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gene regulation. This extreme genome reduction raises questions about the rate and process of genome
deterioration in aphid endosymbionts. The acquisition of complete genome-sequence data from aphid
endosymbionts now provides an opportunity to study these processes at the whole-genome level.
DIVERGENCE DATES FOR
BUCHNERA
The
Aphidae association extends back to the origin of Aphidoidea approximately 150
to 200 million years ago (Moran et al., 1993), and the origin of the aphidiform ancestor can be
traced back to Jurassic or earlier, about 250 million years ago (Heie, 1987). Aphids are a morpho-
logically deÝned group of insects with a rich fossil record, making it possible to assign divergence
dates for the separation of the different species. Since the aphid endosymbionts have been vertically
transmitted, with no evidence of horizontal transmission (Simon et al., 1996), it is in principle
possible to infer divergence dates for the aphid endosymbionts based on dates estimated for their
aphid hosts. Indeed, a comparison of phylogenies based on genes from
BuchneraÏ
and their aphid
hosts conÝrms that the tree topologies are identical, indicative of synchronous divergences (Moran
et al., 1993; Moran and Baumann, 1994). These Ýndings support an exclusively vertical transmission
of
Buchnera
via maternal inheritance.
Other cases of bacteriocyte-associated bacteria showing a phylogenetic congruence with hosts
have also been identiÝed in tsetse Þies, carpenter ants, and members of Blattaria (Bandi et al.,
1995; Chen et al., 1999; Sauer et al., 2000). A well-studied example of an association between
arthropods and bacteria is
Buchnera
spp. that are maintained in a wide range of hosts (Breeuwer
et al., 1992; Vavre et al., 1999). However, unlike
Wolbachia
Buchnera
, the obtained phylogenies for
Wolbachia
and its host are not congruent. This implies a loss of the ancestral bacterium in one or more lineages,
followed by multiple subsequent infections (OÔNeill et al., 1992).
COMPARATIVE GENOMICS OF
BUCHNERA
(Ap) are thought to have
diverged about 50 to 70 million years ago. Thanks to the availability of genome-sequence data for
The aphid
Schizaphis graminum
(Sg) and its relative
Acyrtosyphon pisum
Buchnera aphidicola
(Sg) (Tamas et al., 2002), it is
now possible to quantify the changes introduced during the past 50 million years. The two genomes
are of approximately the same size Ð 641,454 bp for
(Ap) (Shigenobu et al., 2000) and
Buchnera
Buchnera
(Sg) and 640,681 bp for
Buchnera
(Ap) (Table 3.1). The similarity of genome sizes is in accordance with highly conserved genome
sizes among aphid endosymbionts, showing less than a 5% difference even among lineages sepa-
rated by 100 to 200 million years of independent evolution (Wernegreen et al., 2000).
There are 545 identiÝed genes in
(Ap)
(Tamas et al., 2002). A total of 526 orthologous genes were identiÝed, while pseudogenes and
Buchnera
(Sg), compared to 564 genes in
Buchnera
TABLE 3.1
Comparison of Genome Features for
B. aphidicola
(Sg)
and
B. aphidicola
(Ap)
B. aphidicola
B. aphidicola
Feature
(Sg)
(Ap)
Genome size (bp)
641,454
640,681
Genic G + C content (%)
26.2
26.3
Intergenic G + C content (%)
14.8
16.1
Protein coding genes (no.)
545
564
Pseudogenes (no.)
38
13
Orthologous genes (no.)
526
526
