A combination of file photos shows Bertrand Serlet, senior vice president of OSX software at Apple Inc, (L) and Craig Federighi, vice president of Mac OS at Apple Inc (R) speaking at the Apple Inc's Worldwide Developers Conference in San Francisco on June 8, 2009. REUTERS/Robert Galbraith/FilesApple said top software engineer Bertrand Serlet will leave the Mac computer maker after more than a decade spent developing its signature operating system, Mac OS X. Craig Federighi, currently the vice president of Mac Software Engineering, will take over from Serlet and report to CEO Steve Jobs, Apple said in a statement.