Genome-wide characterization and expression profiling of immune genes in the diamondback moth, Plutella xylostella (L.)

Abstract

The diamondback moth, Plutella xylostella (L.), is a destructive pest that attacks cruciferous crops worldwide. Immune responses are important for interactions between insects and pathogens and information on these underpins the development of strategies for biocontrol-based pest management. Little, however, is known about immune genes and their regulation patterns in P. xylostella. A total of 149 immune-related genes in 20 gene families were identified through comparison of P. xylostella genome with the genomes of other insects. Complete and conserved Toll, IMD and JAK-STAT signaling pathways were found in P. xylostella. Genes involved in pathogen recognition were expanded and more diversified than genes associated with intracellular signal transduction. Gene expression profiles showed that the IMD pathway may regulate expression of antimicrobial peptide (AMP) genes in the midgut, and be related to an observed down-regulation of AMPs in experimental lines of insecticide-resistant P. xylostella. A bacterial feeding study demonstrated that P. xylostella could activate different AMPs in response to bacterial infection. This study has established a framework of comprehensive expression profiles that highlight cues for immune regulation in a major pest. Our work provides a foundation for further studies on the functions of P. xylostella immune genes and mechanisms of innate immunity.

Figure 1. Phylogenetic analysis of PGRPs based on the sequences of D. plexippus (Dp), B. mori (Bm), P. xylostella (Px), D. melanogaster (Dm), A. gambiae (Ag) and T. castaneum (Tc).

Figure 2. Phylogenetic analysis of GNBPs/βGRPs based on the sequences of D. plexippus (Dp), B. mori (Bm), P. xylostella (Px), D. melanogaster (Dm), A. gambiae (Ag) and T. castaneum (Tc).

Figure 3. Expression profiling of immune genes at different developmental stages of P. xylostella.

E: Egg; L1: 1^st instar larva; L2: 2^nd instar larva; L3: 3^rd instar larva; L4: 4^th instar larva; P: Pupa; Ma: Male adult; Fa: Female adult. Red represents the up-regulated genes, green indicates the down-regulated genes, and black represents no change in gene expression.

Figure 4

Differential expressions of AMPs induced by bacterial infection. C: Control, Eb: Enterobacter sp., Sm: Serratia sp., Ec: Enterococcus sp. Different letters over the columns within a graph denote significant differences (p < 0.05) among the different bacterial treatments, as determined by one-way ANOVA followed by LSD post hoc test.

Figure 5. Proposed model of the potential immune signaling pathways in P. xylostella

P. xylostella immune genes are indicated with prefix “Px” and are detailed in Table S2. All the putative pathways genes from P. xylostella were predicted based on sequence similarity as compared with other insects. The small question mark (?) next to the MyD88 indicates that the absence of this gene may be caused by the method of homologue search or incomplete genome information, and we cannot rule out the presence of MyD88 in P. xylostella. It is also possible that the function of MyD88 may be substituted by other adaptors, which need to be validated by experiments in future studies.