Biology Reference
In-Depth Information
One might expect the number to vary between species with the size and level of nutrient provisioning
of the egg (Krakauer and Mira, 1999), but this has not been investigated, and informal estimates
of N
range from 50 to 500
Buchnera
cells (Douglas, 1998). These values can be translated to the
e
proportion of
Buchnera
cells in the motherÔs mycetocytes available for transmission. Let us consider
the pea aphid
Acyrthosiphon pisum
, which on reaching adulthood bears 100 mycetocytes with a
total volume of 0.03 mm
3
and has 100 offspring (Wilkinson and Douglas, 1998).
Buchnera
cells
have an average diameter of 2.5
m and occupy 60% of the mycetocyte volume (Whitehead and
Douglas, 1993a), and there are therefore 2.2
n
cells available for transmission. The
50 to 500 cells transmitted per offspring account for 2 to 22% of the
10
Buchnera
6
cells available,
leaving 78 to 98% of the cells to die with the mother. (This calculation is a simpliÝcation because
transmission is initiated in larvae, but it illustrates the point.) Do
Buchnera
cells compete for access
to offspring? The highly ordered process of transmission suggests not;
Buchnera
cells do not
proliferate outside of mycetocytes around the ovaries or invade embryos/eggs beyond a very
restricted developmental stage. The very small
Buchnera
N
may provide the explanation for the altruism of
e
the majority. If few
cells are transmitted and all derived from one mycetocyte (i.e., within
a few cell divisions of a common ancestor), then most of their progeny in the offspring aphid will
be clonemates, closely related to each other by descent. These clonemates are predicted to cooperate,
just as the cells in the aphid body cooperate and do not compete to be a reproductive cell (Maynard
Smith and Szathmary, 1995).
Depressed conÞict among
Buchnera
Buchnera
cells is not the only predicted evolutionary consequence
of the low
N
. The second consequence is genomic deterioration of
Buchnera
. If the complement
e
of
acquired by an aphid from its mother uniformly bears a slightly deleterious mutation,
then there is no way to replace these slightly inferior
Buchnera
cells by superior forms. The resultant
gradual genetic decay in the vertically transmitted cells is expected to be exacerbated if genes
mediating DNA repair and recombination accumulate mutations, reducing the capacity to correct
replication errors and purge mutations by sexual reproduction. There is now overwhelming evidence
that genomic decay has been a major factor in the evolution of
Buchnera
genomes (Moran, 1996;
Rispe and Moran, 2000). Protein-coding genes have a much higher incidence of nonsynonymous
substitutions (i.e., ones that alter the amino acid in the protein product) (Moran, 1996), and the
substitutions in the 16S rRNA gene tend to destabilize the secondary structure of the rRNA molecule
(Lambert and Moran, 1998)
(Figure 2.2).
As Figure 2.2B illustrates, these effects are not unique
to
Buchnera
but are common to microorganisms that are persistently transmitted vertically (Moran
and Wernegreen, 2000).
One would expect pseudogenes to accumulate in the
Buchnera
Buchnera
genome, as has been described,
for example, in
Mycobacterium leprae
(Cole et al., 2001). Certainly, pseudogenes have been
described in certain
genes (Lai et al., 1996; Baumann et al., 1997; van Ham et al., 1999),
but the strong deletional bias in bacterial genomes presumably results in the total elimination of
most genes that have become nonfunctional (Lawrence et al., 2001). The predicted consequence is
genome reduction; only those genes whose function is under strong selection pressure are retained.
Consistent with this prediction, the genome size of
Buchnera
is small at 0.45 to 0.64 megabases
(Mb) (Charles and Ishikawa, 1999; Wernegreen et al., 2000; Gil et al., 2002). Many genes that
would be essential for independent existence, including almost all genes for phospholipid synthesis
and the TCA cycle, are absent, and many genes for DNA repair, including
Buchnera
recA
and
uvrABC
, are
missing (Shigenobu et al., 2000).
A
CCESSORY
B
ACTERIA
The vertical transmission of accessory bacteria has been demonstrated for multiple aphid species and
bacterial taxa by both microscopic and molecular methods (e.g., Darby et al., 2001; Fukatsu et al., 2000
and 2001; Sandstrm et al., 2001). Buchner (1966, p. 310) comments that the coccoid primary symbionts
(
Buchnera
) and
rod-shaped accessory bacteria transmitted to the winter eggs of
Drepanosiphum
Ñappear
