Phenoloxidase activity in isoptera

Abstract

Insects are an evolutionarily and ecologically successful taxonomic group. To survive and be competitive, insects must have the ability to defend against the assault of pathogens and parasites. Arthropod immunity is comprised of two major components, the humoral and cellular responses. The latter is heavily dependent on the phenoloxidase cascade, although both components utilize this cascade. Phenoloxidase (PO) is a "versatile" enzyme, responsible for the process of melanization in arthropods. It plays a role in the sclerotization and tanning of newly molted exoskeleton while also being involved in several immune responses such as encapsulation, nodulation and wound healing. The quantification of PO activity has been used as a proxy for estimating the investment in immunity by a variety of species. Relatively few studies have focused on how sociality influences PO activity in social Hymenopterans (i.e. ants, bees and wasps) and fewer published studies exist regarding Isopterans (i.e. termites). Termites are faced with a unique set of challenges while living under significant pathogenic constraints. Their nesting, feeding and foraging ecologies favor exposure to varied microbial pathogenic/parasitic communities and their sociality can exacerbate disease transmission within a colony. Yet, these social insects thrive in microbial rich environments. In this study, PO activity was quantified and used to estimate immune investment as a function of termite phylogeny (and the accompanying nesting, feeding, foraging habits), gender and caste. This work presents evidence that PO activity varies across species and their corresponding specific life history attributes. Taxa that nest in microbial rich environments (i.e. soil) exhibit significantly higher PO levels than arboreal species. In terms of caste membership, those individuals with greater reproductive potential exhibit higher PO investment, although no sex-based differences in PO activity exist in most species tested. The quantification of PO activity during the course of fungal infection by lethal dosages (2x106 and 2x108 conidia/ml) of the entomopathogenic fungus Metarhizium anisopliae indicates that the termite immune system responds with an up-regulation of PO activity two days after infection relative to controls. Termites provide a unique perspective on the evolution of the immune systems within the eusocial insects. These comparative data indicate for the first time that termite immune investment (measured in terms of PO activity) varies in accordance with predictions made by life history theory.