Hierarchical clustering of transcripts defined as being differentially expressed under iron limitation (FL) in P. tricornutum , along with the hierarchical clustering of corresponding orthologs expressed under iron limitation (FL) in T. pseudonana. Expression levels are shown in an increasing scale from grey to dark blue, and are based on frequencies of ESTs in each library (see Materials and methods). For two-letter library codes for P. tricornutum, see Table 1. T. pseudonana library codes are TL, temperature limited; FL, iron limited; CL, carbon dioxide limited; SL, silicate limited; NL, nitrate limited; OL, old library; NP, nitrate plus . The red letter 'D' in the T. pseudonana cluster denotes the diatom-specific transcripts.