Retrotransposons are mobile genetic elements that encode for their own replication. Many studies have linked their expression to stress caused by environmental factors. Genome sequencing and EST libraries of the coastal diatom Phaeodactylum tricornutum indicate that this organism has an active copia-like retrotransposon (Blackbeard), which is variably expressed under different culture conditions. In this study, we induce physiologic stress in P. tricornutum and measure Blackbeard expression over time. However, we find the dominant pattern of Blackbeard expression is related to cell culture density, not short-term physiologic stress. Density dependent expression of a retrotransposon in a diatom provides significant insight into the biogeography of diatom genome mutation. We suggest the shallow coastal ocean, where diatom densities are high, may be the geographic locus for generating genomic diversity in diatom lineages.
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