Biology Reference
In-Depth Information
Buchnera
PASS
PABS
FIGURE 2.3
Bacteria detected in pea aphids
Acyrthosiphon pisum.
US clones (
n
= 16) from various sources.
[Redrawn from Sandstrm, J.P., Russell, J.A., White, J.P., and Moran, N.A. (2001).
Mol. Ecol.
10:
217Ï228:
Table 2.]
parasitoid whose ovipositor may become contaminated with accessory bacteria at a previous
oviposition event. Since the successful transmission of accessory bacteria requires the parasitoid
infection to abort (essentially all successfully parasitized aphids produce no offspring), accessory
bacteria may be predicted to accumulate in aphids with physiological resistance to parasitoids. This
expectation, however, remains to be tested.
An alternative transmission route for accessory bacteria is oral. Any accessory bacterial cells
released into the environment via aphid secretions (honeydew, cornicle secretions, saliva) are
potentially available for ingestion by other aphids. However, the incidence of accessory bacteria
in these aphid secretions, bacterial persistence in the free-living condition, and the capacity of the
bacteria to survive in aphid guts after ingestion all remain unknown.
The expected evolutionary consequences of mixed vertical/horizontal transmission are that
the phylogenies of accessory bacteria and aphids will not be congruent and that their genomes
will not be subject to decay. Consistent with the Ýrst expectation, the 16S rRNA sequence of
PABS (= T type) has >98% similarity to that of bacteria described in the whiteÞy
Bemisia tabaci
(Darby et al., 2001; Sandstrm et al., 2001). Using published substitution rates for 16S rRNA
genes of bacteria, PABS and the bacteria in
B. tabaci
are estimated to have diverged 17 to 34
million years ago, considerably more recently than the likely common ancestor of aphids and
whiteÞy (Darby et al., 2001), indicating that these two bacteria have not evolved from a common
bacterial ancestor in the common ancestor of aphids and whiteÞy. It is unknown whether the
bacteria have switched directly between these two insect families or whether their common
ancestor was a free-living form.
MUTUAL DEPENDENCE
This section considers whether bacteria and aphids are dependent on their association and the
underlying causes of the observed level of dependence.
Buchnera
and accessory bacteria will be
considered Ýrst and then the aphid.
B
ACTERIA
Buchnera
cells are absolutely dependent on their intracellular habitat in aphids. As considered in the
section entitled Modes of Transmission of Bacteria, their genome is very small, close to the size of the
