Biology Reference
In-Depth Information
Morphological differences among prokaryotic symbionts in mycetocytes form different insect
taxa, and differences in location of their mycetocytes suggest that these symbionts are probably
not monophyletic. Indeed, comparative sequence analysis of 16S rDNA has revealed that myc-
etocyte bacteria appear to be specialized with respect to their hosts (Baumann et al., 1993). For
example, among sternorrhynchous homopterans, aphids, whiteÞies, and mealybugs, each harbors
its own lineage of mycetocyte bacteria, suggesting either that symbiosis took place multiple
times in these insect lineages or that symbionts were replaced in certain lineages. Mycetocyte
symbionts from aphids and whiteÞies are members of the
h
-subdivision of the proteobacteria,
whereas those from mealybugs are afÝliated with the
-subdivision (Munson et al., 1992; Clark
et al., 1992). As for nonhomopteran insects, tsetse Þies (Aksoy et al., 1995), carpenter ants
(Schroeder et al., 1996), and weevils (Campbell et al., 1992) also share members of the
c
-
subdivision of the proteobacteria with the mycetocyte symbionts, and cockroaches (Bandi et al.,
1994) and termites (Bandi et al., 1995) speciÝcally harbor bacteria of the Flavobacterium-
Bacteroides group in the mycetocyte.
That some lineages of homopteran insects acquired symbionts independently of one another is
best illustrated by the fact that a few groups contain fungi instead of bacteria as mycetocyte
symbionts. A well-studied example is planthoppers, which harbor yeast-like symbionts in the
mycetocyte (Noda, 1974). Molecular studies have placed the symbionts in the class Pyrenomycetes
in the subphylum Ascomycotina (Noda et al., 1995).
h
B
UCHNERA
Of the mycetocyte symbioses, the best-studied example is that of aphids. To date, the 16S rDNA
sequences of mycetocyte symbionts of numerous aphid species have been obtained and analyzed
phylogenetically together with sequences of other representative prokaryotes. Results indicate that
all aphid primary symbionts, the inhabitants of the large mycetocytes, belong to a single, well-
supported clade within the
h
-3 subdivision of the proteobacteria and have
Escherichia coli
and
related bacteria as their closest relatives (Moran et al., 1993). The formal designation,
Buchnera
aphidicola
, was claimed to apply to this symbiont clade (Munson et al., 1991), whereas the species
name,
, has for the most part fallen into disuse. This is because these symbionts are too
diversiÝed to be regarded as a single species.
It turned out that the sequence-based phylogeny of
aphidicola
was completely concordant with
the morphology-based phylogeny of the corresponding aphid hosts. Since the probability that such
a concordance would occur by chance is inÝnitesimally small, this Ýnding implies a single original
infection in the common ancestor of aphid species, followed by cospeciation of aphids and
Buchnera
Buch-
nera
. The distribution of modern symbionts among their host aphids thus reÞects parallel clado-
genesis through consistent and long-term transmission from mother to daughter. Horizontal transfer
of these symbionts apparently did not take place or was very rare (Moran et al., 1993; Moran and
Baumann, 1994). The aphid fossil record implies that the symbiotic association between aphid and
Buchnera
is ancient. Several aphid lineages date from an 80-million-year-old amber deposit,
implying that this is the minimum age of the most recent common ancestor of this set of aphids
as well as of the original infection with
Buchnera
(Heie, 1987). Comparative sequence analyses
of 16S rDNA of
from a dozen aphid species suggest that the original infection dates to
200 to 250 million years ago (Moran and Baumann, 1994).
The radiation of homopteran insects into the sap-feeding niches provided by vascular plants
was probably dependent on the early acquisition of bacterial mutualists. This possibility is supported
by the fact that most plant saps lack nutrients essential for insects and that almost all modern
Homopterae harbor symbionts (Buchner, 1965; Douglas, 1989). These considerations suggest that
the infection that led to modern
Buchnera
might have occurred in the ancestor common to aphids
and related insects. However, sequence analyses have indicated that symbionts have been acquired
more than once within the sternorrhynchous Homopterae.
Buchnera