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, 12 (5), 342-56

The MADS and the Beauty: Genes Involved in the Development of Orchid Flowers

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The MADS and the Beauty: Genes Involved in the Development of Orchid Flowers

Serena Aceto et al. Curr Genomics.

Abstract

Since the time of Darwin, biologists have studied the origin and evolution of the Orchidaceae, one of the largest families of flowering plants. In the last two decades, the extreme diversity and specialization of floral morphology and the uncoupled rate of morphological and molecular evolution that have been observed in some orchid species have spurred interest in the study of the genes involved in flower development in this plant family. As part of the complex network of regulatory genes driving the formation of flower organs, the MADS-box represents the most studied gene family, both from functional and evolutionary perspectives. Despite the absence of a published genome for orchids, comparative genetic analyses are clarifying the functional role and the evolutionary pattern of the MADS-box genes in orchids. Various evolutionary forces act on the MADS-box genes in orchids, such as diffuse purifying selection and the relaxation of selective constraints, which sometimes reveals a heterogeneous selective pattern of the coding and non-coding regions. The emerging theory regarding the evolution of floral diversity in orchids proposes that the diversification of the orchid perianth was a consequence of duplication events and changes in the regulatory regions of the MADS-box genes, followed by sub- and neo-functionalization. This specific developmental-genetic code is termed the "orchid code."

Keywords: Flower development; MADS-box genes; Orchid code.; Orchidaceae.

Figures

Fig. (1)
Fig. (1)
Schematic diagram of an orchid flower.
Fig. (2)
Fig. (2)
Time-calibrated phylogenetic relationships of the five sub-families of Orchidaceae, modified from Gustafsson et al. [14]. The numbers below the branches indicate the divergence time, as expressed in millions of years ago (Mya). On the right are the images of orchid species that are representative of each subfamily.
Fig. (3)
Fig. (3)
Diagram of the ABCDE and the quartet models (A) and of the expanded ABCDE model of floral development (B).
Fig. (4)
Fig. (4)
Neighbor-joining tree, obtained from the alignment of the amino-acid sequences of the orchid MADS-box proteins, outlining the various functional classes.
Fig. (5)
Fig. (5)
The orchid code, modified from Mondragon-Palomino and Theissen [133, 134]. The colors indicate the various clades of the AP3/DEF-like genes and their expression profiles in the orchid perianth. A model for the possible initial and intermediate stage of the orchid perianth is also presented. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this paper).

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