Chesapeake Energy Corp.'s top shareholder today urged CEO Aubrey McClendon and the board of directors to consider selling the company, as the second-largest U.S. natural gas producer struggles to dig itself out of a growing $13 billion debt load. Southeastern Asset Management, based in Memphis, Tenn., and led by Mason Hawkins, sent a letter to McClendon and the board that criticized McClendon's plan for paying off debt and transitioning to higher-priced oil. It opened the door to shareholder support for an acquisition of Chesapeake.