Nutrition and microorganisms

Broadly defined, nutrition concerns the nature and processing of foods needed to meet the requirements for growth and development, involving feeding behavior (Chapter 2) and digestion. Insects often have unusual or restricted diets. Sometimes, although only one or a few foods are eaten, the diet provides a complete range of the chemicals essential to metabolism. In these cases, monophagy is a specialization without nutritional limitations. In others, a restricted diet may require utilization of microorganisms in digesting or supplementing the directly available nutrients. In particular, insects cannot synthesize sterols (required for molting hormone) and carotenoids (used in visual pigments), which must come from the diet or microorganisms.

Insects may harbor extracellular or intracellular microorganisms, referred to as symbionts because they are dependent on their insect hosts. These microorganisms contribute to the nutrition of their hosts by functioning in sterol, vitamin, carbohydrate, or amino acid synthesis and/or metabolism. Symbiotic microorganisms may be bacteria or bacteroids, yeasts or other unicellular fungi, or protists. Studies on their function historically were hampered by difficulties in removing them (e.g. with antibiotics, to produce aposymbionts) without harming the host insect, and also in culturing the microorganisms outside the host. The diets of their hosts provided some clues as to the functions of these microorganisms. Insect hosts include many sap-sucking hemipterans (such as aphids, psyllids, whiteflies, scale insects, thrips, leafhoppers, and cicadas) and sap- and blood-sucking heteropterans (Hemiptera), parasitic lice (Psocodea), some wood-feeding insects (such as termites and some longicorn beetles and weevils), many seed- or grain-feeding insects (certain beetles), and some omnivorous insects (such as cockroaches, some termites, and some ants). Predatory insects never seem to contain such symbionts. That microorganisms are required by insects on suboptimal diets has been confirmed by modern studies showing, for example, that critical dietary shortfall in certain essential amino acids in aposymbiotic aphids is compensated for by production by Buchnera symbionts (Gammaproteobacteria). Similarly, the nutritionally poor xylem sap diet of sharpshooters (Hemiptera: Cicadellidae) is augmented by the biosyntheses of its two main symbionts: Sulcia muelleri (Bacteroidetes), which has genes for the synthesis of essential amino acids, and Baumannia cicadellinicola (Gammaproteobac-teria) that provides its host with a number of cofactors including B vitamins. In some termites, spirochete bacteria provide much of a colony's nitrogen, carbon and energy requirements via acetogenesis and nitrogen fixation. Although insects once were presumed to lack cellulases, they are present at least in termite guts, and their role in cellulose digestion relative to that of their symbionts is under active study.

Extracellular symbionts may be free in the gut lumen or housed in diverticula or pockets of the midgut or hindgut. For example, termite hindguts contain a veritable fermenter comprising many bacteria, fungi, and protists, including flagellates, which assist in the degradation of the otherwise refractory dietary ligno-

cellulose, and in the fixation of atmospheric nitrogen. The process involves generation of methane, and calculations suggest that tropical termites' symbiont-assisted cellulose digestion produces a significant proportion of the world's methane (a greenhouse gas) production.

Transmission of extracellular symbionts from an individual insect to another involves one of two main methods, depending upon where the symbionts are located within the insect. The first mode of transmission, by oral uptake by the offspring, is appropriate for insects with gut symbionts. Microorganisms may be acquired from the anus or the excreta of other individuals or eaten at a specific time, as in some bugs, in which the newly hatched young eat the contents of special symbiont-containing capsules deposited with the eggs.

Intracellular symbionts (endosymbionts) may occur in as many as 70% of all insect species. Endosym-bionts probably mostly have a mutualistic association with their host insect, but some are best referred to as "guest microbes" because they appear parasitic on their host. Examples of the latter include Wolbachia (section 5.10.4), Spiroplasma, and microsporidia. Endo-symbionts may be housed in the gut epithelium, as in lygaeid bugs and some weevils; however, most insects with intracellular microorganisms house them in symbiont-containing cells called mycetocytes or bacteriocytes, according to the identity of the symbiont. These cells are in the body cavity, usually associated with the fat body or the gonads, and often in special aggregations of mycetocytes, forming an organ called a mycetome or bacteriome. In such insects, the symbionts are transferred to the ovary and then to the eggs or embryos prior to oviposition or parturition: a process referred to as vertical or transovarial transmission. Lacking evidence for lateral transfer (to an unrelated host), this method of transmission found in many Hemiptera and cockroaches indicates a very close association or coevolution of the insects and their microorganisms. Evidence for benefits of endosymbionts to hosts is accruing rapidly due to genome sequencing and studies of gene expression. For example, as mentioned above, the provision of the otherwise dietar-ily scarce essential amino acids to aphids by their bacteriocyte-associated Buchnera symbiont is well substantiated. Of interest for further research is the suggestion that aphid biotypes with Buchnera bacteriocytes show enhanced ability to transmit certain plant viruses of the genus Luteovirus relative to antibiotic-treated, symbiont-free individuals. The relationship between bacteriocyte endosymbionts and their phloem-feeding host insects is a very tight phylogenetic association (see also Wolbachia infections, section 5.10.4), suggesting a very old association with co-diversification.

Some insects that maintain fungi essential to their diet cultivate them external to their body as a means of converting woody substances to an assimilable form. Examples are the fungus gardens of some ants (Formicidae) and termites (Termitidae) (sections 9.5.2 & 9.5.3) and the fungi transmitted by certain timber pests, namely, wood wasps (Hymenoptera: Siricidae) and ambrosia beetles (Coleoptera: Scolytinae).

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