trans

PLASTIDS 291
recently trees incorporating plastid rRNA genes
show dinoflagellate plastids to be most closely
related to plastids of apicomplexan plastids. At
face value this relationship suggests that the
plastids were acquired by a common secondary
endosymbiotic event, but the trees are not
clear-cut. The branches of dinoflagellate and
apicomplexan plastids are extraordinarily long
and, as discussed above, their grouping must
be regarded with abject caution. A potentially
more telling piece of evidence for a common
origin comes not from the difficult-to-tree plastid
genes but from the nuclei of the hosts.
Trees of glyceraldehyde-3-phosphate dehydrogenase
(GAPDH) genes provide striking evidence
that the plastids of dinoflagellates and
Apicomplexa (as well as other algal groups)
have acquired their plastids in one common
secondary endosymbiosis of a red alga.
Understanding the origin of the apicoplast
is more than just an academic exercise. Because
the apicomplexan plastid is potentially an
excellent target for anti-parasite drugs (see
below), it is important that we understand its
evolutionary history. If a common origin for
dinoflagellate and apicomplexan plastids is
confirmed, we will have deepened our understanding
of the origins of one of the world’s
deadliest parasites. Dinoflagellates and Apicomplexa
diverged at least 400 million years
ago, but despite outward appearances they
are not so fundamentally different. Both have
the ability to associate closely with animals,
dinoflagellates as endosymbionts of corals
and other invertebrates and Apicomplexa as
intracellular parasites. An attractive scenario
is that this ability to associate with animals
goes back to their common ancestor, and
that one lineage (dinoflagellates) persisted with
photosynthesis and commensal interactions
while another (Apicomplexa) abandoned
photosynthesis, instead converting to parasitism
to exploit the host. This presumably
happened quite early in animal evolution, but
the parasites are still with us. Why Apicomplexa
keep a vestige of their plastid is the next
question.
Apicoplast function
Why do these parasites still have a vestigial
plastid? As far as is known, apicomplexan parasites
are not photosynthetic, but the plastid is
indispensable. Clearly, evolution has failed to
expunge the organelle despite several hundred
million years of parasitic living. Moreover,
lab-generated mutants in which the plastid is
unable to divide (and therefore unable to be
passed on to daughter parasites) are not viable.
Clearly, the parasites depend on the plastid for
some service or function. Interestingly, plants
are dependent on their plastids too. Some
plants, such as beechdrops or Indian pipe, have
abandoned photosynthesis and are wholly parasitic.
Importantly, these non-photosynthetic
plant parasites always keep their plastids,
although they are much reduced and lack
chlorophyll. Why is this?
Although the main role of plastids in photosynthesis
is well recognized, it is less well appreciated
that a plant plastid is the sole site for
several essential cellular processes such as
heme, isoprenoid, essential amino acid and
lipid synthesis. These functions probably make
plastids indispensable for plants and many
algae. Could parasites be dependent on their
apicoplasts for one or more of these functions?
It is a pervasive paradigm in parasitology
that parasites don’t make anything that
they can procure from their hapless hosts. For
instance, because malaria parasites can scavenge
and modify lipids from the erythrocyte, it
was believed that malarial parasites are unable
to synthesise fatty acids de novo. This dogma
has recently been overturned with the
identification of a fatty acid synthesis pathway
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA