Upon the initial formation of a county annexation review board the governor shall give written notice of such formation to all the nominating sources designated therein and nominations must be filed with the office of the governor within fifteen days after receipt of such notice. Nominations to fill vacancies caused by expiration of terms must be filed at least thirty days preceding the expiration of the terms. When vacancies occur in the membership of the board, the governor shall solicit nominations from the appropriate source and if none are filed within fifteen days thereafter, the governor shall fill the vacancy by an independent appointment.