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.