1 Introduction
Many aphid species are economically important plant pests that feed on plant sap and also caused damage by transmitting plant viruses. Around 100 aphid species have been identified as significant agricultural pests among the approximately 5,000 known species (Blackman & Eastop, 2020). Up to this point, studies on aphid genomes have mostly focused on the subfamily Aphidinae (International Aphid Genomics Consortium, 2010; Li et al., 2019; Mathers, 2020; Mathers et al., 2017; Mathers, Mugford, et al., 2020; Mathers, Wouters, et al., 2020; Nicholson et al., 2015; Thorpe et al., 2018; Wenger et al., 2016). Genome sequencing efforts on other subfamilies that are distantly related to Aphidinae are very limited (Julca et al., 2020; Biallo et al., 2020). Unlike most free-living aphids, galling aphids induce the formation of galls on their primary host plants and live in the galls. Galling aphids may serve as good models for the study of unique ecological and behavioral phenomena for insect-plant interaction and coevolution (Moran, 1989; Wool, 2004). Up to now, galling aphids whose genomes have been sequenced include Eriosoma lanigerum and Hormaphis corn . The aphidE. lanigerum often causes bark deformation and cancer-like swellings on roots, trunk or brunches of apple, and sometimes induces leaf-rosette galls on American elm (Ulmus americana ) (Blackman and Eastop, 2020). Another aphid H. corn induces a gall which has an ostiole on the underside of the leaf (Kurosu et al., 1992). The galls induced by E. lanigerum and H. corn are quite different to the completely closed gall since its aphids have peculiar strategies to adapt the specific characteristics such as high CO2concentration, honeydew treatment and nutrition exchange of the closed internal environment (Chen et al., 2020).
The horned gall aphid,Schlechtendalia chinensis (Hemiptera: Aphididae: Eriosomatinae: Fordini), is one of the most economically valuable insects. Its gallnuts are valuable for medicinal purposes and in chemical industries. Some of the gallnuts components such as tannins are useful in producing inks, wine, food, cosmetic antioxidants, and animal feed. High levels of tannins ranged from 50 to 70% have been found in horned galls (Zhang, Tang, & Cheng, 2008). The annual yield of gallnuts in China is 8,000-10,000 tons, accounting for >90% of the total yield worldwide (Zhang, Tang, & Cheng, 2008).
S. chinensis has a complex life cycle involving both sexual and asexual reproduction stages with a host alternation between the Chinese sumac (Rhus chinensis , Anacardiaceae) and mosses (Plagiomnium spp.Mniaceae). In this holocyclic life cycle, a fundatrix produced by a mated female crawls along the trunk and feeds on a new leaf, where it induces the formation of a horned-gall. The fundatrix produces wingless fundatrigeniae via parthenogenesis in gall. In autumn, wingless fundatrigeniae produce winged fundatrigeniae named autumn migrants. When galls become mature and burst open, alate autumn migrants fly to nearby mosses and produce nymphs for overwintering. In the following spring, nymphs on mosses develop into spring migrants that fly back to the primary host, R. chinensis and produce female and male offspring (sexuales). After mating, each female reproduces only a fundatrix, starting the cycle again (Figure 1) (Zhang, Qiao, Zhong & Zhang, 1999; Blackman and Eastop, 2020). This unusual life cycle with various morphologically distant aphids at different stages is likely driven by adaptation to different environmental conditions. Unlike most free-living aphids from the Aphidinae taxon, galling aphids have many distinct biological characteristics. The most striking characteristic is that the feeding of most galling aphids does not seriously affect the health of their host plants. In fact, the formation of galls can provide some benefits to the primary host plant (Chen et al., 2020).
The complexity both in its developmental process and in the structure of its induced galls implies that S. chinensis may have unique gene sets that regulate its development and manipulate its host plants (Takeda et al., 2019; Hirano et al., 2020). The underlying molecular mechanisms for its complex life cycle remain largely unknown. Galls result from dramatic reprogramming of plant cell biology driven by insect molecules. Previous studies have shown that gall induction is highly, species-specific and different galling insects deliver unique sets of effectors into plant tissues, resulting in gall formation (Zhao et al., 2015; Alibory et al., 2018). The underlying mechanisms for the ability of the galling aphid to parasitize on host plants via apparently harmless galls remained unknown either. To understand the genetic basis of the complex lifestyle, we generated a high-quality chromosome-level genome assembly of S. chinensis , representing the first genome sequence of aphids that induces the formation of completely closed galls. In addition, we analyzed the phylogenetic relationship betweenS. chinensis and closely related species to give a better understanding of the unique biological characteristics of S. chinensis , such as suppressing plant defense and inducing gall formation.