Genome wide regulation of transcription by the photoperiod. Hierarchical clustering of Ostreococcus genes according to their expression patterns. Because of the wide range amplitudes of amplitudes observed (1 <Δlog2 ratio <9) different sets of probes are represented: PCA set (2038 probes), ANOVA (non present in PCA) Set1 with 2 <lΔlog2 ratio <6 (1684 probes); ANOVA (non present in PCA) set 2 with 1 <lΔlog2 ratio <2 (2412 probes). Note that except for two, all genes with 6<Δlog2 ratio <9 are in the PCA set. High values of expression are represented in red, low values in green. The signals were normalized to the average level of the 3 L/D (12:12) cycles. Night periods are in black, light periods in yellow.