Evolutionary genetics of eelgrass clones in the Baltic Sea

Seagrasses are a group of marine flowering plants thriving in shallow coastal waters worldwide. The study species of this thesis, eelgrass (Zostera marina), is the dominant seagrass species of the northern temperate zone. As most other seagrass species, eelgrass shows extensive clonal growth and in the non tidal Baltic Sea its clones can persist over many years. During growth, eelgrass clones become fragmented at several spatial scales because the root connections between ramets (rhizome units) breaks over time. This fragmentation prohibits clone identification in the field, in particular in dense meadows. However, as clones are genetic individuals and hence ultimately subject to selection, they represent the relevant level to address questions in an evolutionary context. In this thesis I studied the clonal structure in dense eelgrass meadows in the Baltic Sea in the context of the mating system, inbreeding depression, local adaptation, kinship structure and genet dynamics. I used microsatellite markers to access the fine scale (1-m) clonal structure in four 15-m x 15-m plots located within dense eelgrass meadows in two populations on the Baltic Coast (1.5 – 3.5 m water depth). These plots served as templates for the selection of replicated transplants for laboratory and field experiments, as permanently marked areas for resampling and tracking genets and as database for the calculation of genetic parameters. The key questions investigated and their answers were the following: (i) Can flowering ramets recognize their genetic neighbourhood through pollen and/or growth interactions? - Addition of self versus cross pollen affected the inflorescence sex ratio. This can only be explained by the presence of a cryptic self-incompatibility system. (ii) Does inbreeding depression influence the size distribution of eelgrass clones? - The level of heterozygosity was higher in larger clones. Together with measures of reproductive output this suggests that large dominant clones outcompete their relatively inbred neighbours in an environment with low levels of disturbance. (iii) Are eelgrass clones locally adapted? - Transplantation of replicated genets between two populations showed significant local adaptation and dominance at one site. (iv) Does limited gene flow lead to kinship structure beyond the spatial spread of eelgrass clones? - Spatial autocorrelation, modified for a clonal species, revealed significant coancestry (fij) for the neighbourhood of ramets, clone fragments and entire clones. (v) What are the demographic parameters of eelgrass genets? - Eelgrass clones showed surprisingly high turnover across genets but almost constant patterns within genets between the years 2000 and 2001. This suggests a genetic component to flowering intensity and the production of vegetative shoots. This last study was designed to continue for several more years. In conclusion the clonal patterns and the mating landscape in eelgrass meadows are profoundly influenced by limited seed and pollen dispersal and by selection for outbred clones. If cryptic self-incompatibility is adaptive, then eelgrass individuals may be well equipped to buffer negative effects of the changing geometry in their genetic neighbourhood on sexual reproduction. The geometry of the genetic neighbourhood is changing surprisingly fast. Within clones however, patterns of reproductive output remain constant over time. Such an evolutionary view into eelgrass genetics does also have implications in a conservation restoration context. Genetic erosion, the loss of genetic diversity in an eelgrass meadow will ultimately lead to increased homozygosity. In such a scenario, selection for outbred clones and delayed selfing will cease. The depletion of genotypes will also increase kinship structure and hence the negative effects of biparental inbreeding. The import of new genetic material from foreign sites through long range dispersal is unlikely to be sufficient in order to stabilize the process of genetic erosion because most dispersal seems to be limited to only a few meters. Finally the selection of suitable transplants in eelgrass restoration can benefit from a combination of genetic marker data and phenotypic measurements to consider both genetic diversity and the degree of local adaptation in management decisions.Summary 7 Zusammenfassung 9 Introduction 1 Seagrasses: not merely a green frill on the shoreline 13 2 What are eelgrass individuals? - The dandelion concept 17 3 Mapping eelgrass clones with microsatellite markers 19 4 Thesis outline 23 Chapters I Flexible mating: Experimentally induced sex-ratio shift in a marine clonal plant. 29 II Inbreeding depression influences genet size distribution in a marine angiosperm. 45 III Local adaptation and transplant dominance in genets of the marine clonal plant Zostera marina. 61 IV Spatial autocorrelation of microsatellites reveals kinship structure in a marine angiosperm. 75 V Genet demography of a marine clonal plant based on marker assisted clone identification. 91 Conclusion 105 Danksagung 109 References 110 Glossary Often used definitions and synonyms 127 Appendix 1 MATLAB codes for clone assignment; startup.m & scissors.m 128 Appendix 2 MATLAB codes for cellular automaton; zosgrow.m 135 Curriculum 141 Erklärung 14