Evolution of obligate nutritional endosymbionts

Obligate nutritional symbioses are almost countless, especially in insects (Buchner, 1965; Moya et al , 2008) Associations can be extracorporeal and environmental like fungi and ambrosia beetles or intracellular and linked to transovarial transmission Complex micro-bial communities spanning three bacterial phyla including a-Proteobacteria are vertically transmitted between juveniles, larvae, and adults by a single sponge species (Schmitt et al , 2007) Cospeciation has been reported for a range of diverse endosymbionts and hosts such as sulfur-oxidizing endosymbionts and deep sea clams (Peek et al , 1998), Blat-tabacterium (Flavobacteria) and cockroaches and the termite Mastotermes darwiniensis (Lo et al ., 2003), Buchnera in aphids (y-Proteobacteria) (Clark et al ., 2000), Cand. Carsonella (y-Proteobacteria) in psyllids (Thao et al ., 2000), Cand. Portiera (y-Proteobacteria) in white-flies (Thao and Baumann, 2004), Cand. Blochmannia (y-Proteobacteria) in carpenter ants (Degnan et al., 2004), Wigglesworthia (y-Proteobacteria) in tsetse flies (Chen et al ., 1999), Cand. Nardonella (y-Proteobacteria) in palm weevils (Lefevre et al ., 2004), Cand. Tremblaya (P-Proteobacteria) in mealybugs (Baumann and Baumann, 2005; Downie and Gullan, 2005), Cand. Uzinura (Flavobacteria) in armored scale insects (Gruwell et al , 2007), and the coprimary symbionts Cand. Sulcia (Flavobacteria) and Cand. Baumannia (y-Proteobacteria) in leafhoppers and sharpshooters (Takiya et al , 2006) Cospeciation between endosymbi-onts and insect hosts might also be erroneously rejected because of particularities associated with host mitochondrial trees (Clark et al , 2000) Interestingly, most of the primary endosymbionts in the y-Proteobacteria might have evolved independently from each other (Herbeck et al , 2005; Novakova and Hypsa, 2007) Cospeciation of nutritional symbionts does not depend on intracellular or extracellular localization or on horizontal or vertical transmission Encapsulation of the gut symbionts in the stinkbug family Plataspidae (Heteroptera) makes horizontal transmission of an obligate nutritional gut bacterium, Cand. Ishikawaella capsulata (y-Proteobacteria), as restrictive as transovarial transmission for endosymbionts The gut symbiont exhibits AT-biased nucleotide composition, accelerated molecular evolution, and reduced genome size (Hosokawa et al , 2006) Without encapsulation, the triplex symbiosis of nutritional eukaryotic protists and their bacterial endosymbionts in the guts of termites show almost complete codivergence with the host termites (Noda et al ., 2007) . The purpose of this listing is not only to illustrate the diversity of associations but also to emphasize the paucity of nutritional endosymbionts in the a-Proteobacteria The only nutritional symbiont of an animal in the a-Proteobacteria is found in leeches belonging to the genus Placobdella (Glossiphoniidae, Rhynchobdellida) (Siddall et al ., 2004) . These leeches harbor their intracellular bacteria in mycetomal organs attached to the esophagus that form a pair of pear-shaped blind sacs or caeca lined with large mycetocytes The leeches hold vertebrate blood for digestion in these ceca The endo-symbionts, Cand Reichenowia, are closely related to the nitrogen-fixing, nodule-forming Rhizobiaceae of plants, which belong to the order Rhizobiales that is like the Rickettsiales in the a-Proteobacteria Transovarial transmission of Reichenowia is assumed Other leech families have differently styled mycetomes occupied with members of the y-Proteobacteria (Graf et al ., 2007) . Cand. Reichenowia and the psocid Rickettsia are so far the only two bacteria in the a-Proteobacteria that inhabit animal mycetomes by themselves .

