Sphagnum-dominated peatlands cover vast expanses in both the Northern and Southern Hemispheres and almost one-third of all soil organic carbon is stored in peatland ecosystems –455 to 547 Gt C in boreal bogs, fens and tundra wetlands. Working in collaboration with the Joint Genome Institute (JGI: DOE) we are currently sequencing genomes from several Sphagnum species as a new model for research in ecological genomics. Sphagnum is especially valuable in this regard because (1) up to 25 species can occur sympatrically within wetland communities and are differentiated with regard to narrow ranges of abiotic variables (e.g., pH, ion concentrations, height above the water table); (2) the genus occurs from arctic to tropical and subantarctic regions/climates; and (3) species differ with regard to traits that scale up to impact global biogeochemical cycles. The Sphagnum genome project is a genus-wide effort to understand genomic changes associated with interspecific adaptation to regional climates as the genus diversified through deep time, comparable climate adaptation within individual species that range from arctic to tropical areas, and niche differentiation among co-occurring species within peatland communities.
Results/Conclusions
Genomes from two species, S. fallax and S. magellanicum, are currently in draft stage. Organellar (plastid, mitochondrial) genome sequences resolve the major lineages within Sphagnum and indicate that divergence between primarily hummock-forming species (subgenera Sphagnum and Acutifolia) and hollow-inhabiting species (subgenera Cuspidata and Subsecunda) occurred early during evolution of Sphagnum. Biogeographic analyses indicate that early diversification of Sphagnum was in the boreal zone and range expansions to tropical regions occurred at least 8 times in various subgeneric lineages. Phylogenetic analyses of realized niche evolution across Sphagnum indicate strong phylogenetic signal for interspecific variation along the hummock-hollow gradient (height above water table), suggesting that microhabitat preference shapes peatland physiognomy and leads to local assemblages of related species. Position along the hummock-hollow gradient is associated with physiological/developmental tradeoffs; hollow species tend to concentrate growth in the terminal capitulum, maximizing photosynthesis, grow in loosely packed colonies, and are characterized by higher decomposition rates, whereas hummock species usually have small capitula, slower growth, denser packing within colonies, and decompose more slowly. Comparative genome analyses are aimed at identifying gene families that expanded/contracted in connection with niche shifts and geographic spread, both inter- and intraspecific, changes in gene network structure/function associated with those shifts, and resolving signatures of natural selection over phylogenetic history, as well as among contemporaneous populations.