I found the solution on Gentoo forum :
sys-apps/systemd contains udev and, then, you can safely let sys-fs/udev be removed as systemd will be the provider for virtual/udev.
sys-apps/systemd and sys-apps/dbus have a circular dependency which necessitates a 2-stage installation.
Install sys-apps/dbus with USE="-systemd". This is probably already done if you have a working desktop environment.
Enable the systemd USE flag globally (make.conf) and install sys-apps/systemd. This may also be accomplished by a world update. The consolekit use flag should also be disabled to prevent conflicts with the systemd-logind service. You can also switch to a systemd subprofile to use saner USE flags defaults not needing to change make.conf:
Thanks to eyoung100 : http://forums.gentoo.org/viewtopic-t-977642-start-0.html