Life in unpredictably changing habitats is a great challenge, especially for sessile organisms like plants. Fruit and seed heteromorphism is one way to cope with such variable environmental conditions. It denotes the production of distinct types of fruits and seeds that often mediate distinct life-history strategies in terms of dispersal, germination and seedling establishment. But although the phenomenon can be found in numerous species and apparently evolved several times independently, its developmental time course or molecular regulation remains largely unknown. Here, we studied fruit development in Aethionema arabicum, a dimorphic member of the Brassicaceae family. We characterized fruit morph differentiation by comparatively analyzing discriminating characters like fruit growth, seed abortion and dehiscence zone development. Our data demonstrate that fruit morph determination is a 'last-minute' decision happening in flowers after anthesis directly before the first morphotypical differences start to occur. Several growth experiments in combination with hormone and gene expression analyses further indicate that an accumulation balance of the plant hormones auxin and cytokinin in open flowers together with the transcript abundance of the Ae. arabicum ortholog of BRANCHED1, encoding a transcription factor known for its conserved function as a branching repressor, may guide fruit morph determination. Thus, we hypothesize that the plasticity of the fruit morph ratio in Ae. arabicum may have evolved through the modification of a preexisting network known to govern correlative dominance between shoot organs.

Department of Genetics Friedrich Schiller University Jena Philosophenweg 12 07743 Jena Germany

Laboratory of Growth Regulators Centre of the Region Haná for Biotechnological and Agricultural Research Palacký University and Institute of Experimental Botany Academy of Sciences of the Czech Republic Šlechtitelů 27 CZ 78371 Olomouc Czech Republic

Plant Cell Biology Faculty of Biology University of Marburg Karl von Frisch Str 8 35043 Marburg Germany

School of Biology Faculty of Biological Sciences University of Leeds Leeds LS2 9JT UK

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