[CliffordK (OP)]

I've been thinking of planting a cherry tree (two?)

About half the cherry trees available are self-pollinating, and about half of them require a separate pollinating tree of a slightly different variety.

Why?

I presume part of the issue is the use of clonal lines from grafting, so that two Bing cherries would essentially be genetically identical, something that would not occur in the wild.

What is the mechanism for a tree to reject its own pollen?

[Don_1]

I wonder if this has some evolutionary purpose. It seems all the sweet cherries are self pollinating, but not all the acid cherries. Could it be that once a variety has become sweet, it no longer cross pollinates with other trees in order to ensure that it retains its sweetness, thereby being more attractive to fruit eating animals which will spread its seed. Some of those which are acid may be able to self pollinate, but might be open to cross pollination in order to improve on the sweetness of the fruit.

Other than that, I really have no idea.

[CliffordK (OP)]

I was looking for sweet cherries.  Around here, one of the most popular sweet cherries is the Bing Cherry which requires a pollinator. 

I will likely only get a single tree of any variety, so all it means is that I need to buy at least two trees (or one tree grafted with multiple varieties).  But, a commercial orchard could potentially have hundreds of clonally propagated trees so having additional pollinator trees would be important.

In the wild, one would not have the clonal orchards so it would not be as big of an issue except that a single solitary tree would not be viable.

I will try to look at lists of trees and try to determine the characteristics of the ones requiring a pollinator.  Perhaps the dark colored cherries are more likely to have the requirement. 

Anyway, I find it interesting that they have evolved a mechanism to reject "self pollination".  Perhaps with antigens? 

It apparently is a different mechanism than is used with male sterile corn, as two non-self--pollinating trees of different types can cross pollinate.