The Sixth Circuit has once again answered this question most recently in Louzon v. Ford Motor Company, No 11-2356 (June 4, 2013):
it is not a prerequisite in discrimination cases that a comparator have the same supervisor as the plaintiff.
In addition to reversing the summary judgment granted Ford by the district court, the Sixth Circuit also vacated a discovery order where the district judge had restricted "discovery based on this erroneous same-supervisor requirement" which the Sixth Circuit had previously ruled in Bobo v. UPS, 665 F3d 741 (6th Cir 2012), amounts to "an improper denial of discovery."
The Sixth Circuit's decision in Louzon was also discussed in previous posts: How Important Is Whether A Comparator Was Working For the Same Supervisor? and Once Again, The Sixth Circuit Rules That A Similarly-Situated Comparator Need Not Have Worked For The Same Supervisor.
The Sixth Circuit's opinion in Louzon was written by Judge Karen Nelson Moore joined by Judge Jane Stranch and Senior Judge Joseph M. Hood of the U.S. District Court for the Eastern District of Kentucky.