Symbiotic solutions to nitrogen limitation and amino acid imbalance in insect diets

Although comprising ~ 80% of the Earth's air, nitrogen (N), in a usable form, is a limited resource for herbivorous insects. Additionally, much of the consumed dietary N is lost through-excretion, further challenging such insects, while limiting their growth and reproduction. To meet this challenge, many insect groups have evolved symbiotic relationships with microbes, enabling success within otherwise inhospitable niches. These mutualistic microbes contribute to their hosts' N-economies through nitrogen recycling, fixation and/or upgrading, using divergent metabolic strategies with varying precursors and physiological requirements. In this chapter we highlight each strategy, organizing our discussion by insect taxonomy, while enumerating the various microbes and innovations that have converged upon N-centric, nutritive functions. Our overview places a large emphasis on N-recycling, due in part to a lack of recent reviews on this topic. In reviewing N-fixation, we take a termite-centred approach, capitalizing upon an extensive bank of research performed across several decades. We also emphasize essential amino acid (EAA) and precursor biosynthesis, discussing non-fixing, non-recycling mutualisms in which symbionts make up for dietary shortcomings and missing pathways for nutrient biosynthesis in their hosts. In these explorations, we discuss the specificity and evolutionary histories between herbivorous insect hosts and their often ancient N-metabolizing symbionts. We focus, further, on correlations between dietary evolution and altered symbioses, which cast light on the causes and consequences of these nutritional relationships. We, finally, describe the evidence supporting prior arguments for N-centric mutualisms, emphasizing how a pairing of genomics and experimentation can uncover mechanism, while pinpointing just how symbiont metabolism shapes the fitness and N-budgets of diverse, herbivorous hexapods.