Two other genera, Torix and Hemiclepsis, in the same leech family Glossiphoniidae that carries Reichnowia, are populated by bacteria that are members of the genus Rickettsia The leech Rickettsia are intracellular in various leech tissues such as epidermis, esophagus, and salivary glands (Kikuchi et al ., 2002) . The detection of the bacteria in leech eggs suggests near 100% vertical transmission for most species The leeches Torix tagoi, T. tukubana, Hemiclepsis marginata, and H. japonica exhibit a stable infection frequency of 96, 83, 29, and 0%, respectively (Kikuchi and Fukatsu, 2005) . In T. tagoi and T. tukubana, infected individuals were remarkably larger in size than uninfected individuals, whereas in H. marginata, infected and uninfected individuals were almost comparable in size Rickettsia in T. tuku-bana mark a transition in infection frequency coupled with a nutritional benefit, one step before becoming an obligate nutritional endosymbiont and one step before host provision for symbionts . The Rickettsia in leeches form a sister group to the crane fly and mycetomic Cerobasis Rickettsia .

Aphids have mycetomes and Buchnera aphidicola (Enterobacteriaceae, y-Proteobacteria) as their primary nutritional endosymbiont They also hold several facultative or secondary bacterial endosymbionts, one of which is a Rickettsia, better known as PAR-symbiont (pea aphid Rickettsia) (Chen et al , 1996) The Rickettsia have a negative effect on the host fitness This is interpreted as a probable artifact of laboratory rearing and might disappear under specific environmental conditions in the wild (Montllor et al ., 2002; Sakurai et al ., 2005; Simon et al ., 2007) . Equally possible is that this Rickettsia still has some patho-genicity associated with its ability to infect aphids as a new host However, the Rickettsia do not seem to be easily transinfected artificially to other aphid species (Tsuchida et al , 2006) Remarkably, the Rickettsia in the pea aphid Acyrthosiphon pisum (Aphididae, Hemip-tera) are not only found in the hemolymph but also in secondary mycetocytes and in the sheath cells of the primary mycetome (Sakurai et al ., 2005). The amount of the primary endosymbiont Buchnera was significantly suppressed in the presence of Rickettsia, particularly at the early adult stage when the aphid host actively reproduces and has the highest nutritional demand on its primary endosymbiont A completely opposite situation has been reported for biotypes in Israel of the whitefly Bemisia tabaci (Aleyrodidae, Hemiptera) . Buchnera serves as primary endosymbiont in whiteflies as well In B. tabaci the only stage at which Rickettsia can be seen associated with bacteriocytes is in very young embryos of eggs just having been laid; the Rickettsia seem to leave the bacteriocytes (Gottlieb et al , 2006) The authors now call this phenotype scattered A second phenotype, confined, has been described for other biotypes in Israel In confined, the Rickettsia are strictly localized within the bacteriocytes at all developmental stages (Gottlieb et al , 2008) The signal of the fluorescent probe is the strongest at the circumference of the bacteriocytes This is the first case where secondary Rickettsia have been detected inside the mycetocytes of the primary, obligatory endosymbiont The earlier case of Rickettsia compromising the replication of the primary endosymbiont in the pea aphid might have had Rickettsia in the primary myce-tomic cells after all Rickettsia were not the only secondary endosymbionts detected inside primary mycetocytes of the whitefly Hamiltonella, Arsenophonus (both Enterobacteriaceae), Cardinium (Bacteroidetes), and Wolbachia did share the same cells with Buchnera in addition to Rickettsia (Gottlieb et al , 2008) However, Hamiltonella and Arsenophonus as well as Car-dinium and Rickettsia seem to be mutually exclusive and Cardinium and Wolbachia seem to be rare in the same individual This Rickettsia of aphids and whiteflies are related to R. bellii and the mycetomic Liposcelis Rickettsia. The aphid and whitefly Rickettsia endosymbiosis might be the best example for the transition of a still pathogenic Rickettsia challenging a residing primary endosymbiont and being destined to become the first or second obligate nutritional Rickettsia .

The primary endosymbiont of tsetse flies, Wigglesworthia, has a long association with its host. Applying the dynamics of reductive genome evolution and sequence evolution to Wigglesworthia, both approaches suggest that the bacterium had already been a secondary endosymbiont when it colonized tsetse flies as a primary endosymbiont, so little is known yet about its early evolution toward nutritional symbiosis (Herbeck et al ., 2005; Khachane et al ., 2007) .

