Dan Akerson, General Motors Co’s (GM) outgoing chief executive, on Monday defended his tenure leading the No. 1 U.S. automaker and said his successor will have to build on the turnaround that he guided. GM said last week that Akerson would step down in January and be succeeded by product development chief Mary Barra, the industry’s first woman CEO. Akerson assumed control of the Detroit company shortly before its autumn 2010 reintroduction as a public company and steered its return to profits after the 2009 bankruptcy and $49.5 billion federal bailout. Akerson, speaking in one of his final appearances before he leaves, also celebrated the exit last week of the U.S. Treasury as a shareholder.