Perspectives on MADS-box expression during orchid flower evolution and development

Abstract

The diverse morphology of orchid flowers and their complex, often deceptive strategies to become pollinated have fascinated researchers for a long time. However, it was not until the 20th century that the ontogeny of orchid flowers, the genetic basis of their morphology and the complex phylogeny of Orchidaceae were investigated. In parallel, the improvement of techniques for in vitro seed germination and tissue culture, together with studies on biochemistry, physiology, and cytology supported the progress of what is now a highly productive industry of orchid breeding and propagation. In the present century both basic research in orchid flower evo-devo and the interest for generating novel horticultural varieties have driven the characterization of many members of the MADS-box family encoding key regulators of flower development. This perspective summarizes the picture emerging from these studies and discusses the advantages and limitations of the comparative strategy employed so far. I address the growing role of natural and horticultural mutants in these studies and the emergence of several model species in orchid evo-devo and genomics. In this context, I make a plea for an increasingly integrative approach.

Expression of paralogous SEP-, FUL-, AG- and STK-like MADS-box genes in wild-type and peloric Phalaenopsis flowers

“…On the other hand, classic genetic analysis or genetic transformation (transient or stable) of at least one from the recently advanced model species (reviewed in Mondragón-Palomino, 2013) would offer a more direct approach to their actual function or functions.…”

“…The developmental origins of these transitions, especially the ontogeny of the labellum and its role in orchid reproduction have been recurring topics in botany and evolutionary biology since the 19th century. The finding that MADS-box class B genes specify perianth and stamen identity in Arabidopsis put forward DEFICIENS - and GLOBOSA -like genes as candidates for experimentally addressing some of the key morphological transitions in orchid flower evolution (Mondragón-Palomino, 2013). …”

“…Their phylogeny served as a point of reference to compare the expression of DEF -like genes in wild-type representatives of most orchid subfamilies and flower terata. This analysis yielded the “orchid code,” a model where differential combinatorial expression of DEF -like genes from specific clades is associated with the development of each type of perianth organ: outer tepals, inner lateral tepals and labellum (Mondragón-Palomino and Theißen, 2008, 2011; Mondragón-Palomino, 2013). Namely, specification of labellum identity is associated with higher levels of mRNA from both clade three ( PeMADS3 -like) and clade four genes ( PeMADS4 -like) in this organ (Mondragón-Palomino and Theißen, 2011).…”

“…Previous studies reported the expression of SEP -, FUL -, AG- , and STK- like genes in orchids (Reviewed in Mondragón-Palomino, 2013). According to this work the evolution of these genes is characterized by several instances of gene duplication as well as a conserved pattern of expression of each duplicate gene.…”

Evolution and Expression Patterns of TCP Genes in Asparagales

“…In the inner floral whorls bilateral symmetry is evident by the formation of a gynostemium that results from the congenital fusion between the single fertile stamen (sometimes two fertile stamens) and stigmas (Rudall and Bateman, 2002; Pabón-Mora and González, 2008; Endress, 2016). Such floral elaboration has been linked to extremely specialized pollination mechanisms and the exceedingly high diversification rates in Orchidaceae (Gong and Huang, 2009; Mondragón-Palomino and Theißen, 2009; Mondragón-Palomino, 2013). …”