Lathraea clandestina flowering in the Botanical Gardens Bonn. Photo: Nadja Korotkova.
In cooperation with Claude dePamphilis (Penn State University, USA), we work on analysing selectional shifts on chloroplast genomes of parasitic plants. Parasitic plants are valuable models for studying gene and genome evolution in the absence of photosynthesis. Under relaxed functional constraints, the plastid genomes of some parasitic plant have experienced great reductions in gene content and accelerated rates of evolution for the remaining genes. For example, the fully sequenced plastid genome of Epifagus virginiana (Orobanchaceae) displays extreme genome reduction; it lacks functional copies of all photosynthetic and ndh genes, all four RNA polymerase genes, almost half of the tRNAs, and one third of the ribosomal protein genes. Accelerated evolution is observed in Epifagus, a result of both relaxed selection and an increase in the neutral rate of base substitution. To understand whether genome evolution is similar in independent lineages of nonphotosynthetic plants, the plastid genome of the holoparasite Pholisma arenarium (Lennoaceae/Boraginaceae) was cloned into fosmid vectors, sequenced, and analyzed. The plastid genome of Pholisma shows a pattern of gene loss that is strikingly similar to that observed in Epifagus. All of the photosynthetic genes (with the notable exception of rbcL) and ndh genes are lost, as are the RNA polymerase genes and some components of the translation apparatus. The gene losses in Pholisma are a perfect subset of those observed in Epifagus, and the remaining genes are also evolving at an accelerated rate. The results indicate parallel and convergent evolution of the two independent lineages of nonphotosynthetic plants and also suggest that photosynthesis has been lost more recently in the lineage including Pholisma.
Other studies deal with the phylogeny and taxonomy of Orobanchaceae (joint work with Eberhard Fischer, Koblenz).