Therefore, the microenvironment of host fruits to which flies try to expand will have an important influence on the survival and adaptation of fruit flies. Therefore, when tephritids successfully expand their host range from ancestral host fruits to new hosts, they must adapt well to the chemical and nonchemical properties of the microenvironment from the novel host fruits, including their phytochemicals, color, and phenology. The color of the host fruit is an important cue to many fruit-infesting insects when selecting a new host . The phenology of the novel host, such as the timing of flowering and fruiting, also affects the ability of a tephritid to use a new host . Importantly, host chemicals are key drivers when herbivores encounter a novel host and serve as attractants and barriers to adaptation . Phytochemicals include volatile compounds and secondary metabolites that serve as attractants or defensive compounds to herbivores, such as tephritids. Volatile compounds allow tephritid adults to select among potential hosts while in flight, similar to fruit color. Once tephritid flies overcome the volatile chemicals of a potential new host, they eventually make contact with the host fruit, and then they must adapt to any secondary metabolites present to successfully colonize the host fruit. These chemical and nonchemical cues of a potential novel host fruit act as selective pressures on tephritids when a novel host is encountered . These selective pressures involve visual identification; behavioral selection; and physical, chemical, round planter pot and neurophysiological responses by tephritid flies to the novel host fruit . There is likely a genetic basis for each of these processes, which suggests that various genes are involved in regulating the host plant expansion of tephritids.
Therefore, increasing our knowledge of the categories and roles of these genes in regulating host expansion will deepen our understanding and allow for improved management strategies for tephritid fruit flies. Gene regulation of host plant expansion has been revealed in several herbivorous insects, including Subpsaltria yangi Chen , Drosophila mettleri Heed , and Chilo suppressalis Walker . For example, research on host plant expansion in a cactophilic fly, Drosophila mojavensis , revealed cytochrome P450, glutathione S-transferases, and UDPglycosyl transferases as major gene classes involved in new host use . Therefore, the present review summarizes current knowledge on the categories and roles of the genes involved in host plant expansion in tephritids and the related regulatory mechanisms and relates these findings to the development of new control methods for tephritid species. Volatile chemicals stimulate chemosensory receptors in tephritid flies when assessing a potential novel host and trying to expand . Therefore, chemosensory-related genes are involved in the initial process of host plant expansion for tephritids. Olfactory-related genes of tephritids are one type of chemosensory gene that includes several gene families of odorant-binding proteins , chemosensory proteins , odorant receptors , ionotropic receptors , and sensory neuron membrane proteins , which are primarily involved in the identification of volatile chemicals, including volatiles of host fruits. After receiving odor chemical signals, these olfactory-related genes are triggered to transduce cascades that send information to specific regions of the brain, which ultimately leads to specific behavioral responses . OBP genes play an important role in the first step of chemosensory identification of insects, including tephritids . OBP genes direct odorant-binding proteins to bind volatile odor molecules specifically by distinct expression to related olfactory receptors that are bound to olfactory receptor neurons in antennae . CSP genes are regarded as playing a similar role as OBP genes involved in the initial process of chemosensory signal transmission to corresponding receptors .
OBP and CSP genes are major gene types that lead tephritid flies to respond to different chemosensory chemicals, including volatile chemicals of host plants . Except for these two categories of genes, some odor receptor genes also play important roles in host odor recognition of tephritids, such as genes related to odor receptors and ionotropic receptors . Odorant receptors of insects are composed of at least two proteins: a conserved coreceptor as an ion channel and a specific OR subunit , which determines the ligand specificity and forms structurally ligand-gated ion channels . The OR genes mediate odorant receptors of insects transmitting the odorant molecules they receive into electric signals that are transmitted to a higher-order neural center . IR genes are related to ionotropic glutamate receptors , which are regarded as ion channels . They also play important roles in odor chemical perception . The sensory neuron membrane proteins gene encodes transmembrane domain-containing proteins that belong to a large gene family of CD36 receptors . SNMPs regulates the corresponding proteins to identify chemosensory signals, mainly pheromone chemicals . The GR family is another type of chemosensory protein that is a ligand-gated ion channel broadly expressed in gustatory receptor neurons in taste organs and is mainly involved in taste recognition of CO2 , sugar, and bitterness . When receiving taste signals, GR genes are involved in identifying taste and ingestion. Altering gene expression levels also helps tephritids respond to different host plants and realize host expansion. OR13a and OR82 expression are higher in antennae in B. dorsalis in response to 1-octen-3-ol and geranyl acetate, respectively, which are major volatile components of its host fruits, mango and almond fruit . For B. minax, increasing the expression levels of several GR genes regulate the taste process in response to different chemosensory stimuli of hosts .Once a tephritid adult identifies a potential novel host fruit for oviposition or feeding, the plant fruit must be suitable for larval development, which includes overcoming any secondary toxic chemicals in the novel host fruit . The ABC transporter superfamily belonging to phase III enzymes can be subdivided into eight subfamilies, from ABC-A to ABC-H. The cytochrome p450 gene family of phase I mainly contributes to the catalysis of numerous oxidative reactions during endogenous and exogenous metabolism . The important roles of genes in this family are the metabolism of xenobiotics, plant allelochemicals , and even insecticides. GSTs are multifunctional genes of phase II enzymes that play a crucial role in the detoxification of endogenous and xenobiotic compounds, including plant secondary metabolites and pesticides. CCE families of phase II have been shown to be involved in the detoxification of plant-derived allelochemicals as well as insecticides . The ABC transporter genes of phase III encoding membrane-bound proteins typically function in the ATP-dependent transport of various substrates across biological membranes . These genes can participate in regulating detoxification of host plant secondary metabolites of tephritid flies by coding corresponding enzymes, which help to transform toxins entering the insect system into hydrophilic compounds that can be eliminated and in the adaptability of different hosts . The major digestive-related genes include gene families of cysteine proteases, proteases, lipase, glucosidase, and serine proteases . The serine proteases are members of the supergene family, including chymotrypsin, trypsin, thrombin, subtilisin, plasmin, and elastase. subclasses . Various digestive proteases exert important roles in the nutrition digestion of tephritid flies from novel host plants that they try to expand to. Plant proteins of host plants are an important nutrition source used by tephritid flies. However, round pot for plants protease inhibitors of host plants are a widespread defense against herbivores such as tephritids. Therefore, genes coding various proteases react to protease inhibitors by regulating inhibitor-sensitive proteases or expressing proteases that are not targets of the inhibitors . When expanding to other novel hosts, tephritid flies must adapt to different chemical environments from their native hosts. In fact, nonchemical stimuli, such as color, are associated with vision-related genes that allow the identification of different hosts . The genes responsible for color discrimination in Diptera are primarily related to opsin proteins in the photoreceptor cells of the eye .
