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and several studies have confirmed the extended ABC model as the molecular background of flower development in this plant group. The core eudicots are characterized as having one copy of each of the B-class genes and at least two copies of A-class genes: one is expressed in floral meristems, the other in inflorescence meristems. In monocots and non-core eudicots the validity of the ABC model is under discussion. Generally, more than one functional copy is found of at least one of the B-class genes. The A-class genes apparently are expressed in meristems of both flower and inflorescence. Morphologically petaloid stamens and styles are well known within the petaloid monocots, whereas the phenomenon is rare in core eudicots. A simple model based on the extra copies of B-class genes can explain the molecular background of petaloid stamens in the monocots; the only requirement is that two copies of the same gene have different expression patterns and are responsible for development of petals and stamens, respectively. The formation of petaloid styles can be explained in the same way, but this hypothesis requires that A- and C-class gene expression is not mutually exclusive in monocots. The difference in expression of the A-class genes outside the floral organs shows a fundamental difference between monocot and core eudicot flowers.

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© 2006 Bo Johansen, Signe Frederiksen, Martin Skipper

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