Biology Reference
In-Depth Information
A
0.5
0.4
0.3
0.2
0.1
0
trpa trpb trpc(f) trpd trpe
B
Proteus vulgaris
Escherichia coli
100
Aphid endosymbiont (Sg)
Aphid endosymbiont (Mp)
90
Aphid endosymbiont (Mk)
Aphid endosymbiont (Sc)
Ant endosymbiont
80
Tsetse fly endosymbiont
Oceanospirillum linum
Whitefly endosymbiont
70
Alcaligenes eutrophus
Mealybug endosymbiont
60
Agrobacterium tumefaciens
Wolbachia pipientis
0.0 0.1 0.2 0.3
Distance from E. coli
FIGURE 2.2
The consequences of vertical transmission for the molecular evolution of
Buchnera
. (A) Nonsyn-
onymous substitutions in Ýve protein-coding
trp
genes of
Buchnera
. The number of substitutions between
Buchnera
in the aphids
Schizaphis graminum
and
Schlechtendalia chinensis
(open bars) and between the two
enteric bacteria
E. coli
and
Salmonella typhimurium
(closed bars) is expressed as the ratio of nonsynonymous:syn-
onymous substitutions per nucleotide site. [From Hurst and McVean (1996).
Nature
381:
650Ï651: Fig. 1].
(B) The stability of Domain I of the 16S rRNA in symbiotic bacteria, including
Buchnera
,
and free-living bacteria,
showing the relationships of bacteria and stabilities (ÏEG) summed over Domain I for each organism. [From
Lambert, J.D. and Moran, N.A. (1998).
Proc. Natl. Acad. Sci. U.S.A.
95:
4458Ï4462: Fig. 2.]
at approximately the same time at the follice zone [of the ovaries] and are transmitted together to the
oocyte through the gaps,Ò but modern studies are generally lacking. In one recent study on
Aphis fabae
,
the abundance of accessory bacteria relative to
Buchnera
was lower in embryos than in the maternal
tissues (Wilkinson et al., 2001a), suggesting that either very few accessory bacteria are transmitted or
that embryos provide a poor environment for their proliferation.
The dynamics of accessory bacterial populations are, however, fundamentally different from
Buchnera
because the accessory bacteria are not universally present in aphids. For example,
the pea aphid,
A. pisum
, may bear PASS, PABS, PAR, and U-type, but their incidence varies
between different clones (Sandstrm et al., 2001) (
Figure 2.3)
.
These data suggest strongly that
accessory bacteria can be lost through failure of vertical transmission and gained horizontally
by individual aphids. The frequencies of such gains and losses are unknown and may vary with
environmental conditions.
Horizontal transmission can be demonstrated experimentally in the laboratory by the injection
of isolated accessory bacteria from a ÑdonorÒ aphid to a ÑrecipientÒ that lacks the accessory bacteria;
the injected accessory bacteria are stably inherited through multiple generations (e.g., Chen et al.,
2000; Fukatsu et al., 2001). In natural populations, an equivalent process may be mediated by a
