> Not sure we can classify SMR on the main as the result of intense inbreeding.

I am not sure we can rule it out, either. 

It is logical that parasites would seek vigorous hosts and eschew anemic ones. If inbred pupae are substandard in some way that makes them less attractive, that would lower the incidence of varroa in those cells. 

The point is, though, the SMR trait was amplified by researchers by using lineages that expressed this trait, to the point where they were too inbred to survive. Then outcrossed with "wild type" stock to restore viability. 

Similar effects were seen by researchers who developed the Starline and Midnite hybrid bees. They crossed and developed inbred lines, and then crossed separate inbred lines to produce "hybrid vigor" which supposedly expressed itself in highly prolific honey bee colonies. 

PLB

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