Phylogeny reconstructions revealed that the MADS-box genes family is composed of several defined gene clades. Most clade members share highly related functions and similar expression patterns. For example, the MADS-box genes providing the floral homeotic functions A, B and C each fall into separate clades, namely SQUAMOSA- (A), DEFICIENS- or GLOBOSA-(B), and AGAMOUS-like genes (C). The genes determining ovule identity (termed D function genes) also belong to the clade of AGAMOUS- like genes and are not shown in the scetch on the left.
The establishment of the above mentioned gene clades was probably an important event towards the establishment of the floral homeotic functions. Now the question arises as to when these gene clades arose during evolution and how some of their members became the "molecular architects of the flower".
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