Participating today in the 16.04 Beta 1 milestone are Lubuntu, Ubuntu Cloud, Ubuntu GNOME, Ubuntu MATE, Xubuntu, and Ubuntu Kylin. Remaining notably absent from these 16.04 milestone releases is Kubuntu.
Xubuntu 16.04 Beta 1 features various Xfce package updates, Ubuntu GNOME is now mostly riding off a GNOME 3.18 stack, and they're all now utilizing the Linux 4.4 kernel with Mesa 11.1.2 + LLVM 3.8 and GCC 5.3.1 as some of their primary components.
More details on all of today's opt-in flavors of Ubuntu 16.04 Beta 1 can be found via the ubuntu-devel-announce message. Ubuntu 16.04 is scheduled to be officially released on 21 April as the next Long-Term Support release of the operating system. Coming up before then is the UI freeze on 10 March, final beta on 24 March, kernel freeze on 7 April, and final freeze on 14 April.