The phylogenetic context for the origin of a unique purple-green photosymbiosis
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Abstract
Symbioses are widespread in nature and have been the source of much evolutionary innovation. While some types of symbioses evolved multiple times across space and time (e.g., oxygenic photosymbioses or chemosymbioses), others are extremely rare. Purple photosymbioses are an example of such rare associations. Only two purple photosymbioses between heterotrophic eukaryotes and intracellular purple bacteria have been documented. This is in stark contrast to very common oxygenic photosymbioses and poses the question of what factors prevent the more frequent establishment of purple photosymbioses. To shed light on this question, we investigated the evolutionary history of the purple-green ciliate Pseudoblepharisma tenue (Spirostomidae) using a phylogenetic and comparative approach. We sampled about 30 new isolates of spirostomid ciliates, inferred a comprehensive and well-supported phylogenomic tree, and resolved the sister relationship between Pseudoblepharisma and Spirostomum . Furthermore, we characterized P. tenue ’s sister species, here renamed Pseudoblepharisma chlorelligera , and revealed that it constitutes a quadripartite symbiosis between a ciliate, a green alga, and two different non-photosynthetic bacteria. In addition, we discovered three colorless, non-photosymbiotic Pseudoblepharisma species, which branch as sister to the photosymbiotic P. tenue and P. chlorelligera . Our phylogenetic and comparative genomic analyses suggest that the green algal symbionts of P. tenue predated the acquisition of purple bacterial symbionts, and that the ancestor of the extant Pseudoblepharisma species was non-photosymbiotic and facultatively anaerobic. These data allowed us to hypothesize on the evolutionary steps that led to the origin of P. tenue and thus bring us closer to explaining the conditions that led to the evolutionary emergence of a unique purple-green symbiosis.
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This study ha
This paper was exciting to begin with, promising to trace the evolutionary history behind the rare acquisition of a purple endosymbiont. It ended up being even more exciting, expanding our knowledge of Pseudoblepharisma species, piecing together deep evolutionary information cross four different bacterial and eukaryotic families to succinctly paint not just the steps that facilitated the acquisition of a purple endosymbiont, but also the subfunctionalization of two neighboring species adapted to different oxygen environments.
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Close phylogenetic associa
Some of the most important branches on this tree seem to have low support, and the two trees are rather different. Perhaps creating a concatenated alignment and using that to create a tree will help to improve branch support and clarify species' relationships.
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Meanwhile, “Ca. Accumulibacter symbioticus” has retained the complete heme biosynthesis pathway, in particular hemC and hemH, that is incomplete in both the host and “Ca. Propionivibrio subcutaneus” and could be indicative of metabolic complementation (Fig. S10, S11).
The heme can play a role in getting capturing oxygen that might get in the way of anoxic reactions, much like in legume root nodules. It might also be doing the opposite, helping to provide oxygen for other functions matching the more aerobic environment of this species.
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(Fig. 3H, 3L, 3N).
I think you mean 3M, instead of 3N. There are also different colorless elements in these images like the starch granules (which are labelled) but the bacteria are not labelled. Please highlight them.
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to P. tenue is the absence of intracellular purple bacteria and the much larger number of green algae. P. chlorelligera was packed with intracellular green algae with an average of 352 green algal cells per host (n=6) (Fig. 3A)
I think it would be interesting to see some of these values (# of algal cells, length, etc) displayed quantitatively as bar graphs with error bars. It would facilitate comparisons to have a similar figure for P. tenue showing cyst stages, free-swimming cells. etc. It could be provided as a supplement.
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Scale bars: 50 µm.
The scale bar for TBCC008 is missing.
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Maximum likelih
All the important branches are strongly supported, as shown by the bold branches. I appreciate the use of SH-aLRT statistics. The phylogeny clearly points to an evolutionary history: purely heterotrophic > containing green algal endosymbionts > containing purple endosymbionts. Sampling more species would be helpful in confirming whether the taxa are truly cleanly separated as 'obligate heterotrophs' and 'photosynthesic endosymbionts', but expanding the tree this much is already an extraordinary finding.
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