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The diverse flowers of Orchidaceae are the result of several major morphological transitions, among them the most studied is the differentiation of the inner median tepal into the labellum, a perianth organ key in pollinator attraction. Type A peloria lacking stamens and with ectopic labella in place of inner lateral tepals are useful for testing models on the genes specifying these organs by comparing their patterns of expression between wild-type and peloric flowers. Previous studies focused on  DEFICIENS-  and  GLOBOSA -like MADS-box genes because of their conserved role in perianth and stamen development. The “orchid code” model summarizes this work and shows in Orchidaceae there are four paralogous lineages of  DEFICIENS/AP3 -like genes differentially expressed in each floral whorl. Experimental tests of this model showed the conserved, higher expression of genes from two specific  DEF -like gene lineages is associated with labellum development. The present study tests whether eight MADS-box candidate  SEP-, FUL-, AG- , and  STK -like genes have been specifically duplicated in the Orchidaceae and are also differentially expressed in association with the distinct flower organs of  Phalaenopsis  hyb. “Athens.” The gene trees indicate orchid-specific duplications. In a way analogous to what is observed in labellum-specific  DEF -like genes, a two-fold increase in the expression of  SEP3 -like gene  PhaMADS7  was measured in the labellum-like inner lateral tepals of peloric flowers. The overlap between  SEP3 -like and  DEF -like genes suggests both are associated with labellum specification and similar positional cues determine their domains of expression. In contrast, the uniform messenger levels of  FUL-like  genes suggest they are involved in the development of all organs and their expression in the ovary suggests cell differentiation starts before pollination. As previously reported  AG -like and  STK -like genes are exclusively expressed in gynostemium and ovary, however no evidence for transcriptional divergence was found in the stage investigated. Gene expression suggests a developmental regulatory system based on the combined activity of duplicate MADS-box genes. We discuss its feasibility based on documented protein interactions and patterns of expression.

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