**5. Gene Evolution for VOCs**

The evolution of orchids has resulted in an immense diversity of flower traits such as color and scent. Orchidaceae consist of extraordinary adaptations that may have guaranteed its evolutionary success. To date, most of the examined plant gene families originated through gene duplication. Gene duplication plays a key role in species evolution because it provides raw materials for the evolution of new genes and new genetic functions. Multiple mechanisms contribute to gene duplication, including tandem duplication, segmental duplication, transposon-mediated duplication, and retro duplication. Studies of floral scent gene duplications in orchids have been limited. Orchid TPSs are the key enzymes that generate the structure diversity of terpenes. Through the analysis of plant genome, researchers have shown that the plant TPS gene family have their gene numbers ranging from 20 to 150 and thus belong to a mid-size family [70]. *Phalaenopsis equestris* genome has 23 TPSs belonging to TPS-a, -b, -c, e/f, and -g. Twenty-three TPSs found and predicted as having mono-, di- and sesqui-terpene synthase evolutionary relationships among orchids and experiencing duplication and then sub- or neo-functionalization, have occurred during evolution [71]. It has been proposed [72] that diterpene synthases are the origin of mono- and sesqui-terpene synthases during evolution. *P. aphrodite*, *PaCHS3*, *PaCHS4*, and *PaCHS5* formed a tandemly arrayed gene cluster, and the intervals between the three *CHS* genes were approximately 13.3 kb (*PaCHS3* and *PaCHS4*) and 7.7 kb (*PaCHS4 and PaCHS5*). This arrangement was also observed in a closely related orchid, *P. equestris*, in which the three CHS genes were all positioned on Scaffold 000036. Thus, the tandem array of three *CHS* genes was probably present in a common ancestor before speciation within *Phalaenopsis*. Tandem gene duplications represent a substantial proportion of all plant genes [73].
