of diatom types. The Cretaceous–Paleogene extinction event, which in the oceans dramatically affected organisms with calcareous skeletons, appears to have had relatively little impact on diatom evolution. Turnover Although no mass extinctions of marine diatoms have been observed during the Cenozoic, times of relatively rapid evolutionary turnover in marine diatom species assemblages occurred near the Paleocene–Eocene boundary, and at the Eocene–Oligocene boundary. Further turnover of assemblages took place at various times between the middle Miocene and late Pliocene, in response to progressive cooling of polar regions and the development of more endemic diatom assemblages. A global trend toward more delicate diatom frustules has been noted from the Oligocene to the Quaternary. This coincides with an increasingly more vigorous circulation of the ocean's surface and deep waters brought about by increasing latitudinal thermal gradients at the onset of major ice sheet expansion on Antarctica and progressive cooling through the Neogene and Quaternary towards a bipolar glaciated world. This caused diatoms to take in less silica for the formation of their frustules. Increased mixing of the oceans renews silica and other nutrients necessary for diatom growth in surface waters, especially in regions of coastal and oceanic upwelling. Genetics Expressed sequence tagging In 2002, the first insights into the properties of the Phaeodactylum tricornutum gene repertoire were described using 1,000 expressed sequence tags (ESTs). Subsequently, the number of ESTs was extended to 12,000 and the diatom EST database was constructed for functional analyses. These sequences have been used to make a comparative analysis between P. tricornutum and the putative complete proteomes from the green alga Chlamydomonas reinhardtii, the red alga Cyanidioschyzon merolae, and the diatom Thalassiosira pseudonana. The diatom EST database now consists of over 200,000 ESTs from P. tricornutum (16 libraries) and T. pseudonana (7 libraries) cells grown in a range of different conditions, many of which correspond to different abiotic stresses. Genome sequencing In 2004, the entire genome of the centric diatom, Thalassiosira pseudonana (32.4 Mb) was sequenced, followed in 2008 with the sequencing of the pennate diatom, Phaeodactylum tricornutum (27.4 Mb). Comparisons of the two reveal that the P. tricornutum genome includes fewer genes (10,402 opposed to 11,776) than T. pseudonana; no major synteny (gene order) could be detected between the two genomes. T. pseudonana genes show an average of ~1.52 introns per gene as opposed to 0.79 in P. tricornutum, suggesting recent widespread intron gain in the centric diatom.