Auswirkungen der parasitischen Lebensweise auf die Plastidengenome von Cuscuta reflexa und Cuscuta gronovii

The holoparasitic plant genus Cuscuta comprises species with photosynthetic capacity and functional chloroplasts as well as achlorophyllous species and intermediate forms with restricted photosynthetic activity and degenerated chloroplasts. In this thesis the plastid genomes of two parasitic flowering plant species, Cuscuta reflexa and Cuscuta gronovii, were sequenced. Both species are capable of performing photosynthesis, albeit with different efficiencies. Together with the plastid genome of the achlorophyllous parasite Epifagus virginiana, these species represent a series of progression towards total dependency on the host plant. The plastid chromosome sizes with 121.5 kbp and 86.7 kbp for C. reflexa and C. gronovii, respectively, were found to be larger than that of E. virginiana (70 kbp). Although the chromosome structure is similar to that of non-parasitic plants, a number of insertions, deletions (indels) and sequence inversions were found. Gene losses are more pronounced in C. gronovii than in C. reflexa, corresponding to the lower photosynthetic efficiency in C. gronovii. Among the genes that were lost from both species are the ndh genes as well as several genes related to the plastid gene expression such as tRNA genes and genes for ribosomal proteins. Gene losses that occurred specifically in C. gronovii concern the rpo genes for the plastid encoded RNA polymerase and the psaI gene coding for a subunit of photosystem I. Moreover, C. gronovii is the first plant where a loss of the gene for the putative splicing factor MatK was observed. Thus, a step-by-step reduction in the plastid genome may be characteristic for the genus Cuscuta and perhaps for all parasitic plants. This can range from mild changes in C. reflexa, mainly in the non-coding regions, to massive rearrangements of gene expression in C. gronovii to, finally, the loss of all genes for the photosynthetic apparatus as evidenced in E. virginiana.