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‘AMITOCHONDRIATE’ ORGANISMS 283
and retained by a nucleated host, invokes the
existence of an early lineage of eukaryotes without
mitochondria. Although, the mitochondriate
derivatives of the endosymbiotic event
have become more numerous and far more
conspicuous, students of eukaryotic origins
recognized that if any descendants of the
amitochondrial stock are still alive, then these
organisms would be valuable models in interpreting
our origins. The quest was thus on to
find the organisms that best represented the
‘host’, the premitochondrial eukaryote. Amitochondriate
parasites were considered good
candidates at the time.
Reconstruction of eukaryotic evolution
underwent explosive growth with the advent
of DNA sequencing. Most of this activity initially
focused on one gene, rRNA, as a marker
of relationships. Phylogenetic trees inferred
from rRNA gene sequences have yielded valuable
additions to our models of evolution, with
clear depiction of relationships previously recognized
on other grounds and also the revelation
of other relationships not previously well
understood. The initial trees incorporating parasites
like Giardia were exciting for evolutionary
biologists. Giardia emerged at the base of
the eukaryotic tree, exactly where one would
expect to find it if it were truly an early offshoot
of eukaryotes prior to mitochondrial acquisition.
Several other amitochondrial organisms,
principally parasites, were also added
to the phylogenetic trees, and three lineages
(the diplomonads, including Giardia; the
parabasalids, including Trichomonas vaginalis;
and the microsporidia, including Encephalitozoon)
consistently emerged at the base of
these trees. This gene-tree topology was congruent
with the endosymbiotic model of
eukaryogenesis and provided us with several
models to study. Different versions of the rRNA
trees were not always in agreement as to which
amitochondriate protist was the first to emerge
from the eukaryotic lineage, but combined with
the often simple ultrastructure of the organisms
a cohesive story of an ancient relict lineage
was beginning to emerge. These organisms
became known as the kingdom Archezoa,
a group defined primarily by the lack of
mitochondria and their basal position in the
eukaryotic tree. They were considered the first
eukaryotes and an extant version of the kind of
cells from which we ourselves are derived.
However, not all the data sat comfortably
with the ‘Archezoa’ hypothesis. The original
amitochondriate eukaryotes were envisaged
as anaerobes. Indeed, mitochondrial acquisition
is thought to have aided their coping with
increasing O 2 levels. If the amitochondriate
eukaryotes (Archezoa), which were primarily
parasites inhabiting anaerobic niches within
animals, were indeed descendants of the first
eukaryotes, where had they survived prior to
the advent of animal body cavities? These
anaerobes would have had to inhabit anaerobic
zones for many hundreds of millions of
years and then to invade these new environmental
niches as metazoa evolved. This scenario
is plausible, but other arguments against
the Archezoa hypothesis also appeared.
The ascendancy of rRNA as the key phylogenetic
marker was not so much because it is the
ideal marker but because of its practicability.
rRNA genes are relatively easy to sequence
(nowadays at least) and the enormous database
provides a huge head start for interpreting
relationships within the larger context.
But interpreting evolution of organisms from
a single gene sequence places a great deal of
reliance on those sequence data reflecting the
actual pattern of evolutionary branching. It
is clear (with the benefit of hindsight) that factors
other than simple fixation of mutations as
a consequence of evolutionary divergence act
on gene sequences. Thus, patterns in the gene
sequences that do not necessarily reflect the
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA