trans

286 PLASTIDS, MITOCHONDRIA, AND HYDROGENOSOMES
a more efficient mechanism and perhaps a
means to detoxify oxygen, thereby providing a
selective advantage to the partnership. Could
other drivers have been responsible for the
association producing the mitochondrion?
A new hypothesis, namely the hydrogen
hypothesis, adopts this rationale. The hydrogen
hypothesis suggests that the endosymbiont
wasn’t adopted for its ability to perform
oxidative phosphorylation but for its ability
(under anaerobic conditions) to generate
hydrogen. Like the classical formulations of
the endosymbiont hypothesis, the hydrogen
hypothesis invokes a mutual benefit for
the two partners and postulates an alphaproteobacterial
endosymbiont as the partner
producing the mitochondrion. Where the
hydrogen hypothesis departs from the paradigm
is in its proposed host. Rather than a
nucleated, amitochondriate phagotroph as
a host (posited by the classical theory), the
hydrogen hypothesis invokes a hydrogenconsuming,
methane-evolving archaebacterium
as the host. Referred to as a syntrophic
(shared feeding) relationship, the hydrogen
hypothesis has the endosymbiont feeding the
archaebacterium with its wastes (hydrogen
and CO 2 ). Initially the endosymbiont would
have been outside the host (perhaps attached)
and would have scavenged reduced carbon
compounds from the environment. With the
ongoing establishment of the interaction, the
endosymbiont would have gradually become
enclosed within the host, which would eventually
have to assume responsibility for providing
the endosymbiont with reduced carbon
compounds for conversion into hydrogen and
CO 2 that the host could use. The hydrogen
hypothesis then goes on to postulate that the
endosymbiont (now a protomitochondrion)
was modified to activate its oxidative phosphorylation
and perform aerobic respiration
using the supplied carbon substrate (pyruvate).
The consortium is thus envisaged to have
started out in an anaerobic environment but
to have switched to an aerobic metabolism,
perhaps as the concentration of O 2 in the
atmosphere increased.
Hydrogenosomes of parabasalids would thus
represent one branch in this diversification
where anaerobic metabolism was maintained.
Conversely, most eukaryotes utilized the
endosymbiont for aerobic respiration. The fact
that parabasalids emerge early in the eukaryotic
divergence is congruent with this scenario.
A major objection to the hydrogen hypothesis
(when it was first proposed) was that
Archaea are not known to engulf other cells.
However, it was recently demonstrated that
some prokaryotes (beta-proteobacteria) harbor
endosymbiotic gamma-proteobacteria,
so bacteria-within-bacteria is not an impossibility.
Moreover, the recent identification of
cytoskeleton-type proteins equivalent to tubulin
and actin in prokaryotes is making us
rethink our models of prokaryotes as organisms
unlikely to undergo cell-shape changes
conducive to engulfment of other cells.
The hydrogen hypothesis thus places the
origin of the mitochondrial endosymbiont
before the origin of the nucleus, which would
have occurred by an autogenous process similar
to that invoked for nuclear origins in the classical
formulation of the endosymbiotic
hypothesis. This new version of the endosymbiont
hypothesis, which challenges a longstanding
paradigm in cell evolution, owes
its conception to the study of hydrogenosomes
in parabasalid parasites like Trichomonas
vaginalis.
PLASTIDS
This is the first parasitology text to contain a
chapter on plastids. Until recently plastids were
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA