>Or it could have been due to some other problem with the hybrid.

Right. I have long thought that the African and European bees may be
so distantly related that they *almost* constitute different species.
Obviously, speciation occurred in the case of Apis cerana. Someday we
may see a reclassification into Asian, European *and* African species.

Why? Because it has been shown that hybrids between AHB and EHB, if
and when they occur, fail to thrive. The same is true for many other
species crosses. Heterosis can be positive or profoundly negative.

It is entirely plausible that African crosses were tested in the north
and simply vanished. If the cause was some genetic weakness, like
asymmetry, we may never know.

Orley Taylor writes:
> Gene flow between neotropical African  and European bees appears to be  strongly asymmetrical. African bees have maintained their genetic integrity, in spite of hybridizing with European bees, as they have expanded their distribution over the last 40 years. Low acquisition of European traits into the African population can be attributed to pre and post zygotic isolating mechanisms, i.e. mate selection, queen developmental time and hybrid dysfunction.  European bees become rapidly Africanized and nearly all traces of the nuclear and mitochondrial genome disappear from the feral bee populations following the arrival of African bees.  The disappearance of the European traits seems to be due to a lack of pre reproductive isolation which results in extensive mating by European queens with African drones. This is followed by a pattern of queen development which favors hybrid rather than European queens. Matings by these F1 queens to African drones results in colonies with low fitness and the eventual loss of European mtDNA from the population. Displacement of European bees  therefore seems to be due, in part, to a type of "genetic capture" in which one form, A. m. scutellata, eliminates the others by hybridizing with their females. This type of displacement may not be uncommon,  a similar case is known for sulfur butterflies.

* > The genetic and population consequences of the interactions
between A. m. scutellata and A. mellifera subspecies from Europe
suggest that A. m. scutellata deserves the status of a semi-species.

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