Maersk/BA
To give one example that comes to mind straightaway of a route initially started by Maersk and then 'transferred' to BA would be BHX-FCO. I think Maersk operated 737s down to FCO for a while before BA operated it with their AR1s.
I'm pretty sure there were other routes similarly swapped from Maersk to BA but my memory is hazy. I can't think of (m)any that were transferred the other way...
Scott