Evo-devo of flowers and flowering genes: Salvia as a case study

Abstract

The Plantae kingdom and its autotrophic representatives shape the surface of planet Earth and especially flowers contribute to a great amount to the astonishing beauty of nature. Naturally, flowers are not primarily intended to please humans, but they represent the sexual reproductive organs of the angiosperms, the flowering plants. The ingenious interaction of flowers with pollinators is an example for co-evolution and results in fine-scaled adaptations and complex floral shapes and structures. The unique staminal lever mechanism that is limited to the species rich genus Salvia from the Lamiaceae family is an extraordinary example for plant-pollinator interaction. The species of genus Salvia possess zygomorphic flowers, which have an amazing variety of petals and stamens, resulting in melitto- and ornithophily and worldwide distribution. In order to conduct a robust investigation on the evo-devo of Salvia flowers, the aim of the study was to take as holistic a view as possible on the genus Salvia by interweaving morphological, molecular and developmental approaches and put this against the background of speciation. Flowers of 24 Salvia species from different parts of the world were investigated with regard to their floral geometry. Measurements of corolla and reproductive traits were analyzed by principal component analysis, which unveiled the flower size (represented by 79,21% of data in PCA) and bilabiate or tuberous floral shape (represented by 11,34% of data in PCA) as dominating floral features. In this context, the greater floral variety of New World Salvia species (concerning the dimensions size and shape) could be demonstrated, a result that is congruent with the elevated species number of genus Salvia on these continents. Finally, the statistical evaluation of floral traits appears to be a suitable tool to morphologically classify Salvia species. For characteristic genes from the ABCDE model of flowering orthologues were identified for the genus Salvia and used for gene expression experiments that were conducted in two distant (concerning origin, floral habitus or pollinator) related Salvia species, S. pratensis and S. elegans, and here for closed flower buds and fully opened inflorescences, respectively. Key finding in this experiment is a general gene expression pattern that for the most part corresponds with the ABCDE model demonstrating the models conservation for the biggest genus of the Lamiaceae. Additionally, most of the evaluated flowering genes have been shown to be higher expressed in developing tissue, compared to opened flower tissue. B-class genes display huge elevation in expression in Salvia pratensis and Salvia elegans in corolla and stamen tissue, the organs that are of crucial importance for attracting pollinators and the way of reproductive isolation in Salvia. The two B-class genes GLOBOSA and DEFICIENS have been used for an in-depth Bayesian based phylogenetic inference including more than 30 different species, which is the first examination of a floral trait-related marker for the genus Salvia and therefore directly linked with speciation in this genus. Different levels of gene duplication were unraveled, with a duplication event of the GLOBOSA gene that is limited to New World Salvia species, whereas the DEFICIENS gene displays a clear duplication that is evident for the whole genus Salvia. A model based on the multiplication of transcription factor interactions of flowering genes has been elaborated, hypothesizing that this increase of interactions synchronizes with the species radiation of genus Salvia in the New World and thus might one of the driving forces for this radiation. Significant posterior probability (> 95%) values underline the results and also proves the suitability for phylogenetics using these genes in the genus Salvia, especially for the Eurasian Salvia species. Plotting the amino acid sequences of B-class genes in the wider context of ABCDE genes including sequences from Arabidopsis, lead to a strongly supported ABCDE cluster, that shows a remarkable split into a B-class clade and an ACDE-class clade. This split was interpreted as an early divergence of division of responsibility into plant-based intrinsic factors (ACDE-class), like inward seed growth, and extrinsic factors (B-class), like outward interaction with pollinators, in Salvia flowers or maybe for flowers in general. Taken together, the evo-devo approaches conducted in this thesis and applied to the species rich genus Salvia give an insight on the driving forces behind the speciation process.