3.2 ∣ Gene families
We identified 17 gene families that were expanded, and three families were contracted in the WWS genome when compared to the other 15 arthropod species we used in this study (Figure 2c). We analyzed the functional properties of the gene family expanded in the WWS genome and identified 497 unique gene families that might be related to adaptations consistent with plant sap consumption (Figure 1a). InterPro classification of the genes from 17 significantly expanded gene families (276 genes) showed significant enrichment of these genes in categories mainly involved in xenobiotic detoxification, including metabolism of xenobiotics by cytochrome P450 enzymes (P<0.01) and other drug-metabolizing enzymes (P<0.01) (all Fisher’s exact test) (Supplementary Table 14). This finding is consistent with the idea that a parasitic lifestyle, and the concomitant exposure to toxic compounds in the plant sap, would drive the expansion of genes encoding enzymes involved in xenobiotic metabolism.
Four groups of 24 genes involved in fatty acid synthesis and transport also underwent expansion. We examined all genes that may play major roles in fatty acid and lipid metabolic pathways and compared them with those from other insect species. A total of 704 WWS genes are predicted to act in fatty acid and lipid metabolic pathways, compared to only 239 related genes in the Hessian fly genome (Supplementary Figs. 9 and Supplementary Table 14). This result suggests that the expansion of fatty acid and lipid metabolism genes is linked to the production of wax in WWS (Kunst & Samuels, 2003; Teerawanichpan et al., 2010).
Two major groups of genes were found to have been fewer members compared to other insect genomes, namely odorant receptor genes and genes encoding dynein proteins. Specifically, there are seven dynein-encoding genes, 31 genes encoding odorant receptors, and 12 genes encoding odorant binding proteins in the WWS genome while the Hessian fly genome harbors 18 dynein genes, 122 genes encoding odorant receptors, and 32 genes encoding odorant binding proteins (Supplementary Figs. 9). Dynein, a motor protein, converts the chemical energy contained in ATP into the mechanical energy of movement (Supplementary Table 15). The reduction in dynein and the olfactory system for WWS might have coincided with the recession of foraging in this species as it feeds on its host branch without movement from the second instar nymph to adult. The reduction of odorant receptor genes might be explained by its parasitic nature and sessile lifestyle of females.
The expansion of xenobiotic detoxification genes is consistent with the idea that this could lead to the ability of WWS to effectively neutralize secondary compounds present in plant phloem. Similarly, the expansion of genes related to fatty acid and lipid synthesis is likely explained by the high production levels of white wax in WWS, an effective adaption for WWS to evade predators and cope with harsh environmental conditions.