Abstract
Oceanic phages are critical components of the global ecosystem, where they play a role in microbial mortality and evolution. Our understanding of phage diversity is greatly limited by the lack of useful genetic diversity measures. Previous studies, focusing on myophages that infect the marine cyanobacterium Synechococcus, have used the coliphage T4 portal-protein-encoding homologue, gene 20 (g20), as a diversity marker. These studies revealed 10 sequence clusters, 9 oceanic and 1 freshwater, where only 3 contained cultured representatives. We sequenced g20 from 38 marine myophages isolated using a diversity of Synechococcus and Prochlorococcus hosts to see if any would fall into the clusters that lacked cultured representatives. On the contrary, all fell into the three clusters that already contained sequences from cultured phages. Further, there was no obvious relationship between host of isolation, or host range, and g20 sequence similarity. We next expanded our analyses to all available g20 sequences (769 sequences), which include PCR amplicons from wild uncultured phages, non-PCR amplified sequences identified in the Global Ocean Survey (GOS) metagenomic database, as well as sequences from cultured phages, to evaluate the relationship between g20 sequence clusters and habitat features from which the phage sequences were isolated. Even in this meta-data set, very few sequences fell into the sequence clusters without cultured representatives, suggesting that the latter are very rare, or sequencing artefacts. In contrast, sequences most similar to the culture-containing clusters, the freshwater cluster and two novel clusters, were more highly represented, with one particular culture-containing cluster representing the dominant g20 genotype in the unamplified GOS sequence data. Finally, while some g20 sequences were non-randomly distributed with respect to habitat, there were always numerous exceptions to general patterns, indicating that phage portal proteins are not good predictors of a phage's host or the habitat in which a particular phage may thrive.
Highlights
Oceanic phages are critical components of the global ecosystem, where they play a role in microbial mortality and evolution
Given our extensive host range information for these cyanobacteria phage–host systems, we examined g20 clustering patterns for relationships with respect to the host strains upon which the phage were isolated or could cross-infect
First we found that 13 of the 38 Global Ocean Survey (GOS) sample sites included in our analysis lack Prochlorococcus and Synechococcus, yet 75 g20 sequences from these sites fell into clusters I, II and III (Fig. 2), thought, from earlier studies, to represent myophages that infect marine picocyanobacteria
Summary
Oceanic phages are critical components of the global ecosystem, where they play a role in microbial mortality and evolution. Previous studies, focusing on myophages that infect the marine cyanobacterium Synechococcus, have used the coliphage T4 portal-protein-encoding homologue, gene 20 (g20), as a diversity marker These studies revealed 10 sequence clusters, 9 oceanic and 1 freshwater, where only 3 contained cultured representatives. The cultured representatives were isolated using only Synechococcus hosts (7 strains = WH7803, WH7805, WH8007, WH8012, WH8018, WH8101, WH8113), which undoubtedly limits the diversity represented considering the larger diversity of Synechococcus strains (Rocap et al, 2002; Fuller et al, 2003; Ahlgren and Rocap, 2006) and that the sister genus Prochlorococcus is abundant in open ocean waters This raises the question: could these seven environmental-sequence-only clusters represent novel cyanomyophages that infect this broader diversity of Synechococcus host strains, Prochlorococcus or other cyanobacteria?. This allowed us to ask: do any of the new environmental sequences cluster with the previously observed environmental-sequence-only clusters? are g20 sequence clustering patterns ecologically meaningful? Do they reflect the habitat – and by inference the microbial community – of the site from which they were isolated?
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