IS THE ORIGIN OF THE DIATOMS RELATED TO THE END-PERMIAN MASS EXTINCTION

Small subunit ribosomal RNA (ssu rRNA) coding regions from 30 diatoms, nine other heterokont algae, three oomycetes, one thraustochytrid and one heterotrophic flagellate were used to construct a molecular clock from maximum-likelihood trees, and from linearized trees using a neig hbor-joining analysis. Taxa with fast and/or aberrantly evolving ssu r RNAs were not included in our molecular clock calculations. First appe arance dates of diatom taxa from the fossil record were regressed agai nst their corresponding branch lengths to infer the average and earlie st possible age for the origin of the heterokont algae. The earliest a ge estimates (based on the median-evolving diatom taxon in the maximum -likelihood tree or on the average branch length in a linearized tree) suggest that the secondary endosymbiotic event leading to the diverge nce of pigmented heterokonts from their non-pigmented ancestors is unl ikely to have occurred much before the Permian-Triassic boundary. The trees also show that the diatoms, one of the major groups of pigmented heterkonts, must have inherited their diplont life cycle and the abil ity to form resting stages from the last common ancestor shared with t he oomycetes and the other pigmented heterokonts. We hypothesize that non-pigmented, diploid heterokonts, capable of forming resting stages and of stably maintaining a photosynthetic organism within their cytop lasm, had an adaptive advantage over other organisms during the intens e climatic tectonic and geochemical changes that led to a mass extinct ion close to this boundary. After the mass extinction, many niches in the marine and aquatic realms were opened and the heterokont algae, in cluding the diatoms, appear to have diverged after this time.