When an infectious secondary endosymbiont becomes a primary endosymbiont, it is like a life sentence without parole . The primary endosymbiont loses its ability to infect. It is locked up for life; its vagility is extremely compromised . It is doomed to slowly degenerate in its genome content and in its physiological capabilities . Final death occurs when it again is replaced by another secondary endosymbiont. The evolution to "real mutualism" is only an anthropocentric illusion. There are two options to escape this scenario of annihilation

Before degeneration progresses too far, the symbiont genes have to move to the nucleus and become functional in their new environment . The first part is rare in higher animals but no longer without precedence . A part of the Wolbachia genome moved to the X chromosome in the host nucleus in the adzuki bean beetle, Callosobruchus chinensis (Chrysome-lidae, Coleoptera) (Kondo et al ., 2002). The transferred genes might represent 30% of the genome of Wolbachia and have probably been derived from a single lateral transfer event (Nikoh et al ., 2008) . The genes are not transcriptional active in the nucleus . Around half of the transferred genes, 27 out of 57, have been structurally disrupted by a premature stop codon and pseudogenized, but 34 genes showed background levels of RNA in the nucleus possibly through promoter-less leaky transcription. Unequal crossing over between syn-apsing X chromosomes might have led to a duplication of some of the transferred genes The transfer into the beetle nucleus might have occurred between 0 74 and 2 5 million years ago . Fragments ranging in size between less than 500 bp and more than 1 Mbp have also been detected in the genomes of three Drosophila species, three parasitoid wasp species, one mosquito species, and two filarial nematode species (Dunning Hotopp et al ., 2007) In both nematode species, Wolbachia is already an obligate endosymbiont Regaining function of the prokaryotic genes in the eukaryotic nucleus is the next important step that led nutritional symbionts to become mitochondria, chloroplasts, and apicoplasts in parasites The first step to regaining function in the eukaryotic nucleus is being transcribed Wolbachia genes are being described also in the nuclei of the salivary glands of Anopheles species that do not carry any Wolbachia bacteria (Arca et al., 2005) . Anopheles mosquitoes transcribe Wolbachia genes in the nucleus in the absence of the bacterium in the cytoplasm; the beetles transcribe bacterial genes in the presence of the bacterium However, the transcribed Wolbachia genes have not (yet) been shown to exert any function

An alternative option to survive as a primary endosymbiont continuous degeneration of one's genome is the establishment of coprimary endosymbionts . This occurs in leafhop-pers and sharpshooters where a Flavobacteria and a y-Proteobacteria are both obligate as nutritional endosymbionts for their hosts (Takiya et al ., 2006) . The genome of the Flavobacteria Baumannia cicadellinicola still holds 651 genes, which is 252 genes more than Buchnera aphidicola (394 + 5) in the aphid Cinara cedri. The genome of the y-Proteobacteria Sulcia muel-leri codes for only 263 genes, which is still 50 genes more than Carsonella ruddii (253) in the hackberry petiole gall psyllid Pachypsylla venusta (Psyllidae, Hemiptera) (Nakabachi et al ., 2006; PĂ©rez-Brocal et al., 2006) . S. muelleri seem to provide the sharpshooters with several amino acids and the cofactor menaquinone; B. cicadellinicola contribute the amino acids methionine and histidine that S. muelleri cannot provide, and many cofactors and vitamins B. cicadellinicola has lost its genes for menaquinone; S. muelleri has lost all genes for cofactors and vitamins but menaquinone It becomes obvious that the two endosymbionts most likely complement each other in addition to the host The two bacteria might also reveal metabolic interdependence in the fatty acid, polyisoprenoid, and other biosynthetic pathways (McCutcheon and Moran, 2007)

Coprimary nutritional endosymbionts might turn out not to be the exemption The secondary endosymbiont of the aphid C. cedri, Serratia symbiotica, is certainly an obligate coprimary endosymbiont C. ruddii is reportedly the only symbiont in psyllids (Thao et al , 2000; Nakabachi et al ., 2006) . Do psyllids harbor an overlooked coprimary endosymbiont or has functional gene transfer to the host taken place? However, coprimary endosymbi-onts will only delay the need for either replacement by a new endosymbiont or functional transfer of the endosymbiont genes to the host nucleus These processes apply very well to animals The ancestor of mitochondria in animals has only lost genes to lead to a mito-chondrial genome that has practically the same size and number of genes for all extant animals from insects to man In plants, mitochondria have obtained genes through lateral gene transfer at a surprisingly high rate . Even in plastids, presumably defect genes become replaced by new bacterial homologues (Rice and Palmer, 2006)

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