title: Slow but not low: genomic comparisons reveal slower evolutionary rate and higher dN/dS in conifers compared to angiosperms.
abstract: BACKGROUND
  Comparative genomics can inform us about the processes of mutation and selection across diverse taxa. Among seed plants, gymnosperms have been lacking in genomic comparisons. Recent EST and full-length cDNA collections for two conifers, Sitka spruce (Picea sitchensis) and loblolly pine (Pinus taeda), together with full genome sequences for two angiosperms, Arabidopsis thaliana and poplar (Populus trichocarpa), offer an opportunity to infer the evolutionary processes underlying thousands of orthologous protein-coding genes in gymnosperms compared with an angiosperm orthologue set.
  
  
  RESULTS
  Based upon pairwise comparisons of 3,723 spruce and pine orthologues, we found an average synonymous genetic distance (dS) of 0.191, and an average dN/dS ratio of 0.314. Using a fossil-established divergence time of 140 million years between spruce and pine, we extrapolated a nucleotide substitution rate of 0.68 × 10(-9) synonymous substitutions per site per year. When compared to angiosperms, this indicates a dramatically slower rate of nucleotide substitution rates in conifers: on average 15-fold. Coincidentally, we found a three-fold higher dN/dS for the spruce-pine lineage compared to the poplar-Arabidopsis lineage. This joint occurrence of a slower evolutionary rate in conifers with higher dN/dS, and possibly positive selection, showcases the uniqueness of conifer genome evolution.
  
  
  CONCLUSIONS
  Our results are in line with documented reduced nucleotide diversity, conservative genome evolution and low rates of diversification in conifers on the one hand and numerous examples of local adaptation in conifers on the other hand. We propose that reduced levels of nucleotide mutation in large and long-lived conifer trees, coupled with large effective population size, were the main factors leading to slow substitution rates but retention of beneficial mutations.
authors.affiliation: Department of Forest Sciences, University of British Columbia, 2424 Main Mall, Vancouver, BC V6T 1Z4, Canada. elbuzzo@gmail.com
authors.forename: Emmanuel
authors.initials: E
authors.lastname: Buschiazzo
authors.forename: Carol
authors.initials: C
authors.lastname: Ritland
authors.forename: Jörg
authors.initials: J
authors.lastname: Bohlmann
authors.forename: Kermit
authors.initials: K
authors.lastname: Ritland
mesh_terms: D019943:Amino Acid Substitution; D019143:Evolution, Molecular; D017343:Genes, Plant / Q000235:genetics; D018745:Genome, Plant / Q000235:genetics*; D023281:Genomics; D019684:Magnoliopsida / Q000235:genetics*; D064028:Tracheophyta / Q000235:genetics*