Rickettsia as obligate and mycetomic endosymbionts

In the booklouse Liposcelis bostrychophila (Liposcelidae, Psocoptera), Rickettsia-like organisms were first observed by electron microscopy in the ovaries, oocytes, abdominal sub-epidermal tissues, esophageal epithelial cells, and subesophageal ganglion cells (Yusuf et al , 2000; Yusuf and Turner, 2004; Chapman, 2005) Yusuf and Turner performed the first molecular characterization of the intracellular bacteria (Yusuf and Turner, 2004) These bacteria have now been confirmed to be Rickettsia by sequencing and fluorescent in situ hybridization (Perotti et al , 2006) No pathology was evident in infected somatic tissues by electron microscopy No bacteria have been seen in the salivary glands as might be expected from Rickettsia that are obligate for oogenesis . The Rickettsia in the partheno-genetic psocids Cerobasis guestfalica (Trogiidae, Psocoptera) and L. bostrychophila occupy single cell mycetocytes (bacteriocytes) and mycetomes (bacteriomes, bacteriotomes). The first association with dedicated host-provided structures becomes evident in first instar nymphs . Two large mycetocytes start to migrate into the fat body, one on each side of the body. These single cell mycetocytes are heavily infected with Rickettsia . During the final instar and teneral stage, infected cells aggregate to build a paired mycetome in between future ovaries and midgut (Figure 10.1). Each mycetome is put together by 4-8 big and 8-12 small mycetocytes, which become surrounded by a dense, uniform anucleate layer. A few cell-free Rickettsia were also observed in the hemolymph close to the mycetomes . The behavior of Rickettsia during early development in psocids resembles that of the yet unidentified primary endosymbionts of cattle and pig lice, Haematopinus eurysternus and H. suis (Haematopinidae, Phthiraptera)

A characteristic that Rickettsia share with endosymbionts of ciliates and termites is their presence and replication in host nuclei Rickettsia can replicate in the nuclei of invertebrate and vertebrate host cells although the majority of bacterial replication occurs in the cytoplasm Nuclear presence is much more common for psocid Rickettsia and R. felis than for other Rickettsia and Orientia

Mycetomes have so far only been described for nutritional symbioses . Does this suggest a nutritional role for Rickettsia? A nutritional role would fit very well for L. bostrychophila as an ancestor to the Phthiraptera, most if not all of which have mycetomes . One might then expect to find Rickettsia in biting and sucking lice . Primary endosymbionts of lice all belong so far to the Enterobacteriaceae and Legionellaceae of the y-Proteobacteria (Burkhart and Burkhart, 2006; Sasaki-Fukatsu et al , 2006; Allen et al , 2007; Fukatsu et al , 2007; Hypsa and Krizek, 2007; Perotti et al , 2007, 2008) No other indications support a nutritional angle at the moment Mycetomes have not (yet) been described for other psocid species Nutri-

Pectoralis Muscle Tear Mri Axial

Figure 10.1 (Color figure follows p. 238.) Rickettsia in a paired mycetome at both sides of the body between ovaries and midgut in a teneral of the booklouse L. bostrychophila (Psocoptera) Ventral view of a horizontal section Extracellular Rickettsia are visible in the hemolymph as well as Rickett-sia-carrying mycetocytes in other tissues The mycetome on the left side of the animal is duplicated Confocal microscopy picture with a Rickettsia-specific probe (yellow channel) . Bar 100 pm. (Modified from Perotti, M A , Clarke, H K , Turner, B D , and Braig, H R [2006] Rickettsia as obligate and mycetomic bacteria . FASEB J. 20: 2372-2374 and E1646-E1656 . With permission from FASEB.)

Figure 10.1 (Color figure follows p. 238.) Rickettsia in a paired mycetome at both sides of the body between ovaries and midgut in a teneral of the booklouse L. bostrychophila (Psocoptera) Ventral view of a horizontal section Extracellular Rickettsia are visible in the hemolymph as well as Rickett-sia-carrying mycetocytes in other tissues The mycetome on the left side of the animal is duplicated Confocal microscopy picture with a Rickettsia-specific probe (yellow channel) . Bar 100 pm. (Modified from Perotti, M A , Clarke, H K , Turner, B D , and Braig, H R [2006] Rickettsia as obligate and mycetomic bacteria . FASEB J. 20: 2372-2374 and E1646-E1656 . With permission from FASEB.)

tional symbioses are not known from isolated species If nutritional symbiosis occurs, it is spread over an entire family or even an order So why do these two psocid species have mycetomes? No other bacteria were detected in the mycetomes . Assuming that the two Rickettsia of psocids are not nutritional, are we looking at the first cases were hosts lost their nutritional symbionts through a change in diet and the Rickettsia took advantage of the left over structures? If not, these Rickettsia have to be considered as organ-forming Rickettsia for the time being

0 0

Post a comment