Annotation of detoxification-related gene families
The detoxification of xenobiotics is essential for the environmental adaptation of arthropods, including collembolans. The expansion of detoxification-related gene families is often observed in highly polyphagous species. We manually annotated ABC, CCE, CYP, GST and UGT genes in the two F. candidastrains (Table 2, Figure 4, Figure S4). In contrast to D. melanogaster , the ABC, CCE, CYP and GST families were greatly expanded in the two F. candida strains. FCDK showed greater expansion of the ABC, CYP and GST families than FCSH, consistent with the wider distribution of the former strain.
ABC transporters function as primary active transporters that transport diverse substrates across lipid membranes and include eight subfamilies, A‒H (Dermauw & Van Leeuwen, 2014). The gene numbers of ABCB, ABCD, ABCE and ABCF subfamilies are similar among FCDK, FCSH and D .melanogaster (Table 2, Figure 4a). D. melanogaster shows the fewest ABCG and ABCH genes but the greatest number of ABCA genes, which are related to lipid trafficking (Wenzel et al., 2007). ABCG genes exhibit diverse functions in eye colouration determination (Ewart et al., 1994; Mackenzie et al., 1999), moulting hormone regulation (Hock et al., 2000; Broehan et al., 2013) and xenobiotic resistance (Labbé et al., 2011). In contrast to FCSH, FCDK shows a major expansion of ABCC subfamily, which are functionally diverse, playing roles in processes comprising ion transport, cell-surface receptor activity and the translocation of a broad range of substrates (Dean et al., 2001; Kruh & Belinsky, 2003; Moreau et al., 2005).
CCEs hydrolysing carboxylic esters can be classified into three larger classes: dietary/detoxification, hormone/semiochemical processing and neurodevelopmental functions (Oakeshott et al., 2005). Dietary/detoxification CCEs accounted for 2/3 of all annotated CCEs in FCDK and FCSH. The two F. candida strains possessed five times as many dietary/detoxification-related and two times as many hormone/semiochemical CCEs as D. melanogaster (Table 2). No significant difference in CCE copy numbers was observed between FCDK and FCSH (Figure S4a).
The CYP superfamily is one of the most groups of important xenobiotic metabolism enzymes in arthropods and is usually classified into CYP2, CYP3, CYP4 and mitochondrial clans (Feyereisen 2006, 2012). For the CYPs identified in both F. candida strains, approximately 1/2 of the genes were clustered into the CYP2 clan, whereas few genes were included in the mitochondrial clan (Table 2, Figure 4b). The large number of CYP2 genes found in F. candida was similar to that reported in Chelicerata (Grbić et al., 2011; Fan et al., 2021) but different from that in insects. CYP2 enzymes are associated with the detoxification and bioactivation of certain exogenous chemicals (Feyereisen 2006). An obvious expansion of the clan CYP3 was also observed in FCDK and FCSH; these enzymes have been found to be major participants associated with xenobiotic metabolism and insecticide resistance across a wide range of artificial or natural chemicals. FCSH showed a contraction in the CYP4 clan relative to FCDK and D. melanogaster .
GST enzymes play a crucial role in the detoxification of a wide range of exogenous and endogenous compounds, particularly in insecticide resistance (Enayati et al., 2005; Li et al., 2007). GSTs can be classified into three superfamilies: cytosolic (13 classes), mitochondrial (kappa), and microsomal (MAPEG) (Oakley, 2011). Few microsomal GSTs were observed, and mitochondrial GSTs were completely lacking in the three taxa. Seven cytosolic classes were observed, among which the epsilon class was absent in FCDK and FCSH and mu was absent inD. melanogaster (Table 2, Figure S4b). The GST class pattern observed in the two F. candida strains strongly conformed to that observed in noninsect arthropods (Labbé et al., 2011; Roncalli et al., 2015; Fan et al., 2021). Folsomia showed a great expansion of the sigma class, which accounted for more than half of the annotated GSTs. Sigma GSTs have anti- and proinflammatory functions in mammals, immune response functions in helminth parasites, and lipid oxidation product detoxification functions in insects (Flanagan & Smythe, 2011).
Insect UGTs have diverse functions, including roles in processes such as detoxification (Smith, 1955), insecticide resistance (Lee et al., 2005), pigmentation (Hopkins & Ahmad, 1991), and sclerotization (Hopkins, 1992). FCDK and FCSH each had 48 UGTs, which was slightly greater than the number in D. melanogaster (Table 2). Most FolsomiaUGTs were clustered into two clades with node support of 100 and were isolated from D. melanogaster UGTs, indicating an independent origin of Folsomia UGT families (Figure S4c).