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PROJECT SUMMARY:
The
ability to engulf and incorporate a previously free-living bacterial
(primary symbiosis) or eukaryotic (secondary symbiosis) cell and
transform it into an organelle is one of the defining characteristics
of eukaryotes. Understanding the process of organelle genesis provides,
therefore, fundamental insights into eukaryotic evolution. All plastids
likely trace their origin to a single cyanobacterial primary symbiosis
in the common ancestor of the red, green, and glaucophyte algae
(Fig. A). In contrast, secondary symbiosis has led to the origin
of photosynthesis in evolutionarily distantly related protists multiple
independent times. The number of secondary symbioses and the specific
source(s) of the different secondary plastids is, however, unknown
in most cases. The red algae, composed of the subclasses Bangiophycidae
and Florideophycidae, offer a model for understanding secondary
symbiosis because Bangiophycidae are the sources of secondary plastids
in four major algal groups (Cryptophyta, Haptophyta, Heterokonta,
Dinophyceae). This proposal addresses the phylogenetic relationship
between plastids in Bangiophycidae and their descendants (through
secondary symbiosis) in Cryptophyta, Haptophyta, and Heterokonta
(Fig. B), and the systematics of the Bangiophycidae. We will determine
the phylogeny of plastids in the Bangiophycidae, Cryptophyta, Haptophyta,
and Heterokonta by comparing concatenated plastid small subunit
(SSU) rDNA and RuBisCo large subunit (rbcL) coding regions. The
phylogeny of the Bangiophycidae nuclear lineage will be inferred
by analyzing concatenated nuclear and plastid SSU rDNA, rbcL, and
mitochondrial coxI coding regions.
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Results
of preliminary phylogenetic analyses of nuclear and plastid DNA coding
regions show that one of the major orders of the Bangiophycidae, the
Porphyridiales, is split into at least three different lineages and
that the Florideophycidae is monophyletic with weak support for an
origin from the Bangiales (Fig. A). We have determined that the plastids
of the Heterokonta are likely derived from members of the Cyanidiales
and are not directly related to the plastids in the Cryptophyta and
the Haptophyta.