Genetic diversity, biogeography and evolution of Apomixis in North American Arabis holboellii Hornem. and Arabis drummondii Gray (Brassicaceae)

Research project P 14463 Apomixis in North American Arabis (Brassicaceae) Marcus KOCH 08.05.2000 Apomixis, the production of seeds by vegetative means, is a trait that has `greatly attracted the interest of numerous botanists since decades. -It is widely distributed among flowering plants and the responsible genetic background might involve only one or a few genes. The agronomical importance of this trait is enormous, because of the greatly enhanced possibilities to propagate seed material, but also to open a new way to produce seeds with missing or greatly reduced geneflow between individuals, which might even help to solve problems of introgression of genes from genetically manipulated plants into wildtype individuals. Among dicots apomixis has been analyzed mostly on a descriptional level, and less attention has been paid on the evolution of this trait. Among the family of Brassicaceae the dicot model plant for molecular biologists, Arabidopsis thaliana, do not show this trait, and, therefore, one could not use the enormous molecular knowledge from this plant to analyze the. genetic background in this species directly. However, among North American Arabis species apomixis is widespread and has been described to occur even on the diploid level. North American Arabis consists of more than 50 species depending on the underlying species concept. More than 10% of these taxa are known only as triploids and apomixis is the assumed way of seed production. Additional 20% are known as diploids as well as triploids, which might indicate varying breeding systems within a single taxon. Therefore, apomixis played a major role as a mechanism of population differentiation and speciation within North American Arabis. In this project the genetic diversity of Arabis holboelfii and closely related Arabis drummondii should be analyzed on the molecular level. The degree of apomixis within a population has greater impact on the distribution of its total genetic variation. Therefore, populational genetic diversity and the intra- populational degree of apomixis should be analyzed using molecular markers and cytological investigations. Genetic variation on the populational level should be used to estimate inter populational genetic distances and to infer a phylogenetic network of Arabis populations. Analyses of correlations between geographic distribution of molecular variation, apomixis and genetic distances between populations should help to answer principial questions of phytobiogeography and evolution of Arabis holboellii and Arabis drummondii in the light of apomixis.

Analysis of molecular DNA markers allow to draw far reaching conclusion on organismic evolution. In case of Arabis drummondii, Arabis holboellii and their hybrid Arabis divaricarpa (Brassicaceae), investigated here, a combination of plastid (trnL intron, trnL-trnF IGS, rpoC1 intron), nuclear markers (nuklear ribosomale ITS, microsatellites), and data on pollen size and quality was chosen in order to elucidate the evolution of this species group. This approach revealed detailed information on the phylogeny of the species group, its phylogeography (i.e. recognizing geographical patterns of genealogical structures across the range of a species), the occurence of hybridizaion in time and space, genetic differentation between species and on their genetic diversity, genomic constitution and their breeding system. Thus, it could be shown that hybridization and apomixis are the major forces driving speciation processes in this species complex. Multiple intra-individual ITS copies have been detected in numerous accessions of A. divaricarpa, and, to a minor extent, in A. holboellii. Analysis of conserved regions and secondary structures of the ITS region provided no evidence that in this system hybrid ITS evolution is predominantly driven in a particular direction. It was suggested by all data sets that A. drummondii and A. holboellii are genetically distinct from each other, although the data also indicated occasional introgression of both parental species into each other. A phylogenetic analysis based on noncoding chloroplast DNA sequences (trnL intron and trnL/F intergenic spacer) resolved A. drummondii as monophyletic (i.e. of single origin) taxon, but found A. holboellii to bear chloroplast haplotypes from highly diverged evolutionary lineages. Based on chloroplast and both nuclear molecular marker sets A. divaricarpa was found to be of recent and polytopic origin. Phylogeographic analysis using nested clade analysis (NCA), suggested for A. holboellii (i) past fragmentation events, partitioning genetic variation in several instances between the Sierra Nevada, the Southern Rocky Mountains and the Colorado Plateau on one hand and the Central to Northern Rockies of the United States and adjacent Cascades on the other; and for both parental species (ii) recolonization of major areas formerly covered by the Wisconsin glaciation by three haplotypes; and (iii) restricted gene flow indicating isolation by distance in areas south of the last glacial maximum. Highest genetic diversity was found in the Rocky Mountains from Idaho and Montana south to Utah and Colorado. This area was further hypothesized to have played a major role in the origin of both parental species and probably represented an important glacial refugium. However, evidence for glacial refugia was also found in arctic and boreal regions of Alaska and near the Great Lakes. Microsatellite data found A. drummondii from Northeastern North America to be genetically distinct from western populations. Hybridization of genetically distinct lineage(s) was assumed to have played an important role in the establishment of polyploid A. holboellii. Frequency of polyploid A. holboellii increased substantially from the Northern towards the Southern United States. This trend corresponds to the southerly distribution of derived chloroplast haplotypes and suggests an evolutionary advantage of polyploidy and presumably associated apomixis in the colonization of the Sierra Nevada and the Southern Rocky Mountains. An approach to estimate somatic ploidy levels of herbarium specimens was successfully applied comparing pollen size and number of alleles from seven microsatellite loci.