EVOLUTION AND FLORAL DIVERSITY - THE PHYLOGENETIC SURROUNDINGS OF ARABIDOPSIS AND ANTIRRHINUM

The two most prominent model species for the study of different aspects of floral development, Arabidopsis thaliana and Antirrhinum majus, are members of large families, Brassicaceae and Scrophulariaceae, and orders, Capparales and Scrophulariales. In both groups flowers play an important role in evolutionary diversification, but the evolutionary trends are different. In the Capparales changes in floral symmetry and organ number and in the Scrophulariales changes in organ elaboration and synorganization are especially prominent. Certain evolutionary changes of the floral ground plan have occurred many times in parallel in different genera and tribes. Further, a substantial part of floral diversity is not directly related to the ground plan but superimposed on it as diversification of the floral organs in size, shape, color, and scent, all features that may be related to pollination biology. At this level parallel evolutionary trends also abound. My review comprises this diversity with special emphasis on such multiple parallel changes. I suggest that they are based on genetic changes that may be acquired relatively easily and that they are not as deeply rooted as changes that occurred only once. They may be expected to be at a lower hierarchical level in the (epi)genetic system. If change occurs through mutation in the model species that is similar to the variation that is found in the wild types of deviating, related genera, then these changes would be especially interesting to analyze. A better understanding of these multiple parallel changes would also be crucial for an improved phylogenetic analysis of both orders. Since both model species have relatively simple floral ground plans in contrast to the vast diversity that exists in the flowering plants between extreme taxa, such as Magnoliales and Orchidales, generalizations on principles of genetics of floral development should include glimpses of these extreme groups.