The world’s biggest bond fund, which is run by Bill Gross at Pimco, has opened the door to start buying equity-linked securities, further fuelling the debate about the direction of bond markets.The $250 billion Total Return fund is expanding its guidelines so it will be able to invest up to 10 per cent of its total assets in preferred stock, convertible securities and other equity-related securities, Pimco said in a regulatory filing. The fund will not invest in common stock.