The notion of the "biological individual" is crucial to studies of genetics, immunology, evolution, development, anatomy, and physiology. Each of these biological subdisciplines has a specific conception of individuality, which has historically provided conceptual contexts for integrating newly acquired data. During the past decade, nucleic acid analysis, especially genomic sequencing and high-throughput RNA techniques, has challenged each of these disciplinary definitions by finding significant interactions of animals and plants with symbiotic microorganisms that disrupt the boundaries that heretofore had characterized the biological individual. Animals cannot be considered individuals by anatomical or physiological criteria because a diversity of symbionts are both present and functional in completing metabolic pathways and serving other physiological functions. Similarly, these new studies have shown that animal development is incomplete without symbionts. Symbionts also constitute a second mode of genetic inheritance, providing selectable genetic variation for natural selection. The immune system also develops, in part, in dialogue with symbionts and thereby functions as a mechanism for integrating microbes into the animal-cell community. Recognizing the "holobiont"--the multicellular eukaryote plus its colonies of persistent symbionts--as a critically important unit of anatomy, development, physiology, immunology, and evolution opens up new investigative avenues and conceptually challenges the ways in which the biological subdisciplines have heretofore characterized living entities.

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A symbiotic view of life: we have never been individuals.

1. Bacterial symbionts play a prominent role in insect nutritional ecology by aiding in digestion of food or providing nutrients that are limited or lacking in the diet. Thereby, endosymbionts open niches to their insect host that would otherwise be unavailable.

2. Currently, several other ecologically relevant traits mediated by endosymbionts are being investigated, including enhanced parasite resistance, enhanced heat tolerance, and influences on insect–plant interactions such as manipulation of plant physiology to the benefit of the insect.

3. Traits mediated by endosymbionts are often identified by correlative studies where traits are found to be altered in the presence of a particular symbiont. Recent developments in genomic tools offer the opportunity for studying the impact of bacteria–insect symbioses under natural conditions in a population and community ecology context. In vivo experiments specifically testing putative functions of endosymbionts in parallel to population-level studies on the prevalence of endosymbionts allow disentangling host versus symbiont contribution to phenotypic variability observed in individuals. Effects of symbionts on host phenotype are often large and relevant to host fitness, e.g. by significantly enhancing survival or fecundity in a context-dependent manner.

4. Predominantly vertically transmitted endosymbionts contribute to the heritable genetic variation present in a host species. Phenotypic variation on which selection can act may be due to differences either among host genomes, symbiont genomes, or genotype × genotype interactions. Therefore the holobiont, i.e. the host including all symbionts, should be regarded as the unit of selection as the association between host and symbionts may affect the fitness of the holobiont depending on the environment.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2311.2011.01318.x

Bacterial symbionts as mediators of ecologically important traits of insect hosts

Carotenoids are tetraterpene pigments that are distributed in photosynthetic bacteria, some species of archaea and fungi, algae, plants, and animals. About 850 naturally occurring carotenoids had been reported up until 2018. Photosynthetic bacteria, fungi, algae, and plants can synthesize carotenoids de novo. Carotenoids are essential pigments in photosynthetic organs along with chlorophylls. Carotenoids also act as photo-protectors, antioxidants, color attractants, and precursors of plant hormones in non-photosynthetic organs of plants. Animals cannot synthesize carotenoids de novo, and so those found in animals are either directly accumulated from food or partly modified through metabolic reactions. So, animal carotenoids show structural diversity. Carotenoids in animals play important roles such precursors of vitamin A, photo-protectors, antioxidants, enhancers of immunity, and contributors to reproduction. In the present review, I describe the structural diversity, function, biosyntheses, and metabolism of natural carotenoids.

/pdf/carotenoids-as-natural-functional-pigments-559vno3tuz.pdf

Carotenoids as natural functional pigments.

Polyphenism, in which multiple discrete phenotypes develop from a single genotype, is considered to have contributed to the evolutionary success of aphids. Of the various polyphenisms observed in the complex life cycle of aphids, the reproductive and wing polyphenisms seen in most aphid species are conspicuous. In reproductive polyphenism, the reproductive modes can change between viviparous parthenogenesis and sexual reproduction in response to the photoperiod. Under short-day conditions in autumn, sexual morphs (males and oviparous females) are produced parthenogenetically. Winged polyphenism is observed in viviparous generations during summer, when winged or wingless (flightless) aphids are produced depending on a variety of environmental conditions (e.g. density, predators). Here, we review the physiological mechanisms underlying reproductive and wing polyphenism in aphids. In reproductive polyphenism, morph determination (male, oviparous or viviparous female) within mother aphids is regulated by juvenile hormone (JH) titers in the mothers. In wing polyphenism, although JH is considered to play an important role in phenotype determination (winged or wingless), the role is still controversial. In both cases, the acquisition of viviparity in Aphididae is considered to be the basis for maternal regulation of these polyphenisms, and through which environmental cues can be transferred to developing embryos through the physiological state of the mother. Although the mechanisms by which mothers alter the developmental programs of their progeny have not yet been clarified, continued developments in molecular biology will likely unravel these questions.

