**1. Introduction**

To withstand the ingestion of insects, plants have developed a defense mechanism against insects through various methods such as morphology, biochemistry, and molecular regulation in the long-term evolution process [1]. The production of secondary metabolites in plants is a way of defending themselves against insects. Various secondary metabolites in plants can affect the feeding and food utilization of insects, and toxic secondary metabolites can lead directly to insect death or stunted growth [2,3].

However, in the long-term evolutionary process, insects are constantly adapting to plant defense mechanisms. Genetic variation in morphology, including polymorphic wing growth, and differentiation of different types of mouthparts, such as chewing, stabbing, and sucking [4]. Insects can also use the enzymatic detoxification system, which mainly includes cytochrome P450 monooxygenases (CYPs), glutathione S-transferases (GSTs), and

**Citation:** Li, W.; Yang, B.; Liu, N.; Zhu, J.; Li, Z.; Ze, S.; Yu, J.; Zhao, N. Identification and Characterization of the Detoxification Genes Based on the Transcriptome of *Tomicus yunnanensis*. *Diversity* **2022**, *14*, 23. https://doi.org/10.3390/d14010023

Academic Editor: Luc Legal

Received: 27 November 2021 Accepted: 29 December 2021 Published: 31 December 2021

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carboxylesterases (CCEs), to improve the ability of insects to metabolize secondary substances of plants, and cause insects to adapt to the defense mechanism of the host plant [5].

CYPs are a very large and diverse group of enzymes found in all living things. They constitute a very important system involving endogenous compounds and xenobiotics such as drugs, pesticides, secondary plant metabolites, mutagens, and hormones [6,7]. In addition, CYPs can reflect different levels of phylogenetic information such as subtypes, population, and species. It is a good molecular marker to explore the phylogenetic relationship between populations and population genetic diversity [8,9]. According to the similarity of amino acids, the CYPs of insects can be divided into four clades: CYP2, CYP3, CYP4, and mitochondrial CYP clades [10,11]. When exposed to toxic plant compounds, insects will induce reactions to other exogenous substances. For example, CYP6AE14 in *Helicoverpa armigera* Hübner, 1809 (Lepidoptera, Noctuidae) is involved in the metabolism of gossypol (plant toxins) [12]. GST is a dimeric protein, which plays an important role in intracellular metabolism, chemical cycle, and body defense [13]. Insect GST can be divided into six categories (delta, epsilon, omega, sigma, theta, and zeta). Among them, the delta and epsilon subtypes are unique to insects, and some GSTs play an important role in plant antitoxin [14,15]. In research on *Dendroctonus armandi* Tsai and Li, 1959 (Coleoptera, Scolytinae), it is assumed that DaGSTe1 catalyzes the combination of glutathione with terpenes and phenolic substances from *D. armandi*, so that toxic substances are transported out of the cell, thereby reducing the damage of exogenous toxins from the host to *D. armandi* [16]. In addition to CYPs and GSTs, another detoxification enzyme is CCE. Insect CCEs belong to the carboxyl/cholinesterase family, a branch of the α/β-hydrolase fold superfamily in which enzymes hydrolyze ester bonds, which are present in many plant volatiles, insect pheromones, and hormones, as well as pesticides [17].

*Tomicus yunnanensis* Kirkendall and Faccoli, 2008 (Coleoptera, Scolytinae) is one of the main pests of *Pinus yunnanensis*, which is widely distributed in Yunnan, Sichuan, and Guizhou. The whole life cycle of *T. yunnanensis* is almost entirely on *P. yunnanensis* [18]. The damage to *P. yunnanensis* by *T. yunnanensis* can be divided into two periods: feeding on treetops and trunks. When feeding on the treetops, the adult worms feed on the pith tissues of the branches and supplement nutrients to complete their sexual development; when feeding on the trunk, the mature adult worms transfer from the treetop to the trunk and feed on phloem, lay eggs, and form mother tunnel and sub tunnel. Then, larvae develop under the bark, pupate, and emerge, and adults will burrow out of the phloem and return to the treetops to feed [19,20]. This concealed lifestyle brings many inconveniences to the study of its habits, prevention, and treatment. *T. yunnanensis* first broke out in central Yunnan Province in the 1980s, and then spread to 65 counties in 15 regions of Yunnan Province, infesting more than 200,000 ha of Yunnan pine forests and blighting more than 93,000 ha of *P. yunnanensis*, and seriously damaged the mountain forest ecosystem [21]. To mitigate the damage of this insect, some common measures can play a certain role for a short time, such as biological control, chemical control and tending, and mixed forest [22]. Among them, the mixed forest has achieved remarkable results in insect resistance and contributed to the protection of mountain forest ecosystem, but its anti-insect mechanism still needs further research. The identification of the detoxification genes of *T. yunnanensis* enriches the detoxification gene family in Coleoptera and provides data support for future research on the function of detoxification genes. We hope that the study can provide a theoretical basis for insect resistance in mixed forests.
