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Method in the MADS-boxes: ESTimation of evolutionary relatedness
Alterations in anatomy, morphology, cell biology and biochemistry are significant events in the remarkable transition from a vegetative to a floral apex. Underlying these are changes in the expression of a hierarchy of homeotic genes that are activated in an ordered sequence culminating in the formation of functional flowers. The most well known of these are the ‘ABCD’ genes that set up the floral organ identities after the establishment of the floral apex. These homeotic genes are classified as MADS-box genes in respect of a sequence motif that they possess and that is involved in their function as transcription factors. Yinhe Zhao et al. (Kunming and Beijing, China; Köln, Germany; Houston, Texas, pp. 157–163) are interested in the origin and evolution of this system. They have looked at the expression of flowering-related genes in Asarum caudigerum, a herb with simple flowers and regarded as primitive. They constructed a floral cDNA library and then sequenced 1920 randomly selected cDNA clones. The sequences were aligned with each other and with known sequences in the databases, resulting in the identification of 567 separate cDNAs. About 40 % of these represented either novel genes or genes encoding unknown proteins. The remainder could be assigned with confidence to different functional groups. Here we concentrate on the MADS-box homeotic genes, of which 38 were identified. Twenty-nine of these were of the APETALA3 family, corresponding to the B-genes in the ABCD model and very similar in sequence to the corresponding genes in A. europaeum. Very interestingly, the other nine formed a cluster in the phylogenetic sequence comparison with MADS-box genes from Amborella, regarded as the most basal living angiosperm, and from gymnosperms. The authors suggest that this indicates an origin of A. europaeum intermediate between the gymnosperms and angiosperms, and that study of flower development in this species will provide new insights into the evolution of angiosperm flowers.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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