Which microbes are symbionts

The discovery of symbionts through microscopy and phenotype rather than through culture, reflects the fastidious nature of the symbionts. The co-adaptation of symbiont to host environment has led to these bacteria commonly being hard to establish in culture (although there have been several notable successes, and cell culture has also proven useful). The first robust taxonomic insights into the diversity of bacteria that have evolved into insect symbionts followed the advent of PCR amplification of 16S rDNA, and its sequencing. This development also allowed the creation of PCR assays that permitted survey for the presence of candidate bacteria in a wide range of insects. Bacteria from many clades have been demonstrated to have evolved symbiosis with insects, and in many cases are inherited symbionts.

Within the proteobacteria, many members of the Enterobacteriacae (gamma proteobacteria) have independently evolved into symbiotic interactions with their host, presumably evolving from a gut association into one that existed within the body of the insect. One particular gamma proteobacterium, Arsenophonus, is known to be widespread, being found in about 5% of all insect species (Duron et al., 2008a), and there are implications (from repeated individual records) that others such as Sodalis may also be widespread (Fukatsu et al., 2007; Novakova and Hypsa, 2007). Within the alpha proteobacteria, members of the genera Rickettsia and Wolbachia are obligate symbionts of arthropods. Rickettsia often combine horizontal transmission via an arthropod vector with maternal transmission to host progeny, but many are now thought to be dedicated arthropod symbionts (Perlman et al., 2006). The population biology of Wolbachia, in contrast, is strongly based on maternal transmission, with occasional horizontal transfer events creating new infected lineages or species. Both Wolbachia and Rickettsia exhibit a number of so-called reproductive parasite phenotypes, manipulating sex ratio, and in the case of Wolbachia also induce incompatibility. Other alpha proteobacteria members have also entered into symbiosis with insects, albeit a more narrow subset. Ants, for instance, have recently been observed to carry vertically transmitted Bartonella relatives (Stoll et al., 2007).

Outside of the proteobacteria, there are two clades commonly found in a range of arthropods: the spiroplasmas and members of the genus Cardinium. Cardinium hertigii, a member of the Bacteroidetes-Flavobacteria-Cytophaga, was found in 5-7% of arthropod species sampled (Weeks et al., 2003; Zchori-Fein and Perlman, 2004), and has a population biology akin to Wolbachia, dominated by maternal transmission with occasional horizontal transmission between species. Like Wolbachia, it exhibits a variety of parasitic manipulation pheno-types (Weeks et al., 2001; Zchori-Fein et al., 2001; Hunter et al., 2003).

Members of the genus Spiroplasma represent common associates of arthropods. Derived from the mollicutes (mycoplasma relatives), these are arthropod specialists similar to Rickettsia in population biology. Some strains are propagated only by vertical transmission; others combine vertical and horizontal transfer, with some potentially being horizontally transmitted only (and causing insect disease) (Ammar and Hougenhout, 2006). Where Spiroplasma differ from Rickettsia is their locale in the host: whereas Rickettsia are almost completely confined to the intracellular milieu, spiroplasmas commonly additionally live outside of cells, free in the haemolymph.

A clade of bacteria that are emerging as partners of insects are the actinomycetes. These bacteria are often found on the exterior of insects in specialized glands. They are commonly placed into the local environment of the insect, where they act in a defence role against fungi that degrade either scavenged food provided for the insects' young (Kaltenpoth et al., 2005) or mutualistic fungal gardens provided likewise (Currie et al., 1999, 2003; Scott et al, 2008). They are commonly observed to show maternal inheritance. Although this is often contact-based, without internalization of the bacteria, transmission through feeding in the larval stage is suggested in one case (Kaltenpoth et al, 2005).

There are a range of other bacteria that are important in particular host groups. Chlamydia relatives have been found in symbiosis with the plant-sucking bug Bemisia tabaci and scale insects (Thao et al., 2003). Cockroaches, termites, and many scale insects require the presence of a Flavobacterium, specifically Blattabacterium in the case of cockroaches and termites (Bandi et al., 1995). Mealybugs carry a member of the beta division of proteobacte-ria, Tremblaya (Baumann and Baumann, 2005).

Aside from bacteria, eukaryotes of the phylum Microspora are commonly associated within insects. Whereas many microsporidia are oral pathogens that invade through the gut wall, propagate intracellularly, and kill their host, others are obligately vertically transmitted and have very weak (if any) pathology, or combine vertical and horizontal transmission with the timing of pathogen-esis and horizontal transmission strictly controlled (Terry et al, 2004). All members of the Microspora appear able to co-exist over significant periods with their host individual. Yeasts are also found in obligate interactions with insects, again with vertical transmission (Noda and Kodama, 1996).

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