Transcriptome age index analysis of the total placodal and neural crest tissues. Phylogenetic age of the transcriptome measured by TAI is compared between the placodes and the neural crest. TAI values were calculated as previously described  using the zebrafish microarray expression data (see Methods). Note that a lower TAI value marks a phylogenetically older transcriptome. For every gene included in the analysis, microarray expression levels were averaged over ontogeny. Bars on the left compare all expressed genes in the placodes and the neural crest, whereas the comparison on the right is restricted to those genes with exclusive expression in these structures. The significance of differences was tested using the Student’s t-test. Error bars represent ±1 standard error of mean.