An influential meal: host plant dependent transcriptional variation in the beet armyworm, Spodoptera exigua (Lepidoptera: Noctuidae)

Abstract

Background: To understand the genetic mechanisms of insect herbivory, the transcriptional response of insects feeding on different host plant species has to be studied. Here, we generated gene expression data of the generalist herbivore Spodoptera exigua (Hübner) feeding on three selected host plant species and a control (artificial diet). The host plant species used in this study -cabbage (Brassica oleracea), maize (Zea mays) and tobacco (Nicotiana tabacum)- are members of different plant families that each employ specific defence mechanisms and toxins.

Results: Spodoptera exigua larvae had a higher growth rate, indicator for herbivore success, when feeding on Z. mays compared to larvae feeding on B. oleracea or N. tabacum. Larvae feeding on the different host plant species showed divergent transcriptional responses. We identified shared and unique gene expression patterns dependent of the host plant species the larvae fed on. Unique gene expression patterns, containing uniquely upregulated transcripts including specific detoxification genes, were found for larvae feeding on either B. oleracea or N. tabacum. No diet-specific gene cluster was identified for larvae feeding on the host for which larvae showed optimal herbivore success, Z. mays, or artificial diet. In contrast, for larvae feeding on hosts for which they showed low herbivore success, specific diet-dependent gene clusters were identified. Functional annotation of these clusters indicates that S. exigua larvae deploy particular host plant-specific genes for digestion and detoxification.

Conclusions: The lack of a host plant-specific gene activity for larvae feeding on Z. mays and the artificial diet suggest a general and non-specific gene activity for host plants with optimal herbivore success. Whereas the finding of specific gene clusters containing particular digestion and detoxifying genes expressed in larvae feeding on B. oleracea and N. tabacum, with low herbivore success, imply a host plant-specific gene activity for larvae feeding on host plants with suboptimal herbivore success. This observation leads to the conclusion that a polyphagous herbivore is able to feed on a large variation of host plants due to the flexibility and diversity of genes involved in digestion and detoxification that are deployed in response to particular host plant species.

Keywords: Detoxification; Gene expression; Generalist; Herbivory; Host specialization; Polyphagy; RNAseq; Transcriptomics.

Fig. 1

Overview of the experimental design to study the gene expression differences in Spodoptera exigua larvae feeding on different host plants. a Feeding assay: larvae hatched and developed on three different host plants (Brassica oleracea, Nicotiana tabacum and Zea mays) and artificial diet as control. b larval weight and developmental time was recorded to quantify herbivore success (growth rate in mg/day). c RNAseq data was generated of each diet treatment, followed by de novo assembly and differential gene expression analyses

Fig. 2

Graph showing the growth rates of Spodoptera exigua larvae collected in the third larval stage developing on different hosts, Zea mays, Brassica oleracea, Nicotiana tabacum and artificial diet. Growth rate (mg/day) was calculated by dividing the total larval weight (mg) by the number of days the larvae were allowed to feed (point of collecting: first observed larva reaching third larval phase). Asterisks (*) indicate significant different treatments compared to all others. The hashtag (#) indicate treatments that are not significantly different from each other. The graph was created with the R function ggplot2 [87] with spacing between datapoints

Fig. 3

Hierarchical cluster dendrogram using multiscale bootstrap resampling of Spodoptera exigua larvae samples with different diet treatments each consisting of three replicates. The filtered (gene count of ≥10 and occurrence ≥2 samples) and normalized (CPM + TMM normalization) gene count matrix was used as input, including 58,749 genes. The number of bootstrap replications was 10,000. Given are Bootstrap Probability (BP) and the Approximately Unbiased (AU) values based on complete hierarchical clustering using the correlation distance measure

Fig. 4

Hierarchical clustering dendrogram of all differentially expressed genes (DE) (left) in Spodoptera exigua larvae with different diet treatments clusters the 2585 DE in 6 clusters using a cut-of at 50% (red line). Expression patterns were visualized and the number of genes (N = #) per cluster is indicated (middle). Green colour coding indicates host diet treatment, larvae fed with B. oleracea, N. tabacum and Z. mays are shown with increasing darker shades of green. Each diet treatment consisted of three replicates. The heatmap (right) shows the expression pattern of the DE genes, black lines indicate the cluster devision. Each column corresponds to larvae from different diet treatments as indicated by the colour scheme