Evolution of host plant use and glucosinolate sulfatases in Psylliodes species

The leaf beetle genus Psylliodes comprises about 200 species that are associated with host plants in at least 24 different families. Diet breadth varies among Psylliodes species but the majority use host plants belonging to a single plant family, predominantly within the Brassicaceae, Solanaceae, or Fagaceae. These plant families produce diverse defence metabolites such as glucosinolates, alkaloids, and tannins respectively, raising questions on how their host use evolved and how they adapted to such different hosts. To understand the evolution of host plant use in this genus, we use eight single-copy nuclear genes identified from the transcriptomes of eight Psylliodes species as well as Cytochrome-c-oxidase 1 to reconstruct their phylogenetic relationships. Using the reconstructed phylogeny we inferred the diversification patterns of Psylliodes species and found that they were predicted to have evolved from a polyphagous ancestor, with the descendants specializing on the host plants used by their ancestors and subsequently diversifying on them. Host specialization followed by species diversification can be facilitated by a novel counteradaptation to a host-specific chemical defence. We thus analysed the evolution of a detoxification mechanism that deactivates glucosinolates, the characteristic defence metabolites in Brassicaceae, by enzymatic desulfation using glucosinolate sulfatases. To elucidate the evolution of glucosinolate sulfatases in Psylliodes, we identified putative arylsulfatases from the transcriptomes of eight Psylliodes species and using phylogenetic analysis, identified glucosinolate sulfatases in the Brassicaceae-feeding species. We demonstrate that a conserved arylsulfatase gene had duplicated in the Psylliodes species specialization on Brassicaceae, enabling the evolution of a novel detoxifying function in the context of adaptation to a specific host. Together, our results suggest that changes in diet breadth and the evolution of specialized detoxification strategies played a role in the diversification of Psylliodes flea beetles. Overall design: Total RNA was extracted from the larval gut, remaining larval body and pupa of Psylliodes chrysocephala; the gut, remaining body tissue and whole body of adult P. affinis, P. dulcamarae, P. attenuata, P. hospes and P. laticollis; and the whole body tissue of adult P. crambicola and P. kiesenwetteri. RNASeq was performed using Illumina HiSeq 2500.