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.”
Highlights
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
Class A genes belong to the AP1/SQUA-like subfamily, which is further divided into the paleoAP1-like and the euAP1-like clades [48, 49]
Summary
Mya, covered with pollinia from the orchid species Meliorchis caribea has enabled researchers to narrow the timeframe of the orchid family’s origin, estimating it at 76-84 Mya in the Late Cretaceous [12]. The spatial and functional activity of the floral homeotic genes is exemplified by the elegant ABCDE model of flower development (Fig. 3A) [36, 37] This model was initially developed on the basis of mutant analyses of the model species Arabidopsis thaliana, which exhibits a flower consisting of four concentric whorls of floral organs. The ABCDE model is generally conserved [4044], the increasing identification and functional and evolutionary analysis of the MADS-box genes is highlighting relevant differences in the mechanisms leading to flower development in non-model species, such as orchids, often emphasizing instances in which the MADS-box gene’s function could not be extrapolated from structural orthology [45].
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