On July 21, 2006, PED Seminar Series Presents

Microbial diversity in the wild: genomes, populations and species

by Professor Martin Polz

Microbial diversity in the ocean is far greater than anticipated and we currently face the challenge of defining what may constitute meaningful ecological and evolutionary populations with predictable function. SSU rRNA gene sequencing, the current standard for measuring microbial diversity in environmental samples, indicates the presence of microbial taxa that are not only globally distributed but seem to be able to colonize diverse niche spaces. Genomics of closely related strains suggests a genetic mechanism: even phylogenetically closely related strains can differ dramatically in their gene content, which may facilitate ecologically differentiation. However, our recent observation of diversity among ocean bacterioplankton raises questions to what extent differences in genome architecture should always be judged as adaptive in a given environmental context. At a coastal ocean site, we observed at least 1,600 unique rRNA sequences, which represented ~500 microdiverse (<1% divergent) sequence clusters; when examined in detail, one such cluster (related to Vibrio splendidus) contained over a thousand well differentiated (in size and gene sequences) genome types, each occurring on average at such low environmental concentration that unique traits among them must have negligible impact on overall population function. Indeed, these strains display remarkable diversity in ability to grow on distinct carbon substrates despite isolation at a single site, so it is difficult to imagine that each arose in adaptation to patterns of co-occurrence of carbon substrate availability in the plankton. These observations will be discussed in the context of mechanisms of genome evolution and differentiation.

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