Biology Reference
In-Depth Information
FIGURE 2.1
bacteria and corresponding aphid hosts, as determined from 16S rRNA
sequence and morphology, respectively. [From Moran, N.A. and Baumann, P. (1994).
Phylogeny of
Buchnera
Trends Ecol. Evol.
9:
15Ï20:
Fig. 1. With permission.]
conÝrmed using sequences from an array of
and aphid genes at phylogenetic scales
ranging from the family Aphididae to intraspecies variation (e.g., Clark et al., 2000; Funk et al.,
2000 and 2001).
The processes underpinning the obligate vertical transmission and congruent phylogenies have
been described at the electron microscopic level, especially by Hinde (1971b) and Brough and
Dixon (1990). Mycetocytes become associated with the germarium of the aphid ovaries. As the
parthenogenetic embryos in viviparous aphids develop to the blastoderm stage and eggs in oviparous
aphids initiate vitellogenesis,
Buchnera
cells are expelled from mycetocytes by exocytosis, pass
between follicle cells of the ovaries, and become incorporated into the embryo or unfertilized egg.
Thereafter, they become incorporated into the mycetocytes as these cells differentiate.
An added level of complexity is generated by the telescoping of generations in parthenogenetic
aphids. The capacity of aphids to initiate larviposition within a few hours of reaching adulthood
derives from their initiation of embryogenesis as larvae or even as embryos. Because transmission
of
Buchnera
occurs early in embryo development, an adult aphid may contain multiple symbioses
at different developmental stages: one in her hemocoel and one in each of her daughter embryos
and some of her granddaughter embryos.
At present, essentially no information is available on the cellular and molecular interactions
underpinning vertical transmission of
Buchnera
Buchnera
. For example, what is the nature of the signal
exchange triggering exocytosis of
Buchnera
from mycetocytes and their passage to embryos and
eggs? Do
cells participate in this intercell communication, and is there any Ñquality
controlÒ by which the condition of the transferred
Buchnera
Buchnera
is tested? In connection with this, are
the transmitted
cells, or do they display
modiÝed surface features or metabolic traits linked to their brief extracellular status? With the
increasing availability of speciÝc probes to interrogate gene expression, signaling events, and
metabolites in single cells, these questions can, in principle, be answered.
A further point of ignorance is the number of
Buchnera
cells functionally equivalent to other
Buchnera
Buchnera
cells acquired by each embryo/egg
from its mother, which is equivalent to the effective size (
N
) of the
Buchnera
population in aphids.
e
