Did an ancient chlamydial endosymbiosis facilitate the establishment of primary plastids?
Citable URI
https://hdl.handle.net/1912/1820As published
https://doi.org/10.1186/gb-2007-8-6-r99DOI
10.1186/gb-2007-8-6-r99Abstract
Ancient endosymbioses are responsible for the origins of mitochondria and plastids,
and they contribute to the divergence of several major eukaryotic groups. Although chlamydiae, a
group of obligate intracellular bacteria, are not found in plants, an unexpected number of chlamydial
genes are most similar to plant homologs, which, interestingly, often contain a plastid-targeting
signal. This observation has prompted several hypotheses, including gene transfer between
chlamydiae and plant-related groups and an ancestral relationship between chlamydiae and
cyanobacteria.
We conducted phylogenomic analyses of the red alga Cyanidioschyzon merolae to identify
genes specifically related to chlamydial homologs. We show that at least 21 genes were transferred
between chlamydiae and primary photosynthetic eukaryotes, with the donor most similar to the
environmental Protochlamydia. Such an unusually high number of transferred genes suggests an
ancient chlamydial endosymbiosis with the ancestral primary photosynthetic eukaryote. We
hypothesize that three organisms were involved in establishing the primary photosynthetic lineage:
the eukaryotic host cell, the cyanobacterial endosymbiont that provided photosynthetic capability,
and a chlamydial endosymbiont or parasite that facilitated the establishment of the cyanobacterial
endosymbiont.
Our findings provide a glimpse into the complex interactions that were necessary to
establish the primary endosymbiotic relationship between plastid and host cytoplasms, and thereby
explain the rarity with which long-term successful endosymbiotic relationships between
heterotrophs and photoautotrophs were established. Our data also provide strong and
independent support for a common origin of all primary photosynthetic eukaryotes and of the
plastids they harbor.
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© 2007 Huang and Gogarten. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The definitive version was published in Genome Biology 8 (2007): R99, doi:10.1186/gb-2007-8-6-r99.
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