The present members of the commission shall hold office for the balance of their tenure. Of the members of the commission newly appointed, pursuant to this chapter, the first member appointed, if one be appointed, shall hold office for the term of one year, if a second member is appointed that member shall hold office for the term of two years, if a third member is appointed that member shall hold office for the term of three years, if a fourth member is appointed that member shall hold office for the term of four years, and if a fifth member is appointed that member shall hold office for the term of five years from and after that member’s appointment. Thereafter, the members shall be appointed for terms of five years. The terms of the ex officio members of the commission shall correspond to their respective official tenures. If a vacancy     occurs otherwise than by expiration of a term, it shall be filled by appointment for the unexpired portion of the term.