(1) The pension trust fund must be invested by the board of investments as part of the unified investment program described in Title 17, chapter 6, part 2.
(2) All income earned on any assets constituting a part of the pension trust fund must be paid into the pension trust fund as received.
(3) For investment purposes, the pension trust fund may be commingled with other pension funds administered by the board, but a separate account must be maintained for each system.