The Board of Selectmen and the Planning Board will meet jointly Tuesday to fill an open seat on the Planning Board on an interim basis. The seat had been held by Ron Gleason, who resigned this month from the Planning Board and from his selectman’s seat. The two boards will meet at 6 p.m. at Town Hall in the main meeting room. The interim member will serve until the annual town election on April 27. Candidates to run for the unexpired portion of Gleason’s terms on both boards in the town election can take out nomination papers at the town clerk’s office until March 7.