trans

290 PLASTIDS, MITOCHONDRIA, AND HYDROGENOSOMES
Gene
transfer
Gene
loss
Gene
loss
Gene
transfer
FIGURE 12.3 Sequential endosymbiotic events (primary and secondary) producing plastids. A primary plastid
arises from engulfment and retention of a photosynthetic prokaryote. The phagosomal membrane ruptures releasing
the endosymbiont, with its two membranes derived from the Gram-negative membranes, into the cytoplasm.
Genes transferred from the prokaryote to the eukaryote host nucleus are targeted back to the endosymbiont using
a transit peptide. Plastids acquired by secondary endosymbiosis typically have four bounding membranes. The
inner pair originates from the primary plastid membranes (i.e. the two membranes from the bacterium). The outer
membrane derives from the phagosome (food vacuole) and the membrane between the outer one and the innermost
pair derives from the endosymbiont plasma membrane. Genes transfer to the secondary host nucleus from
the endosymbiont nucleus, which eventually disappears leaving only the four membranes. Algal examples of each
stage are known. In Apicomplexa the eventual loss of photosynthesis resulted in a four membrane-bounded, relict
plastid.
equation (photosynthetic eukaryote) is the first
component of the second equation (Figure
12.3). While primary endosymbiosis happened
(to the best of our knowledge) only once, secondary
endosymbiosis occurred at least twice
and, some would argue, perhaps numerous
times. Secondary endosymbiosis results in
plastids with more than two membranes; three
or four membranes is typical (Figure 12.3).
Although dinoflagellates and Apicomplexa
are close relatives, they both have secondary
plastids, which leaves open the possibility that
they have two separate secondary acquisitions.
Dinoflagellates almost certainly contain a
red algal endosymbiont but an early analysis
of apicomplexan plastid genes concluded
that the apicomplexan plastid derived from
a secondary endosymbiotic green alga. More
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA