It "can" be anything if the person at the airline reservation office are willing.

AA have issued me a Oneworld RTW starting on BA with QF as the first intercontinental sector. Similarly AA were happy to have my travel agent in CPT issue a ticket on AA stock with the first segment CPT-LHR on BA. BA had already applied a price rise that AA had not yet enacted and AA were OK with having it issued on their stock at the lower price