The stat gene family diversified during early vertebrate evolution thanks to two rounds of whole genome duplication (WGD) to produce a typical repertoire composed of 6 STAT factors (named 1-6). In contrast, only one or two stat genes have been reported in C. elegans and in D. melanogaster. The main types of STAT found from bony fish to mammals are present in Agnathan genomes, but a typical STAT1-6 repertoire is only observed in jawed vertebrates. Comparative syntenies showed that STAT6 was the closest to the ancestor of the family. An extensive survey of stat genes across fish including polyploid species showed that whole genome duplications did not lead to a uniform expansion of stat genes. While 2 to 5 stat1 are present in salmonids, whose genome duplicated about 35My ago, only one copy of stat2 and stat6 is retained. In contrast, common carp, with a recent whole genome duplication (5-10My), possesses a doubled stat repertoire indicating that the elimination of stat2 and stat6 additional copies is not immediate. Altogether our data shed light on the multiplicity of evolutionary pathways followed by key components of the canonical cytokine receptor signalling pathway, and point to differential selective constraints exerted on these factors.