Abstract
Regulatory divergence is likely a major driving force in evolution. Comparative transcriptomics provides a new glimpse into the evolution of gene regulation. Ascomycota fungi are uniquely suited among eukaryotes for studies of regulatory evolution, because of broad phylogenetic scope, many sequenced genomes, and facility of genomic analysis. Here we review the substantial divergence in gene expression in Ascomycota and how this is reconciled with the modular organization of transcriptional networks. We show that flexibility and redundancy in both cis-regulation and trans-regulation can lead to changes from altered expression of single genes to wholesale rewiring of regulatory modules. Redundancy thus emerges as a major driving force facilitating expression divergence while preserving the coherent functional organization of a transcriptional response.