Section 12. In the regional planning and economic development district established pursuant to section nine, there shall be a regional planning and economic development commission which shall consist of the mayor of each city, or in a city that has a Plan D or Plan E form of government the city manager, or their designees, a member appointed by the board of selectmen of each town included within said district who need not be a member of the board of selectmen, a member appointed by the planning board of each such city or town who need not be a member of the planning board. Each member shall be appointed for a term of one year commencing June first of the year appointed to May thirty-first of the succeeding year, and the appointment or reappointment of each member shall be certified annually to the commission by the appointing authority in the month of May. Notwithstanding any of the foregoing provisions, the commission may increase its membership by not more than six in order to provide representation to low-income and minority groups.