Comparison of the expression of medaka (Oryzias latipes) pitx genes with other vertebrates shows high conservation and a case of functional shuffling in the pituitary.

With the availability of an increasing number of whole genome sequences in chordates, exhaustive comparisons of multigene families become feasible. Relationships of orthology/paralogy can not only be inferred from sequence similarity but also by comparing synteny conservation on chromosomes. More accurate scenarios for gene and expression domain gain or loss can now be proposed. Here, we take benefit from the recent release of the medaka (Oryzias latipes) genome to analyse the orthology relationships and expression patterns of the three different sub-families of the pitx homeobox genes belonging to the paired class. They are involved in a wide variety of developmental processes and have pleiotropic expression patterns, especially in the case of the pitx2 sub-family. The emerging picture is a strong conservation of expression domains, suggesting that most functions have been present in the common ancestor of actinopterygians and sarcopterygians. Almost all pitx genes are expressed in anterior placodes in all species studied so far, including medaka. It has previously been shown that in mammals, pitx1 and 2 are expressed in the pituitary. Interestingly we demonstrate here that only pitx3 is expressed in medaka pituitary. It will be interesting to analyze what are the corresponding changes in the regulatory elements of pitx genes.