Host-specific gene expression as a tool for introduction success in Naupactus parthenogenetic weevils

Abstract

Food resource access can mediate establishment success in invasive species, and generalist herbivorous insects are thought to rely on mechanisms of transcriptional plasticity to respond to dietary variation. While asexually reproducing invasives typically have low genetic variation, the twofold reproductive capacity of asexual organisms is a marked advantage for colonization. We studied host-related transcriptional acclimation in parthenogenetic, invasive, and polyphagous weevils: Naupactus cervinus and N. leucoloma. We analyzed patterns of gene expression in three gene categories that can mediate weevil-host plant interactions through identification of suitable host plants, short-term acclimation to host plant defenses, and long-term adaptation to host plant defenses and their pathogens. This approach employed comparative transcriptomic methods to investigate differentially expressed host detection, detoxification, immune defense genes, and pathway-level gene set enrichment. Our results show that weevil gene expression responses can be host plant-specific, and that elements of that response can be maintained in the offspring. Some host plant groups, such as legumes, appear to be more taxing as they elicit a complex gene expression response which is both strong in intensity and specific in identity. However, the weevil response to taxing host plants shares many differentially expressed genes with other stressful situations, such as host plant cultivation conditions and transition to novel host, suggesting that there is an evolutionarily favorable shared gene expression regime for responding to different types of stressful situations. Modulating gene expression in the absence of other avenues for phenotypic adaptation may be an important mechanism of successful colonization for these introduced insects.

Fig 1. Number of differentially upregulated genes in three targeted gene categories from weevil species feeding on different host plants or in different experimental conditions.

Categories analyzed include genes related to host plant detection (HD), host plant detoxification (DTX) and immune defense (IM) when comparing: (i) weevils feeding on Legume vs. Other in N. cervinus and N. leucoloma; (ii) N. cervinus weevils feeding on Legume vs. Citrus; (iii) N. cervinus weevils feeding on oranges grown under Conventional vs. Organic farming methods; (iv) weevils feeding within the same host plant family: Citrus (Rutaceae: Citrinae), Legume (Fabaceae), or Other (Asteraceae) host plants; (v) N. cervinus weevils maintained on the natal host plant or switched from that host to a novel host—Switch vs. Maintain. Each point represents a separate pairwise comparison in the set; i.e. one triangle represents the number of DEGs from a head tissue comparison that belongs in the group ‘Legume vs. Citrus’ for each gene group within the three targeted categories. “+” after a gene group name indicates significant differences in the number of DEGs between the two host plants or treatments for that particular gene group.

Fig 2. Composite heatmap showing expression intensity of significantly up- and downregulated genes in three gene categories including all available tissue types for weevils feeding on different host plants or in different experimental conditions.

Results are displayed by prediction, species and tissue for each gene category (host detection (HD), host detoxification (DTX) and immune defense (IM)) and each direction of expression compiled from individual heatmaps (S1 Fig). (i) contrasts when feeding on different plant families, farming methods and experimental conditions (S2ia–S2id Fig), (ii) feeding within the same host plant family: Citrus (Rutaceae: Citrinae) (c vs. c); Legume (Fabaceae) (l vs. l); and Other (Asteraceae) (o vs. o) (S2ii Fig). Shades of red indicate relative levels of upregulation in Group 1 while shades of blue indicate relative levels of upregulation in Group 2.

Fig 3. Number of unique and shared differentially expressed genes (DEGs) associated with host detection, host detoxification and immune defense between comparisons.

Venn diagrams show overlaps or uniqueness in the identity of differentially expressed transcripts in either direction between comparisons. (i) four-way Venn diagrams including: Legume vs. Other, Legume vs. Citrus, Conventional vs Organic, and Switch vs. Maintain for host detection-related DEGs (HD), host detoxification-related DEGs (DTX) and immune defense-related DEGs (IM). (ii) three-way Venn diagrams including comparisons within the same host plant family: Citrus (Rutaceae: Citrinae); Legumes (Fabaceae); and Others (Asteraceae).

Fig 4. Exploration of generalized expression changes specific to particular host plants or experimental conditions.

(i) Identity of upregulated Gene Ontology (GO) terms in G1; (ii) Identity of upregulated GO terms in G2, unique and/or shared between host plant contrasts and experimental conditions. Bold indicates the direction of the contrast, or which host plant group is overexpressing transcripts in that GO term. Parentheses after GO term descriptions contain the tissues where DE was found for each GO term (h: Head, a: Abdomen, and i: Immature). Numerical identifiers, GO categories and statistics for GO term abbreviations can be found in S4 Table.