The practice of lining-out may be found now in some parts of the outer Hebrides, and in some parts of Alabama. There is a logical fallacy in assuming a specific influence from one to the other, transmitted around 1740. The question that needs to be asked is, who was singing in this call and response fashion in the eighteenth century in the southern United States.
    This all reminds me of the famous assumption that there is something "Celtic" about bagpipes, because they seem to be found more commonly in Ireland and Scotland than in England. Of course, the historical fact is that they were common in all those countries, but cultural changes caused the use of the pipes to die out in most of England. The same question needs to be asked here: was lining-out unique to Gaelic-speaking Scottish islanders in 1740? And the answer, I would suggest, is probably "no".