Genomic dissection of an extended phenotype: Oak galling by a cynipid gall wasp

Abstract

Galls are plant tissues whose development is induced by another organism for the inducer's benefit. 30,000 arthropod species induce galls, and in most cases the inducing effectors and target plant systems are unknown. Cynipid gall wasps are a speciose monophyletic radiation that induce structurally complex galls on oaks and other plants. We used a model system comprising the gall wasp Biorhiza pallida and the oak Quercus robur to characterise inducer and host plant gene expression at defined stages through the development of galled and ungalled plant tissues, and tested alternative hypotheses for the origin and type of galling effectors and plant metabolic pathways involved. Oak gene expression patterns diverged markedly during development of galled and normal buds. Young galls showed elevated expression of oak genes similar to legume root nodule Nod factor-induced early nodulin (ENOD) genes and developmental parallels with oak buds. In contrast, mature galls showed substantially different patterns of gene expression to mature leaves. While most oak transcripts could be functionally annotated, many gall wasp transcripts of interest were novel. We found no evidence in the gall wasp for involvement of third-party symbionts in gall induction, for effector delivery using virus-like-particles, or for gallwasp expression of genes coding for plant hormones. Many differentially and highly expressed genes in young larvae encoded secretory peptides, which we hypothesise are effector proteins exported to plant tissues. Specifically, we propose that host arabinogalactan proteins and gall wasp chitinases interact in young galls to generate a somatic embryogenesis-like process in oak tissues surrounding the gall wasp larvae. Gall wasp larvae also expressed genes encoding multiple plant cell wall degrading enzymes (PCWDEs). These have functional orthologues in other gall inducing cynipids but not in figitid parasitoid sister groups, suggesting that they may be evolutionary innovations associated with cynipid gall induction.

Tissue-specific gene expression shows cynipid wasps repurpose host gene networks to create complex and novel parasite-specific organs on oaks

“…Increases in sugar metabolism and transport indicate that galled tissue has shifted from an autotrophic to a heterotrophic nutrition source and has become a nutrition sink on the host plant. Increases in ethylene production have been shown previously in wasp, mite, aphid, and sawfly galls, with the hypothesis that it defending the gall against herbivores and pathogens (Hearn et al, 2019;Samsone, Andersone, & Ievinsh, 2012), however polyphenol oxidase and peroxidase activity were shown to decrease in galled tissue (Gailite, Andersone, & Ievinsh, 2005). Additionally, in this study 1-aminocyclopropane-1carboxylate oxidase (ACO), which catalyzes the synthesis of ethylene, shows 151-fold increase in expression in the inner gall tissue and a 31-fold increase in the outer gall tissue, which doesn't support a defensive role as it would directly impact the feeding wasp larvae.…”

“…Increased levels of IAA have been found in galled tissue and in the galling insects themselves in several systems (reviewed in (Tooker & Helms, 2014)). However, previous studies looking at cynipid gallers on Chestnut (Wood & Payne, 1988) and oak (Hearn et al, 2019) hosts found little evidence for the involvement of auxin and IAA in cynipid galls. The current study shows the up-regulation of indole-3acetic acid-amido synthetase and six auxin response factors.…”

“…Another significantly up-regulated gene in both inner and outer gall tissue was early nodulin-93 (ENOD93 ), which is also involved in the induction of root nodules by rhizobia bacteria (Kosslak et al, 1987;You et al, 2003). ENOD genes were also found to be up-regulated in another cynipid/oak galling system (Biorhiza pallida /Quercus robur ), where it has been proposed that somatic embryogenesis is induced by wasp GH18 chitinases cleaving the oak early nodulin genes (Hearn et al, 2019). The expression of ENOD genes in the legume-rhizobia system occurs after the exchange of biochemical signals between the plant and bacteria.…”

Plants make galls to accommodate foreigners: some are friends, most are foes

“…Insect gall-inducing species have been subjects of famous longterm ecological studies (Table 1): true fruit flies (Tephritidae) on Solidago (Abrahamson & Weis, 1997), Euura and Pontania sawflies on Salix (Price, 1992;Hardy & Cook, 2010), Pemphigus aphids on Populus (Whitham, 1992;Larson & Whitham, 1997), and cynipid oak gall wasps on Quercus (Stone & Sch€ onrogge, 2003;Egan et al, 2012;Hearn et al, 2019). Galls of oak gall wasps are attacked by parasitic plants (Egan et al, 2018).…”

Plants use identical inhibitors to protect their cell wall pectin against microbes and insects

“…Whereas amylases and proteases are widespread among insects and their significance has been evident for decades, recent advances in sequencing technologies and bioinformatics analyses of genomes and transcriptomes have revealed the presence of several endogenous genes encoding plant‐cell‐wall‐degrading enzymes (PCWDEs) in insects. These genes include various families of glycoside hydrolases, esterases, and lyases, and have been detected in several herbivorous lineages (Calderon‐Cortes, Quesada, Watanabe, Cano‐Camacho, & Oyama, 2012; Hearn et al, 2019; McKenna et al, 2019; Wybouw, Pauchet, Heckel, & Leeuwen, 2016). PCWDEs break down PCW polysaccharides such as cellulose, hemicelluloses, and pectins.…”