Strong purifying selection in the silicon transporters of marine and freshwater diatoms

Marine and freshwater diatoms show several important differences in silicon physiology. In addition to containing an order of magnitude more silica in their cell walls, freshwater diatoms also appear to have a less efficient silicic acid uptake mechanism. A novel set of silicon transporters ( SITs), encoded by a small gene family, import silicic acid from the environment into the diatom cell. Some evidence suggests that the disparity in uptake efficiency between marine and freshwater diatoms might be attributable to different demands on SITs in the two environments. To test this hypothesis, partial SIT genes were cloned and sequenced from 45 species of Thalassiosirales, a diatom lineage with high diversity in marine and freshwaters. Phylogenetically based codon substitution models were used to test whether SITs from marine and freshwater taxa were under similar selective constraints and whether codons in different structural locations of the protein were under similar functional constraints. Purifying selection is the predominant evolutionary force acting on SITs, irrespective of location in the protein, and differences in efficiency of silicic acid uptake between marine and freshwater diatoms are not due to sequence differences in SITs.