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The fascinating process of evolution leads to both well-adapted organisms with continuous natural variation and to distinct species. Both processes, adaptation and speciation, are key interests of biological research. This work explores flowering time variation in A. thaliana accessions as an adaptive trait and studies the hybrid incompatibility which was observed between the two accessions Uk-1 and Uk-3 and might represent an example of incipient speciation. Flowering time For the success of plant populations, the tight control of flowering time is crucial. In this study, the flowering time of 155 natural accessions together with 32 flowering time mutants was measured in four different environments representing the major environmental cues affecting flowering time: daylength, ambient temperature and vernalization, a prolonged winter-like time of cold. The flowering time distribution under long day conditions showed two peaks and confirmed the important role of FRI and FLC in flowering time regulation. In late-flowering plants, we found no obvious deletions in either gene, but the function of either gene was impaired in all early flowering accessions. Most frequent were the well-known Col-0- or Ler-type deletion in FRI. Additionally, we found 20 accessions with so far unknown, rare alleles of FRI or FLC. FLC expression levels showed a high linear correlation with flowering time. Microarray experiments revealed that FLC is indeed the gene most highly correlated with flowering time, among all genes represented on the ATH1 microarray. Despite the fact that FRI and FLC functionality determines early or late flowering behavior, we found substantial variation within the two classes, reflecting the action of additional signaling pathways. Also, the effect of FRI and FLC is much smaller under short day conditions, accounting for only 23% of the observed variation - in contrast to 63% under long day conditions. Through environmental sensitivity analysis we have identified conspicuous accessions that are promising candidates to identify new natural alleles of genes controlling flowering time in future studies. A new natural allele of PHYD could indeed be identified successfully from a conspicuous accession identified in this flowering time screen. Speciation The most widely accepted species definition by Ernst Mayr defines species as reproductively isolated groups and turned reproductive isolation or hybrid incompatibility into the main target of biological speciation research. We discovered a dominant hybrid incompatibility between the two A. thaliana accessions Uk-1 and Uk-3. The temperature-sensitive, dwarf-like phenotype with leaf malformations and infertility under ecologically relevant conditions (16°C) suggests a severe loss of fitness. The genetic basis of the observed incompatibility fits with a two gene Bateson-Dobzhansky-Muller model: Two derived alleles of distinct genes function well in their own genetic context. however, negative epistatic interaction of these alleles has detrimental effects when they come together in a hybrid plant,. We mapped the two responsible loci to small chromosomal regions on chromosome 3 and 5. The region on chromosome 3 was fine mapped and all candidate genes present within the mapping interval were tested. The gene HIRA is likely to be the incompatibility gene, however final proof is still required. Microarray experiments suggest that the ectopic expression of pathogen response genes underlies the observed incompatibility. This is consistent with other cases of hybrid incompatibilities described in different plant taxa that also involve pathogen response and show symptoms very similar as described here. Taken together, these findings suggest that ectopic activation of pathogen response genes might be a common mechanism involved in the first steps of species separation. To find such an incompatibility between individuals of the same species, gives the opportunity to study speciation at its beginning. Although we will not find out if eventually a new species will form, we can ask questions about speciation that cannot be addressed by the incompatibilities between already existing species that mainly have been used in speciation research so far.
