Is pollinivory in the omnivorous ladybird beetle Micraspis discolor (Coleoptera: Coccinellidae) symbiosis-dependent?

Abstract

The ladybird beetle Micraspis discolor is a potential biological control agent in the rice ecosystems of Asia. Besides attacking a range of arthropod prey the ladybird also feeds on pollen. To explore the mechanism of pollinivory of M. discolor, we compared the transcriptomes and microbiomes of the ladybird when fed rape pollen versus insect foods (the aphid Aphis craccivora, the mealybug Planococcus citri or eggs of the Mediterranean flour moth Ephestia kuehniella). We also set out to identify putative cellulase genes and cellulolytic bacteria that might aid in pollen digestion. From the genome of M. discolor, we only identified glycoside hydrolase family 1 genes as candidate cellulase genes. However, these genes were not differentially expressed under the pollen-fed treatment as compared to the other diets. Transcriptome comparison among different diet treatments of M. discolor showed that differentially expressed genes were mainly related to nutrient metabolism, indicating differences in nutrient and antinutrient composition among the different diets. When comparing the microbiomes of M. discolor under different diet treatments, cellulolytic bacteria, like Siphonobacter aquaeclarae, were detected as discriminative biomarkers of pollen feeding. The findings from transcriptome and microbiome analyses indicate that cellulolytic bacteria might play a more important role than endogenous genes in pollen digestion by this ladybird beetle.