The vertebrate ancestral repertoire of visual opsins, transducin alpha subunits and oxytocin/vasopressin receptors was established by duplication of their shared genomic region in the two rounds of early vertebrate genome duplications.
Lagman D, Ocampo Daza D, Widmark J, Abalo XM, Sundström G, Larhammar D. BMC Evol Biol. 2013 Nov 2; 13:238. Epub 2013 Nov 2.