Genome-wide identification of genes involved in beetle odoriferous defensive stink gland function recognizes Laccase2 as the phenoloxidase responsible for toxic para-benzoquinone synthesis

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Exocrine glands have evolved several times independently in Coleoptera to produce defensive chemical compounds with repellent, antimicrobial, or toxic effects. Research on such glands had focused on morphological or chemical ecology methods. However, modern genetic approaches were missing to better understand this biological process. With the rise of the red flour beetle, Tribolium castaneum, as a model for studies of development and pest biology, molecular genetic tools are now available to also study the safe generation of toxic compounds in defensive stink glands. Using the RNA-interference-based, genome-wide, phenotypic screen "iBeetle" and the re-analysis of gland-specific transcriptomics based on a significantly improved genome annotation, we could identify 490 genes being involved in odoriferous stink gland function. In the iBeetle screen, 247 genes were identified, of which we present here 178 genes identified during iBeetle's 3rd phase, while the transcriptomics analyses identified 249 genes, with six genes being identified in both functional genomics approaches. Of these 490 genes, only about 40% of these genes have molecularly characterized homologs in the vinegar fly, while for 213 genes no fly homologs were recognized and for 13 genes no gene ontology at all was identified. This highlights the importance of genome-wide gene identification in tissues that have not been previously analyzed to recognize potentially new gene functions. Gene ontology analysis revealed "SNARE interactions in vesicular transport", "Lysosome", "Pancreatic secretion", and "MAPK signaling pathway - fly" as key pathways. Additionally, many of the genes are encoding enzymes, transcription factors, transporters, or are involved in membrane trafficking. As the phenoloxidase responsible for generating the toxic para-benzoquinones in the stink glands of the beetle, we could identify laccase2, which is expressed in the last secretory cell in contact with the cuticle-lined vesicular organelle, where the toxic compounds are safely produced before being released into the gland reservoir.

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