Marketing and advertising giant WPP has bought a "strategic" stake in Dolphin Software, the owners of the mySupermarket price comparison service.The $7m outlay by Sir Martin Sorrell's group has bought a minority stake in Dolphin, which has seen mySupermarket's growth at around 100% year on year.WPP says the stake will strengthen its capabilities in eCommerce, particularly in relationship to packaged goods which is important for many of the company's largest clients.WPP shares were down 1.3% by 11:38.BS