Six types of Rh opsin-expressed genes have been identified as major genes involved in color recognition and photoreception in Diptera insects. The Rh1 and Rh2 opsin genes are associated with motion detection and direction, respectively . Rh3 and Rh4 are UV-sensitive opsin genes, Rh5 is a blue-sensitive gene and Rh6 is a green opsin gene . These opsin genes lead the photoreceptor of eyes to receive various chromophore pigments and then activate a series of visual transduction cascades to launch corresponding color identification behavior. In the genome of polyphagous C. capitata, the long wavelength sensitive genes Rh1, Rh2, and Rh6 and the UV-sensitive genes Rh3 and Rh4 were found, while Rh2-4 and Rh6 were found in the photo transduction pathway of oligophagous B. minax . Moreover, the role of Rh6 in modulating green color discrimination was reported in C. capitata and B. minax . In B. minax, the function of Rh6, which is responsible for green spectral sensitivity, has been identified by knockdown of the gene B. minax in female adults, and B. minax flies significantly reduced their preference for green fruit after cutting Rh6 . Absence of a member of the bluesensitive opsin subfamily was found in both tephritid species C. capitata and B. minax, but Rh5 can be specifically expressed in Drosophila . Reports about vision-related genes directly involved in the host expansion of tephritids are still very few.Tephritid fruit fly hosts expand to other new host plants, and the phenology of the new host is another nonchemical stimulus that affects fly adaptation. The phenology of the host plant fruits includes the time of flowering, fruiting, or maturation . Many studies have revealed that dormancy plays a crucial role in assisting insects in responding to various phenological environments, including the phenology of different host fruits . The dormant state of tephritids was determined by the rate of growth and development. Therefore, genes associated with development are crucial factors that regulate the adaptation of phenology of various hosts. For example, genes related to sensing day length or photoperiodism and the central nervous system regulate chronic adaptation . Under the regulation of related genes, diapause may involve the deceleration of the developmental progress of tephritids to synchronize the phenological environment . R. pomonella of tephritids is a typical case. The ancestral host of R. pomonell is the hawthorn Crataegus mollis, but its species host expanded to the domestic apple Malus domestica and subsequently formed a new apple race . Apple fruits ripen earlier than hawthorn. The flies that infest apples and hawthorns must differentially time their life rhythms to match the differences in ripening times of their respective hosts . To realize this process, the flies of the two host races varied their time of overwintering pupal diapause. Under the pressure of different host fruit phenologies, many development-related genes are involved in regulating the adaptation to the different phenologies of two host plant fruits . Functional genes associated with cell/tissue development , metabolism , translation , and cell division are highly enriched . By increasing the expression levels of these genes, the CNS development of apple flies was elevated during their diapausing period compared to that of hawthorn flies. Adult emergence-associated genes, including key hormone signaling genes, the ecdysone receptor partner usp, the ecdysone biosynthesis protein ecd, cell cycling genes Myb and rbf, genes coding Mediator complex proteins, and various genes in the Wnt signaling pathway , etc., were enriched to regulate adult fly eclosion to match their host fruit ripeness .Genes coding for ribosomal proteins are often associated with protein translation by stably expressing ‘housekeeping’ genes. This type of gene is involved in many basic biological processes, such as digestion, detoxification, growth, and development, in most organisms . Therefore, ribosomal genes may also be involved in the host plant expansion of tephritids after receiving chemical and nonchemical stimuli. As mentioned above, ribosomal genes increased their expression level to regulate the growth of R. pomonella in response to the different phenology of its new host apple . The role of ribosomal genes involved in host expansion and new host adaptation of insects, including tephritid flies, is mainly related to the response of ribosome-inactivating proteins in host plants . RIPs have been found to have insecticidal functions in many insects, including beetles, mosquitoes, and moths . Ribosome genes can help insects such as tephritids realize host shifting by regulating their expression levels to counteract the RIPs of various host plants . In addition, ribosome genes interact with some epigenetic factors, which leads to chromatin remodeling to change gene expression and regulate different biological processes, including host plant adaptation .