However, the underlyinggenetic mechanism has not been revealed for Z. cucurbitae. In fact, nonchemical stimuli, such as color, are associated with vision-related genes that allow the identifcation 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, black plastic pots for plants while Rh2-4 and Rh6 were found in the phototransduction 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 identifed 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 blue sensitive 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 fy 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 . Therefore, genes associated with development are crucial factors that regulate the adaptation of phenology of various hosts. For example, genes related to sensing daylength 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 fy 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 . In response to different secondary chemicals, ribosomal genes were also involved in host detoxification of different species of the R.pomonella complex. R. zephyria and R. pomonella are sister species in the R. pomonella complex that specialize in snowberry and domestic apple plants, respectively . In reciprocal transplant tests of these two Rhagoletis taxa, microarray data indicated signifcant enrichment of mitochondrial ribosomal proteins when the two fly species fed on their new hosts, which contain different complements of phenolic and glycosidic in laboratory studies . Several studies on lepidopteran species revealed the role of ribosomal genes in response to host expansion . For example, ribosomal genes were downregulated in C. suppressalis when extended to the novel host water oats , which may be a more suitable host for C. suppressalis than its native host, rice. In contrast, ribosomal genes were upregulated in H. armigera when shifting to unsuitable novel hosts .The role of genes associated with the oxidative phosphorylation pathway is primarily involved in energy metabolism and provides energy in the form of ATP for most organisms and most biological actions . The OXPHOS pathway is coupled with the mitochondrial electron transport chain, and mitochondria are major sites of reactive oxygen species production in the majority of eukaryotic cells . The level of mitochondrial oxygen fow through the OXPHOS pathway influences ROS homeostasis and regulates the energy supply in different biological processes . OXPHOS genes can take part in many biological activities, and therefore they may also be important in the regulation of the response to host plant expansion of tephritid flies. Research on Bactrocera tau reared on two native cucurbit hosts and a novel host showed a large number of upregulated NADH genes in the OXPHOS pathway in transcript data of B. tau when feeding on banana. These results suggest that OXPHOS genes play an important role in the process of novel host fruit use in B. tau . OXPHOS was also involved in the host expansion of R. pomonella in response to the different phenologies of various hosts, as mentioned above. Certain genes in the fat bodies of tephritids are also involved in the energy supply for many biological processes, including digestion, detoxification, drainage pot development, and immunity . Differentially expressed genes, such as the lipase gene, ATP synthase gene, and alpha-amylase genes , were documented in the tephritids B. dorsalis and P. utilis in response to different secondary chemical environments .The various types of genes summarized above led to multilevel responses in tephritids, including nervous-, behavioral-, chemical-, and physical-level responses, when the flies faced different host environments. These multilevel responses to host expansion result in multilevel adaptations in flies, which lead to successful expansion to a novel host . Adaptation to a novel host is a complex process. Multilevel adaptation in fruit flies results from multigene regulation rather than a single gene or several genes performing various regulatory roles. The transcriptome data revealed that olfactory-, digestion- and detoxifcation-related genes and ribosomal genes were all involved in novel host adaptation in R. pomonella . Laboratory strains of B. tau also had activated OXPHOS genes and digestive and detoxification genes when the fy responded to a novel host environment .
The multiple-gene regulation mechanism during host expansion to a novel host was also documented in other insects. For example, C. suppressalis launched three types of genes simultaneously to regulate adaptation to the new novel host water oat . S. yangi differentially expressed genes related to digestion, detoxification, oxidation–reduction, stress response, water deprivation, and osmoregulation during adaptation to the new host Ephedra lepidosperma . Various genes also regulate the adaptation of tephritids to new hosts via multiple mechanisms. As summarized above, the alteration of gene expression levels, gene family expansion, and the use of various gene types or subfamilies are the major mechanisms involved in novel host adaptation.Many tephritid species attack economically important crops, including vegetables and fruits. The economic losses caused by tephritids reach over US$2 billion annually . Control strategies for tephritids primarily involve chemical use in many countries, which may be harmful to the environment and human health. Therefore, more environmentally friendly control methods should be sought and recommended when possible. RNA interference is an effective method to safely control tephritid flies. RNAi control methods suppress the expression of certain target genes by importing dsRNA . Therefore, selecting the target genes to be ‘silenced’ is a key step in the RNAi control method . Some target genes are associated with functions such as temperature sensitivity and sex determination . For tephritid species, including Anastrepha suspense , Anastrepha fraterculus , B. dorsalis , B. minax , Bactrocera tryoni , and C. capitata , effective RNAi controls have been developed based on the suppression of functional genes associated with eye pigmentation, embryonic segmentation regulation, postembryonic growth/development, reproduction, embryonic temperaturesensitive lethality and sex determination . Based on these target genes, RNAi can be applied in pest control not only for tephritid species but also for some Coleopterans and Lepidoptera insects by foliar spays, ingested dsRNA or sterile insect technique application . However, functional genes related to host plant adaptation are also target genes in RNAi control methods for tephritids. For example, the vision-related gene R6 or gustation gene GR59f of B. minax , digestion-related genes try1, try2, try4, and try5 of B. dorsalis , olfactory Orco gene of B. oleae , CSP2 gene of B. dorsalis , and detoxification genes CYP6A41 and CYP6EK1 of B. dorsalis are associated with host adaptation functional genes, and all of these genes possess an exploitable potential as target genes to control fruit flies. More target genes related to host plant expansion for tephritids need to be identified for their major functions and implemented in pest management. Although RNAi is an effective and tractable genetic tool, other novel gene tools, such as clustered regularly interspaced short palindromic repeats and the CRISPR-associated protein 9 gene editing system, can also provide scalable pest control strategies . Compared with traditional RNAi, CRISPR‒Cas9 can knock down or modify the target gene precisely instead of just suppressing the expression of target gene . The target genes edited by the CRISPR‒Cas9 system can create stable and heritable strains, which can be applied in actual tephritid control. Applying the CRISPR‒Cas9-mediated editing system, some target genes in tephritid flies have been evaluated for their potential for functional application, such as the eye pigmentation gene we , embryonic segmentation gene prd , sex-determination gene Astra-2 , tra2 , and pupae color gene wp . CRISPR/Cas9-mediated precise editing is a process in whichCas9 endonuclease recognizes a specific genomic region under the leading of chimeric single guide RNA . The CRISPR/Cas9 system editing the functional target gene shibire, tsl in B. tryoni and the white pupae gene wt in B. dorsalis, C. capitate, and Z. cucurbitae have been applied in the development of genetic sexing strain application in SIT control. This gene tool also has broad application prospects in tephritid management based on host plant adaptation-related genes in the future. Regulation of host adaptation would be an important mechanism to target because this adaptation allows tephritids to expand in new habitats and change to new biotypes. Therefore, developing suitable novel host adaptation functional genes as target genes in genetic disruption control strategies could help prevent tephritids in an environmentally friendly manner.Fruit trees exhibit two major multiannual reproductive strategies . In the first, the amount of fruit produced allows a sufficient amount of vegetative growth to support production of an ample number of flowers during the following year . Such trees, including fig and some orange and grapefruit cultivars, are defined as regular bearers.