From Genes to Traits and Ecosystems: Evolutionary Ecology of Sphagnum (Peat Moss)
<p>Plants in the genus Sphagnum (peat moss) are the dominant biotic features of boreal peatlands that store nearly one-third of Earth’s terrestrial carbon. Peat mosses are ecosystem engineers and create the peatlands that they inhabit through the accumulation of peat, or partially decayed biomass, and the functional traits underlying this extended phenotype. Interspecific functional trait variation is hypothesized to promote niche differentiation through the creation and maintenance of ecological gradients along which species sort within communities. One prominent gradient relates to height-above-water-table wherein some species produce hummocks raised up to a meter above the water table, while others live in hollows at or near the water table. However, it is unclear how these traits evolved during Sphagnum diversification, to what extent natural selection produced functional trait variation, and which genes might contribute to such phenotypes.
In Chapter 2, a meta-analysis of data from recent studies is used to relate patterns of functional trait variation to the phylogeny of Sphagnum. The results suggest that interspecific variation in various measures of growth, decomposability, and litter biochemistry is phylogenetically conserved in Sphagnum, meaning that closely related species tend to be more similar in trait values than species selected at random from the phylogeny. Furthermore, these results suggest that patterns of trait covariation might represent adaptive syndromes related to niche. This is the first study to formally relate functional trait variation in Sphagnum to its evolutionary history.
In Chapter 3, a field experiment and phylogenetic comparative methods are used to show that natural selection is responsible for shaping interspecific variation in Sphagnum decomposability and its coevolution with niche. In the largest experiment of its kind to date, litter decomposability was measured for over 50 species of Sphagnum under natural conditions. Models of trait evolution were competed against one another to determine which best explained the evolution of this important functional trait. The best model was a multiple-peak Ornstein-Uhlenbeck process wherein the predominantly hummock and hollow clades of Sphagnum possess separate adaptive optima towards which trait values are pulled. Furthermore, the results suggest that shifts in trait optima occurred concomitantly with shifts in realized niche along the hummock-hollow gradient.
In Chapter 4, comparative genomics is used to identify genes involved in the biosynthesis of an ecologically-important class of secondary metabolites, the anthocyanins, and determine when this biosynthetic pathway first evolved in embryophyte land plants. In Sphagnum, complex phenolic molecules embedded in the cell walls influence litter decomposability and, in turn, the rate of peat accumulation. One group of such phenolics, the sphagnorubins, confer red-violet pigmentation to some species of Sphagnum. Sphagnorubins are thought to be homologous to anthocyanin pigments, known best from flowering plants, that have numerous roles including mitigation of abiotic stress. Phylogenetic analyses using full genome sequences representing nearly all major green plant lineages show that the entire anthocyanin biosynthetic pathway was not intact until the most recent common ancestor of seed plants. Furthermore, orthologs of many downstream enzymes in the pathway are absent from seedless plants including mosses, liverworts, and ferns. These results suggest that the production of red-violet flavonoid pigments in seedless plants, including sphagnorubins, requires the activity of novel enzymes and represents convergent evolution of red-violet coloration across land plants.
In Chapter 5, comparative genomics is used to test for molecular adaptation in Sphagnum genomes. Two reference-quality peat moss genomes were compared to those from other plants to identify genes bearing signatures of positive selection and gene families marked by significant rates of expansion in the Sphagnum lineage. Gene Ontology enrichment analyses were then used to identify over-represented classes of genes that might have been particularly important during the evolution of peat mosses. The results suggest adaptive evolution largely occurred in genes and gene families related to epigenetic regulation, secondary metabolism, stress response, and transmembrane transport. Together, these data suggest that selection favored changes to genes involved in response to environmental stress and provide candidate loci that might underlie adaptation to the harsh conditions of boreal peatlands.</p>
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