Abstract
The most widely distributed dinoflagellate plastid contains chlorophyll
c
2
and peridinin as the major carotenoid. A second plastid type, found in taxa such as
Karlodinium micrum
and
Karenia
spp., contains chlorophylls
c
1
+
c
2
and 19′-hexanoyloxy-fucoxanthin and/or 19′-butanoyloxy-fucoxanthin but lacks peridinin. Because the presence of chlorophylls
c
1
+
c
2
and fucoxanthin is typical of haptophyte algae, the second plastid type is believed to have originated from a haptophyte tertiary endosymbiosis in an ancestral peridinin-containing dinoflagellate. This hypothesis has, however, never been thoroughly tested in plastid trees that contain genes from both peridinin- and fucoxanthin-containing dinoflagellates. To address this issue, we sequenced the plastid-encoded
psa
A (photosystem I P700 chlorophyll
a
apoprotein A1),
psb
A (photosystem II reaction center protein D1), and “Form I”
rbc
L (ribulose-1,5-bisphosphate carboxylase/oxygenase) genes from various red and dinoflagellate algae. The combined
psa
A +
psb
A tree shows significant support for the monophyly of peridinin- and fucoxanthin-containing dinoflagellates as sister to the haptophytes. The monophyly with haptophytes is robustly recovered in the
psb
A phylogeny in which we increased the sampling of dinoflagellates to 14 species. As expected from previous analyses, the fucoxanthin-containing dinoflagellates formed a well-supported sister group with haptophytes in the
rbc
L tree. Based on these analyses, we postulate that the plastid of peridinin- and fucoxanthin-containing dinoflagellates originated from a haptophyte tertiary endosymbiosis that occurred before the split of these lineages. Our findings imply that the presence of chlorophylls
c
1
+
c
2
and fucoxanthin, and the Form I
rbc
L gene are in fact the primitive (not derived, as widely believed) condition in dinoflagellates.