The loss of the urea cycle and ornithine metabolism in different insect orders: An omics approach.

Previous studies suggest that some insects require dietary arginine because they cannot synthesize this amino acid through the urea cycle. To determine whether this finding applies to all insects and what its metabolic implications are, we analysed the conservation of 20 genes involved in arginine biosynthesis and metabolism in the genomes of 150 species from 11 taxonomic orders. Our results showed that no insect can synthesize arginine via the urea cycle, as ornithine carbamoyltransferase is absent from all genomes analysed. While we found losses in other genes encoding urea cycle enzymes, nitric oxide synthase (NOS) was conserved across orders. However, the citrulline produced by NOS cannot be converted back to arginine in several insects due to the loss of argininosuccinate synthase and argininosuccinate lyase genes. Despite the inability to synthesize arginine, all insects (except some Hemiptera) can degrade it to ornithine and urea, as the arginase (ARG) gene is conserved across the orders analysed. For some Hemiptera that have lost ARG, we investigated how these insects produce or metabolize ornithine. Our results show that the genes for converting ornithine to glutamate, proline and putrescine are conserved across orders. However, while all insects have enzymes to synthesize putrescine and spermidine, some lack the ability to produce spermine due to the absence of the spermine synthase gene. Taken together, our results show that the loss of the urea cycle has led to significant changes in the pathways by which insects metabolize and recover arginine, which is particularly important for the diversification of hemipterans.