But many modern endosymbionts break
those same rules, McCutcheon noted at the
meeting, drawing on his studies of cicadas.
These sap-sucking insects derive amino acids missing from their diet from bacterial
partners that reside in specialized cells. The
cicada Diceroprocta semicincta, for example, harbors two such partners. One, called
Hodgkinia, supplies the insect with two
amino acids it cannot provide on its own,
while the other, Sulcia, provides another
eight. In another cicada species, however,
Hodgkinia has doubled the amount of its
DNA, diverged into two distinct genomes,
and divided up the task of supplying the
two amino acids, McCutcheon and his colleagues reported at the meeting and online
on 28 August in Cell. One
Hodgkinia provides some
of the genes and the other
fills in the gaps in amino
acid production, making
both “species” essential to
has more recently looked
at the endosymbionts
of a Magicicada cicada,
which emerges on a 13- or
17-year cycle and may harbor scores of distinct Hodgkinia genomes, some that seem
to carry very few or no functional genes.
The finding parallels what happened to
the plant mitochondrial genome in Silene,
McCutcheon suggests. It seems that in both
these cases, unchecked mutation rates and
DNA amplification led to greatly expanded
but marginally functional, fragmented genomes. “When things go wonky, they really
go wonky,” Smith says.
The cicada’s unusual lifestyle—long dor-
mancy, followed by a brief burst of activity—
may play a role in this “genomic insanity,”
McCutcheon proposes. The species with a
single Hogkinia genome can take 3 years to
fully develop, and the one with many Hodg-
kinia genomes can take 17. Although cicada
nymphs are basically dormant during most
of that long cycle, the endosymbionts might
be free to replicate with no survival pres-
sures acting to keep their genomes stable.
“Some cicada life histories seem to allow
slop and chance to take over, or maybe just
slop,” McCutcheon told the audience.
Investigators are also examining modern
analogs to the complex evolution of the
chloroplast, which seems to have emerged
once, but then was lost and regained in
different ways in various modern photo-
synthetic organisms. The photosynthetic
organelle was originally a cyanobacterium
that was engulfed by a eukaryote. Plant
biologists have long recognized that in
some branches of the plant and algal fam-
ily tree, this initial chloroplast was lost, but
a new one was acquired when a host cell
swallowed up an alga that in turn had its
At the meeting, Patrick Keeling, a pro-tistologist at the University of British Columbia, Vancouver, in Canada, described
his studies of a more recent example of
this process, called tertiary endosymbiosis.
Some dinoflagellates, single-cell aquatic
protists, no longer have their original chloroplast, relegating some of its light-sensing
apparatus to a cellular
component dubbed an
eyespot. But they have
replaced it by taking in
a diatom, a single-celled
alga that has its own photosynthetic machinery.
The dinoflagellate still
carries the diatom’s nucleus and mitochondria,
but “I would challenge
anyone to say this is not
an organelle,” Keeling says.
The meeting highlighted other similarities between endosymbionts and organelles.
Mitochondria are typically passed only from
the mother to offspring, and some endosymbionts similarly depend on maternal transmission, dwelling in eggs and perhaps even
promoting female progeny over male to perpetuate themselves, says Steve Perlman of
the University of Victoria in Canada.
All these results “make organelles not so
special,” says W. Ford Doolittle, a molecular
evolutionary biologist emeritus at Dalhousie University in Halifax, Canada. Embracing endosymbionts as good models for the
evolution of organelles makes for “an interesting paradigm shift in the field.” ■
Inside a specialized cicada organ, one endosymbiotic
microbe has split into two species (yellow and blue).
They are surrounded by a third (green). Insect nuclei
“The line separating
organelle is very
John McCutcheon, University
of Montana, Missoula
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THE HUMAN PROTEIN ATLAS