scale. We expect the proposed research to add significantly to the existing knowledge in non-trivial processes of plant evolution such as reticulate evolution, introgression, cryptic speciation, and asynchrony between morphological and genetic divergence. These results are also expected to support an updated taxonomy in the genus Gentianella section Gentianella.

In this research project we have investigated plants of more than 100 populations of the genus Gentianella (dwarf gentian) of Middle and Southeastern Europe applying methods of genetics and morphology. This research was aimed at a better understanding of evolutionary processes in this genus in the Alps, Carpathians and their forelands as well as Southeastern Europe. Another goal was to use the results of our studies for updating the taxonomy in this group, which is actually in need of revision. For these purposes AFLPs (Amplyfied DNA-Fragment Length Polymophisms), sequences of the chloroplast DNA and morphometric data from flowers were used. The results gained by these approaches provide evidence that is interpreted with regard to evolution, biogeography, and taxonomy. We found an overall pattern of various stages of evolutionary divergence in this group with a number of old endemics (plants of restricted distribution) in Southeastern Europe and a group of young species at various rather early stages of divergence in northern Middle Europe in and around the Alps and Carpathian mountain range. This evidence was mainly provided by the chloroplast data showing only very low differences in those northern species while the southern ones were found highly differentiated. The highly resolving AFLP data show divergence also in those northern species due to a more rapid evolution in the nuclear genome. Basically they corroborate the patterns found by the chloroplast DNA. On the other hand comparing the two types of DNA data provides also evidence for hybridization and introgression in marginal populations of the abutting species ranges. Such introgression in groups of populations has resulted in summarizing them as transitional species based on morphological evidence by earlier researchers. Our results provide also insights into biogeographical patterns and processes especially considering recent evidence from literature on cryptic refugial areas north of the Alps and in general in more northern latitudes. The results point to a scenario in which those northern refugial areas have played a more prominent role than previously assumed. The rapid recolonization of the more northern latitudes by Gentianella populations may result from survival of the last glaciations in those northern cryptic refugia. In contrast to the northern species still remaining in an early stage of evolutionary divergence and also morphologically poorly characterized the species of Southeastern Europe show also strong morphological differences besides the strong genetic differentiation. In a special case of a southeastern endemic (Gentianella crispate) we investigate by means of ecological modeling also possible causes for the amphi-adriatic distribution of this species that occurs in the Balkans and with only one population in Italy