The Governor shall appoint and designate the terms of seven commissioners to represent this State on the Wabash Valley Interstate Commission. Of the Commissioners first appointed, two shall serve for terms expiring April 1, 1961; two for terms expiring April 1, 1963; and three for terms expiring April 1, 1965. At the expiration of the term of each commissioner and of each succeeding commissioner, the Governor shall appoint a successor who shall hold office for a term of six years beginning April 1 of the year in which the preceding term expired. Each commissioner shall hold office until his successor has been appointed and qualified. The Governor shall fill any vacancy by appointment for the unexpired term.