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A RE S WITCHES BETWEEN P ARASITISM AND M UTUALISM P OSSIBLE ?
The assumption that entry into the intracellular environment is based on the same or related genetic
adaptations has produced some speculation about whether a switch between these two lifestyles is
possible (Moran and Wernegreen, 2000; Goebel and Gross, 2001). Thus, it has been suggested that
invertebrates and their endosymbionts act as a ÑplaygroundÒ for the evolution of virulence factors
in bacteria, which are later used to invade mammals (Goebel and Gross, 2001). This line of thinking
is supported by a substantial overlap in taxonomic designation (b-, c-, or h-proteobacteria) of both
intracellular mutualists found in invertebrates and many pathogenic bacteria in mammals.
For example, several associations involving members of Enterobacteriaceae in tsetse Þies,
psyllids, and ants have been reported (Schrder et al., 1996; Chen et al., 1999; Spaulding and von
Dohlen, 2001). In addition, in some cases the presence of common virulence factors has also been
observed: the maternally transmitted secondary symbiont of tsetse Þies Sodalis glossinidius requires
a type III secretion system to invade the host. The same type of secretion system has been identiÝed
in pathogenic Enterobacteriaceae, including S. typhimurium (Dale et al., 2001). Another common
feature is tissue tropism, which is found in many pathogenic bacteria. Examples among bacterial
mutualists are bacteriomes in insect species and the root nodules of leguminous plants or the crown
gall tumors (Hentschel et al., 2000). Finally, the identiÝcation of a Ñsymbiosis islandÒ in Mesorhizo-
bium loti much resembling in its nature Ñpathogenicity islandsÒ commonly found in pathogenic
bacteria has been described (Sullivan et al., 2002).
However, in stable and well-established associations based on mutualism, a switch toward
pathogenicity is not likely to occur. This is because the amount of genetic change required to reach
this point would probably deter such a redirection. The highly reduced genomes of obligate host-
associated bacteria mean that they have in a sense been Ñpainted into a (metabolic) cornerÒ (Tamas
et al., 2001). Thus, once amino acid biosynthetic genes have been lost, such as in Rickettsia , it
would be impossible for these bacteria to develop a symbiotic relationship based on the supply of
amino acids to a presumptive host. Another major limitation is the impaired ability to import foreign
DNA (Tamas et al., 2002), which could prevent the acquisition of novel virulence factors required
for a pathogenic lifestyle.
A more likely scenario is that intracellular mutualists may in some species be Ñattenuated
pathogensÒ while retaining their pathogenic status in other species (Corsaro et al., 1999). For
example, Bartonella henselae causes no observable disease to the cat that represents its animal
reservoir, while it causes a variety of diseases in humans, depending on the activity of the immune
system. Numerous such cases have been described for a variety of bacterial and viral systems,
including HIV, which is pathogenic for humans, whereas its close relative SIV causes no observable
harm to its animal host. Thus, although adaptations to unusually stable intracellular environments
impose drastic, largely irreversible limits on the direction for further evolution, the borderline
between mutualists, parasites, and commensals may not always be distinct and well deÝned.
CONCLUDING REMARKS
The majority of recently published papers in the Ýeld of bacterial genomics suggest that bacterial
genomes are highly dynamic structures. Large intragenomic alterations, such as chromosomal
rearrangements and insertions/deletions, occur frequently, probably mediated by insertion
sequences, prophages, and repetitive sequences. As a consequence, closely related species may
differ drastically in gene content and gene order. Even very closely related strains of the same
species have been demonstrated to differ by up to 20% in genome size (Boucher at al., 2001).
Therefore, the most provocative Ýnding of the genomic work on aphid endosymbionts is that the
Buchnera genomes are so remarkably conserved in both genome content and gene order. There are
no signs of inversions, translocations, duplications, or horizontal gene transfer (Tamas et al., 2002).
 
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