Microbial Symbionts of Insects: Genetic Organization, Adaptive Role, and Evolution

Abstract

The microorganisms forming symbioses with insects play an important role in nutrition, development and evolution of their hosts. They make it possible for their hosts to use poorly digestible nutrients, to resist the biotic and abiotic stresses, and to regulate the metamorphosis. The microsymbionts of insects may be facultative (genetically specialized for symbiosis but retaining the capacity for autonomous existence; they are usually located extracellularly, in the gut, hemolymph, or salivary glands of the host) or obligatory (incapable of autonomous existence due to the loss of large parts of their genomes; they are usually located inside specialized host cells). The intracellular symbionts (endocytobionts) are capable of vertical transmission during the host reproduction, which determines the loss of many housekeeping genes, including the genes for replication, transcription and translation. In some obligatory symbionts, amplification of genes performing the functions useful for the hosts, such as the synthesis of essential amino acids, was found. These symbionts exhibit increased rates of accumulation of mutations, including non-synonymous nucleotide substitutions, reflecting suppression of the purifying selection and activation of genetic drift stimulating the genome reduction. Transfer of some genes from endocytobionts to the nuclear chromosomes of insects enables them to implement the novel metabolic functions, including assimilation of rare nutrients. The obligatory intracellular insect symbionts may be used as models to reconstruct the early stages of evolution of cellular organelles, which involve reduction of essential genes and the loss of genetic individuality of the symbionts, i.e., the ability for self-maintenance and expression of their residual genomes. Genetic analysis of insect microsymbionts extends the opportunities for their practical application associated with biological control of harmful insects (herbivorous, bloodsucking) and stimulation of the beneficial ones (honey collectors, pollinators, antagonists of pests).