Developmental Cell – Mapping Gene Expression in Two Xenopus Species: Evolutionary Constraints and Developmental Flexibility
http://www.cell.com/developmental-cell/retrieve/pii/S1534580711001213
Expression time course comparison of orthologs, which are closer than random. Uses sigmoid approximations for gene expression levels — on & off. Discussion on purifying selection related to expression.
Some interesting terms :
http://en.wikipedia.org/wiki/Heterochrony
http://medical-dictionary.thefreedictionary.com/heterometric