# Composition Diversity and Expression Specificity of the _Tps_ Gene Family among 24 _Ficus_ Species
## 1 Introduction  
The interaction between plants and pollinating insects is considered to be an important driving force for the biodiversity and evolution of angiosperms in the tropics [1, 2]. While the polymorphism of floral characteristics helps cater to diverse or specific flower visitors, the quantity and quality of pollen transmitted by flower visitors affect the reproductive success of plants. In nature, most of the interactions between plants and pollinators are generalized, except for a few highly obligate mutualisms, such as fig-fig wasp and yucca-yucca moth. _Ficus_, which belongs to the Moraceae family, constitutes one of the largest genera of angiosperms with approximately 750 species, including trees, shrubs, epiphytic vines, andother life forms [3; 4]. The most typical feature of _Ficus_ is its unique enclosed inflorescence commonly known as a syconium or fig. A syconium contains hundreds or thousands of tiny flowers and connects to the external environment via a narrow ostiole at its top. The opening of the ostiole is controlled by bracts, which are the main parts of _Ficus_ that emit volatile organic compounds (VOCs) in receptive-phase syconia to attract obligate pollinators [5; 6]. Only morphologically specialized fig wasps can enter the syconium and pollinate the female flowers within [7; 8]. A species of _Ficus_ can only attract and drive one or a few fig wasp species to pollinate [9]. The key components that attract specific pollinating wasps are different and generally include one or a few common terpenoid compounds or specific proportions of several compounds [10]. These key components strongly attract specific pollinator wasps, but not the wasps that pollinate other _Ficus_ species [11]. Even for the same _Ficus_ species, the specific pollinating wasps attracted by syconia in the receptive phase may be rejected by those in other developmental stages. This \"push and pull\" mechanism maintains the specific symbiotic relationship between pollinating wasps and their host plants [12; 13; 14]. VOCs include terpenoids, fatty acids derivatives, phenylpropanol/benzene ring compounds, and amino acid derivatives. Terpenoids represent the largest and most diverse class of chemicals, among the myriad of compounds produced by plants [15]. In _Ficus_, terpenoids provide key olfactory signals that attract pollinating wasps [10; 16]. Terpenoids can be classified into monoterenes (including 10 carbons), sesquiterpenes (including 15 carbons), diterpenes (including 20 carbons), and triterpenes (including 30 carbons). Most monoterenes and sesquiterpenes are oily liquids that become volatile at room temperature and have special odors [17]. The synthesis of plant terpenoids generally occurs in three stages: (1) isopentenyl diphosphate (IPP) and dimethylallyl pyrophosphate (DMAPP) are synthesized through the mevalonate (MVA) pathway in the cytoplasm and the 2-C-methyl-D-erythnitol 4-phosphate (MEP) pathway in the plastid, respectively; (2) under the action of allyl transferase, geranyl diphosphate (GPP), farnesyl diphosphate (FPP), geranyl diphosphate (GGPP), and intermediates with various molecular weights are formed; (3) GPP, FPP, and GGPP, under the action of various terpene synthases (_TPSs_), generate monoterenes, sesquiterpenes, diterpenes, and their derivatives (Figure S1). Finally, the modified enzymes catalyze hydroxylation, methylation, isomerization, and reduction to further produce terpene alcohols, acids, esters, and other derivatives [15; 18]. The _TPS_ gene family exists widely in plant genomes, most of which are involved in the production of secondary metabolites [19; 20]. At the protein level, each TPS enzyme has two conserved domains, Pfam ID PF01397 and PF03936, distributed at the N- and C-terminals. On the basis of their related catalytic motifs, TPSs can be divided into classes I and II [21; 22; 23]. Class I TPSs, including all monoterenes, all sesquiterpenes, and some diterpenes, contain two conserved DDXXD and NSE/DTE motifs.