In this article, we examine the evolution of Jak and Stat proteins, and of the Jak-Stat pathway. We first introduce the protein families involved and the signaling pathway in general. We then oppose the simply structured pathway in Drosophila melanogaster to the more complex situation in mammals. Furthermore, we compare the Jak-Stat system between mammals and teleost fishes. Finally, we move to the less well investigated roles of Stats in the worm Caenorhabditis elegans and the slime mold Dictyostelium discoideum. We also survey the distribution of Jaks and Stats among metazoans and other eukaryotes. Orthologs of Stats are widely distributed among metazoans and are also found in choanoflagellates and in slime molds. In contrast, Jaks seem to be confined to the bilaterians and are apparently absent in molluscs, round- and flatworms. This indicates that the Jak-Stat pathway evolved at the base of the bilaterians, but has been lost in some invertebrate groups.
- Jak-Stat Signalling