Biology Reference
In-Depth Information
This may be partially explained by the loss of repeated sequences and
recA
, which is likely to
have had a profound effect on DNA-repair and recombination abilities. The presence of more than
100 genomic copies per cell may represent a compensatory phenomenon directly related to defective
DNA repair. In contrast to the extreme stability in structure, nucleotide substitutions occur at high
rates, and gene loss has been estimated to be about one eliminated gene per 5 to 10 million years
(Tamas et al., 2002).
The observation that the
BuchneraÏ
host association is stable indicates that these endosymbionts
are at an advanced, later stage of the internalization process. Most of the large-scale modiÝcations,
primarily deletions, already happened far back in evolutionary time. Although
Buchnera
are no
longer a source of ecological novelties for their host (Tamas et al., 2002), the converse might still
be true. Changes in the feeding behavior of aphids could induce a genetic change in the
Buchnera
genome, as seen in the case of
cys
genes. These genomes are therefore excellent model systems
for tracing individual genetic changes that can be directly correlated to phenotypes of a particular
host or have occurred as a response to a changing environment.
An evolutionary reconstruction of the events leading to the colonization of intracellular growth
habitats that is applicable to both mutualists and parasites is as follows. The ancestral free-living
bacterium Ýrst learned how to enter a host cell, then how to multiply within it, and Ýnally how to
exit again, maybe via the acquisition of virulence genes on pathogenicity islands. The initial
association may have been either parasitic or mutualistic or it may have had no effect on the selected
host cells. During the early stages of integration, the bacteria were most likely sporadic cell-surface-
associated organisms that mostly reproduced in a free-living manner.
At a later stage, the bacteria invaded the intracellular environment regularly while still main-
taining their free-living growth capabilities; they developed a facultative intracellular lifestyle. For
example,
F. tularensis
is an intracellular parasite of both macrophages and parenchymal cells,
although it can be still cultivated
in vitro
on complex, cysteine-supplemented media (Conlan and
North, 1992; Maurin et al., 2000). However, most facultative intracellular bacteria grew substantially
better within their new host-associated habitat than in a free-living mode. As these bacteria gradually
deepened their relationship with the eukaryotic host cell, the obligatory dimension of the association
started to dominate in such a way that it became increasingly difÝcult for the bacterium to grow
outside of its host. Finally, the process of reductive evolution, Ýrst based on rapid, large-scale
deletions mediated by repeated sequences and later on small intragenic deletion mutations, pushed
the organism to the Ñpoint of no return.Ò Genome reduction eventually became so extensive that a
free-living lifestyle was no longer an option.
Thus, the cost of colonizing a host-associated environment is extreme sequence loss; the genome
is in all cases studied so far reduced to a fraction of what it was at the start of the internalization
process. The reward is a minimal genome stripped of everything not essential for survival Ð and
access to a protective nutrient-rich environment. At a certain stage of this mingling process it may
no longer be meaningful to speak about an insect host and a bacterial guest; the two have merged
to become a new, single organism.
REFERENCES
Andersson, J.O. and Andersson, S.G.E. (1999a). Genome degradation is an ongoing process in
Rickettsia
.
Mol. Biol. Evol.
16:
1178Ï1191.
Andersson, J.O. and Andersson, S.G.E. (1999b). Insights into the evolutionary process of genome degradation.
Curr. Opinions Genet. Dev.
9:
664Ï671.
Andersson, J.O. and Andersson, S.G.E. (2001). Pseudogenes, junk DNA and the dynamics of
Rickettsia
genomes.
Mol. Biol. Evol
.
18:
829Ï839.
Andersson, S.G.E. and Kurland, C.G. (1998). Reductive evolution of resident genomes.
Trends Microbiol.
6:
263Ï278.
