Trends in the molecular evolution of DEF - and GLO -like genes from Orchidaceae and Poales. In this schematic phylogeny the major morphological and evolutionary features of the two lineages of class B genes from Orchidaceae and Poales are summarized: (a) Three gene duplications resulted in four DEF-like genes in the Orchidaceae. The points where these events happened are labeled 1 to 3 without assuming any specific order for duplication events 2 and 3. This includes the reasonable possibility that duplications 2 and 3 occurred simultaneously (e.g. by partial or complete genome duplication). We propose that duplication of DEF-like genes is associated to the emergence of new perianth organ identities. Specifically, from an ancestor with an actinomorphic perianth evolved after the first round of DEF-like gene duplication an intermediate morphology where inner and outer tepals were different. Following a final round of duplication and functional divergence of clade 4 genes the lip or labellum organ identity emerged and made the perianth zygomorphic. The arrow indicates a 180° rotation of the pedicel and or ovary (resupination) that in most orchids sets the lip, the developmentally uppermost tepal, in a lowermost portion of the perianth. (b) Purifying selection prevails in DEF- and GLO-like genes (ω << 1), but is less stringent in Orchidaceae DEF-like genes from clade 2 (ω < 1); (c) Some proteins encoded by DEF-like genes from clade 2 miss part of the C-terminal domain; (d) Positive selection (ω>1) took place at the base of the Poales DEF-like genes. Most of the positions involved are in the K-domain. Positive selection was also detected, although not statistically significant, on the lineage linking DEF-like genes from Restionaceae with the rest of the Poales. The highly reduced and zygomorphic perianth or grasses evolved from a common ancestor with the "typical" monocot perianth morphology. Here represented by black half-moons are the inner perianth organs of Streptochaeta angustifolia (Anomochlooideae) and Oryza sativa (Pooideae) to illustrate the morphological transition that was preceded by the process of positive selection here documented. In this diagram the palea is in gray and the outer perianth whorl is absent.