/pdf/aphid-polyphenisms-trans-generational-developmental-1svzoa7zui.pdf

Aphid polyphenisms: trans-generational developmental regulation through viviparity.

Summary
Symbiotic microbes have become increasingly recognized to mediate interactions between natural enemies and their hosts. The ecologies of these symbioses, however, are poorly understood in many systems, and a predictive framework is needed to guide future studies. To achieve this, we focus on heritable, defensive microbes of insects. Our review of laboratory-based studies identifies diverse bacterial species that have independently evolved to protect a range of insects against parasitoids, parasites, predators and pathogens. Although defensive mechanisms are typically unknown, some involve toxins or the upregulation of host immunity.
Despite substantial benefits of infection in the presence of natural enemies, the protective symbionts of insects are often found at intermediate levels in natural populations. Using a host-centred population genetics approach made possible by the host restriction and cytoplasmic inheritance of these microbes, we propose that balancing selection plays a major role in symbiont maintenance, with protective benefits in the presence of enemies and infection costs in their absence. Other mediating factors are likely to be important, including temperature, superinfections and transmission dynamics.
While few studies have provided evidence for defence in the field, several studies have shown symbiont infection frequencies to be dynamic, varying across temporal and spatial gradients and food–plant associations. Newly presented data from our pea aphid research reveal that temporal shifts in defensive symbiont prevalence can be quite rapid, with Hamiltonella defensa showing 10–20% shifts around a seasonal average of c. 50%. Such findings contrast with more unidirectional changes seen in laboratory population cages, suggesting temporal changes in the costs and benefits of symbionts in the field.
To frame future research on defensive symbiont ecology, we briefly consider a range of studies needed to test laboratory- and field-derived predictions on defensive symbiosis. Included are investigations of defensive mechanisms, symbiont-driven co-evolution and community-level effects. We also consider the need for more thorough and highly resolved molecular diagnostics of natural infections, laboratory studies on functional differences between symbiont strains and species and studies on the relative costs and benefits of defenders in nature.
The emerging theme of symbiont-mediated defence across eukaryotes suggests that knowledge of the functional mechanisms behind protection and natural symbiont dynamics could be key to understanding many of the world's antagonistic species interactions. Thus, the development of insects as a model for such studies holds promise for these organisms and beyond.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/1365-2435.12133

Defensive symbiosis in the real world – advancing ecological studies of heritable, protective bacteria in aphids and beyond

Color variation within populations of the pea aphid influences relative susceptibility to predators and parasites. We have discovered that infection with a facultative endosymbiont of the genus Rickettsiella changes the insects' body color from red to green in natural populations. Approximately 8% of pea aphids collected in Western Europe carried the Rickettsiella infection. The infection increased amounts of blue-green polycyclic quinones, whereas it had less of an effect on yellow-red carotenoid pigments. The effect of the endosymbiont on body color is expected to influence prey-predator interactions, as well as interactions with other endosymbionts.

https://science.sciencemag.org/content/sci/330/6007/1102.full.pdf

Symbiotic bacterium modifies aphid body color.

In Japan, pea aphids Acyrthosiphon pisum mainly feed on vetch and clover, and many aphid clones produce more progeny on vetch than on clover. In this context, particular genotypes of the facultative symbiont Regiella insecticola enhance reproduction of infected pea aphids specifically on clover, thereby broadening the suitable food plant range of the insect. A species that is sympatric to A. pisum, vetch aphids Megoura crassicauda, are commonly found on vetch but not on clover. Laboratory rearing of M. crassicauda strains revealed active reproduction on vetch but substantially no reproduction on clover. Experimental transfection of Regiella from A. pisum to M. crassicauda by haemolymph injection established stable and heritable infection in the recipients, although no Regiella infection has been detected in natural populations of M. crassicauda. Different strains of Regiella-transfected M. crassicauda grew and reproduced on vetch, but exhibited lower fitness in comparison with corresponding uninfected aphid strains. Strikingly, the Regiella-transfected M. crassicauda exhibited improved survival and some reproduction on clover. These results suggest that Regiella has the potential to confer an ecological trait, adaptation to clover, on novel insect hosts, and also account for why Regiella is able to infect M. crassicauda but is scarcely found in these aphid populations.

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Shogo Matsumoto

Papers

Symbiotic bacterium modifies aphid body color.

Interspecific symbiont transfection confers a novel ecological trait to the recipient insect