This has been decided via a number of lawsuits through the years in a number of states. Also every state nursing practice law recognizes nurse anesthesia as the practice of nursing.
A Nurse Anesthetist does NOT have to have an MDA supervising them as they are considered independent practitioners in most states.
In some areas you have all CRNA groups practicing with no MDAs even on staff at the facility.
In the military you often find CRNAs working without MDAs. For example, prior to the BRAC, many USAF hospitals were staffed with two CRNAs and no MDAs. Another example, during the Gulf war at Jeddah, Saudi Arabia there were 8 CRNAs and no MDAs. CRNAs were also heavily used for air evac due to their increased critical care skills.