Preview at WWDC likely in June, followed by September launch.
Apple Seeds Fourth OS X 10.10.3 Yosemite Beta to Developers, Public Beta Testers
Today's beta appears to be nearly identical to the third 10.10.3 beta released last week, but it appears to solve a problem that prevented beta 3 from working with the new 2015 Retina MacBook Pro and MacBook Air.
The new beta, build 14D105g, is available for registered developers through the Software Update mechanism in the Mac App Store and through the Mac Dev Center.
Earlier betas of OS X 10.10.3 have brought several new features to Yosemite, including the new Photos for OS X app. Designed to integrate with iCloud Photo Library, Photos for OS X is a replacement for both iPhoto and Aperture and is on track to see an early 2015 public launch. Reviews have suggested that while Photos is a suitable replacement for iPhoto, with more advanced tools and performance optimizations, it may leave professional users disappointed.
Along with the new Photos for OS X app, the first three OS X 10.10.3 betas introduced a redesigned emoji picker that consolidates emoji into a single page with clear labels, new diversified emoji and emoji skin tone modifiers, new flag emoji and updated emoji for the iPhone, iMac, and Apple Watch. There's also support for Google 2-step verification when setting up accounts in System Preferences, Force Touch APIs for developers, and new data sources for the "Look Up" feature.