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Summary: The phylogenetic distribution of chromalveolate
plastids favors independent tertiary origins
Red algal descended plastids are found in clearly polyphy-
letic taxa, each nested within lineages containing aplasti-
dic relatives (Figure 1). Moreover, photosynthetic taxa
frequently occur at derived positions, with multiple hetero-
trophic groups branching ancestrally. The number of inde-
pendent losses required to account for this pattern is
unreasonable (see above); it is far more parsimonious to
presume several tertiary transfers (Figure 1).
It is clear that plastid evolution in eukaryotes has
resulted in a complex phylogenetic pattern with many con-
flicts among molecular data. Although the chromalveolate
model cannot be rejected formally, neither should it serve as
the a priori framework for interpreting plastid character
evolution, endosymbiotic gene transfers or other intrage-
nomic phylogenetic conflicts. We argue that an alternative
model of serial tertiary endosymbioses is more consistent
with available data, and should be taken into account in
phylogenomic investigations of eukaryotic diversity.
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