%0 Generic
%A Zielinski, Marie-Luise
%D 2009
%F heidok:9843
%K Boechera , Hybridizierung , Polyploidie ,  Samenanlagenentwicklung , SAGEApomixis , polyploidy , hybridization , SAGE , gametophyte development
%R 10.11588/heidok.00009843
%T From a Phenotype to Transcriptomics Apomixis Initiation in the genus Boechera
%U https://archiv.ub.uni-heidelberg.de/volltextserver/9843/
%X The North American genus Boechera provides a highly polymorphic source of natural variation for studies on the regulation of apomictic development (asexual reproduction via seed). The aim of the research conducted was to qualify and quantify naturally-occurring reproductive variation in apomictic and sexual accessions (genotypes), and to further elucidate molecular factors responsible for the initiation of the first step in the apomictic pathway, apomeiosis. Sexual reproduction is characterized by double fertilization (i.e. a pair of sperm nuclei fertilize both the egg and central cell), whereby apomictic plants produce seeds without fertilization of the egg cell (yielding maternal genetic clones). In apomictic Boechera accessions the sexual pathway is altered for three traits: i) formation of an unreduced embryo sac, e.g. through meiotically-unreduced megaspore formation (apomeiosis), ii) development of an embryo from an unfertilized and unreduced egg cell (parthenogenesis), and iii) formation of functional endosperm (embryo nourishing tissue), e.g. fertilization of the binucleate central cell (pseudogamy). This type of apomixis is called “diplospory”. In general, apomixis is correlated with polyploidy. The genus Boechera contains one of the rare cases of diploid apomixis, and thus provides an ideal model whereby both diploid and polyploid apomicts can be compared. We began by examining both pollen and seed formation in a number of ecotypes in order to identify variation in the apomictic phenotype. Both apomicts showed flexibility with regards to combinations of the apomixis components, but diploid apomicts were characterized by higher flexibility to variant ploidy ratios in embryo : endosperm, which can affect seed development. In performing a comprehensive comparative study between reproduction traits of both apomictic karyotypes, I show that most traits exhibit lineage-specificity rather than correlations with ploidy. This could reflect the consequences of natural hybridization (we tested independently-evolved apomictic lineages), where natural selection acts upon novel variation in several traits to allow their establishment in specific niches, thus obscuring the effects of ploidy on reproductive success.  The phenotypic data were used to select highly expressive diploid apomicts for a deep transcriptomic comparison between microdissected live sexual and apomeiotic ovules. This approach was taken to elucidate the first step in apomixis, apomeiosis, which is proposed to be the key factor for stable apomixis expression in Boechera. Approximately, 4 000 differentially expressed mRNAs were identified between sexual and apomictic ovules at the megaspore mother cell stage (MMC), the hypothesised stage of apomeiosis initiation, indicating : i) heterochronic expression of genes, ii) differential gene expression, and iii) a parent-of origin effect. In a following approach I preselected A. thaliana genes and performed a sequence homology search between Arabidopsis thaliana and Boechera. Identified sequences in Boechera were analysed with respect to their expression profiles generated from a SuperSAGE (serial analysis of gene expression) experiments. This study demonstrated that apomixis in Boechera might be influenced by chromatin remodelling, which could suppress or enhance gene transcription, and the cause for chromatin remodelling could be the heterozygous (i.e. hybrid) state of apomictic Boechera. This study yielded a set of promising apomixis initiation candidates, which could be used as a first subset for confirmation and functional studies. These data furthermore give deeper insight into the apomixis pathway and its complexity in the genus Boechera.