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The molecular basis of evolutionary novelties is a highly contentious issue. While homeotic mutants have been of tremendous value for a better understanding of animal as well as plant development and form, their role in evolution is highly controversial. It is often maintained that homeotic mutants have such a drastic effect on the phenotype that fitness is always heavily reduced, so that evolution generally proceeds in a gradualistic and never in a saltational way. On the other hand there is both molecular and morphological evidence that homeotic (or heterotopic) changes occurred during evolution and resulted in the establishment of new lineages with novel morphological features. To better understand the evolutionary significance of homeotic mutants we are studying a floral homeotic variety of Capsella bursa-pastoris (shepherd's purse) in which all petals are transformed into stamens. In contrast to most other homeotic mutants this "Staminoid petals" (Spe) variety occurs on several locations in larger populations and for quite a long time in the wild. It thus qualifies as a drastic morphological variant that might have the potential to establish a new evolutionary lineage (a 'hopeful monster' in Richard Goldschmidt's provocative terminology). Due to its close relationship to the model plant Arabidopsis, the Spe variant of Capsella can be rigorously studied from the molecular level to its ecology in the field. Currently, we are investigating the molecular genetic basis of the phenotype by determining the expression patterns of informative developmental control genes in Capsella wild-type (Fig. 1) and Spe mutants (Fig. 2). Our cooperation partners in Osnabrück will map the Spe locus, and will employ molecular marker technology to determine as to whether Spe variants at different places in Europe are monophyletic or polyphyletic. We started to clone the Spe locus by combining candidate gene approaches with map based approaches. The effects of the Spe phenotype on attraction of pollinators and outcrossing will be studied in garden plots and in natural populations (cooperation with partners in Halle). Our studies may help to answer the question as to whether non-gradualistic changes at the phenotypic level, such as homeotic transformations, have the potential to contribute to macroevolution.
Grant: TH 417/4-1 and -2 of the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) This project is carried out in cooperation with groups at the University of Osnabrück (Prof. Barbara Neuffer, DFG NE 314/7-1), at the Max Planck Institute for Chemical Ecology, Jena (Dr. Jürgen Kroymann) and the University of Halle (Dr. Matthias Hoffmann). Suggested reading: Bartholmes, C., Nutt, P. and Theißen G. (2008). |
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