When I was at school (many years ago!) we were taught that the difference between a species and a strain is that the former can't cross and produce fertile offspring, horse X donkey = mule being the usual example. So while Apis mellifera mellifera can cross with Apis mellifera ligustica and produce fertile drones and queens, a cross between, say, Apis florea and Apis dorsata, even if done in a laboratory, won't work.
Chris
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