Ipomoea purpurea will only cross fertilize with other Ipomoea purpurea or possibly with the youjiro hybrids which have some I.purpurea in them...

The seedpods of Ipomoea purpurea reflex shortly after fertilization so that the top of the seedpod capsule faces the ground and this feature makes it relatively easy to identify I.purpurea species...

Hope that helps...

Ron

Here is a link that might be useful: Ipomoea purpurea seedpods reflexing to face the ground

How did they produce hybrids of I.purpurea if it won't cross fertilize with other Ipomoea species?

timtheimplanter - My response was intended to indicate what will cross fertilize under ordinary open pollinated conditions...

The 'how' of true inter-specific Ipomoea purpurea hybrids = Dr.Yoshiaki Yoneda who successfully hybridized Ipomoea purpurea with Ipomoea nil under very professional conditions in a highly funded laboratory in 1995...

Check out Dr.Yoneda's website for a fascinating look into the depths of the Morning Glory World...check out the interspecific hybrid section...

Cheers...

Ron

Here is a link that might be useful: Dr.Yoshiaki Yonedas Morning Glory Encyclopedia database website