The new study goes beyond past investigations by showing that the hybrid offspring produced by mating these beetle types with each other in the lab grow faster than their willow parent but slower than their maple parent when raised on maple leaves, and faster than their maple parent but slower than their willow parent when raised on willow leaves. Evolutionary biologists consider growth rate as a measure of survivability or fitness. So the hybrids' lower fitness reduces opportunities for genetic exchange between beetle types by way of matings between surviving hybrids and the maple and willow beetles themselves. As a result, it promotes the "reproductive isolation" that pushes the two groups toward becoming distinct species.
In addition, the scientists examined what happens when the hybrids mate with pure willow beetles and pure maple beetles. The hybrids all receive half their genes from one parent and half from the other. So, when a hybrid mates with a willow beetle, the genome of their offspring, called backcrosses, have 75 percent willow beetle and 25 percent maple beetle genes. Similarly, when a hybrid is mated with a maple beetle, the genome of their offspring is 75 percent maple beetle and 25 percent willow beetle.
"If the lower fitness of the hybrids were simply caused by overall genetic incompatibility, then both backcrosses should use the two tree types equally well," says Funk. "On the other hand, if genes involved in adapting to one tree type or the other were the cause, then maple backcross individuals should do better when raised on maple than on willow and willow backcrosses should do better on willow than on
|Contact: David F. Salisbury|