There is a body of literature on pollen abnormalities of hybrid plants. The mechanism that causes deformed pollen is also very well known, being abnormalities during meiosis such as nondisjunction of homologous chromosomes, non-pairing of the same, faulty synapsis, etc.

One of the questions often asked in Botany PhD exams is, Why is the pollen of hybrids often not fully fertile, with many malformed grains? Another question asked of botany students is, What mechanism could explain 50% pollen sterility in a hybrid plant?

The general answer is: Various chromosome abnormalities, and depending on the degree of abnormalities, the developing spore may abort (undergo apoptosis, cell suicide) early on or late, potentially yielding a variety of sizes and deformations, as well as full-size apparently normal-looking grains that are actually dead inside.
Measuring the percentage of non-living (= often distorted, malformed, or small) pollen to estimate meiotic incompatibility/ hybridization has been a standard technique for more than 50 years [references to follow] to detect hybrids. The stain lactophenol-cotton blue is most usually used for this [reference?].

A recently published species (2004, in journal Novon 14), Passiflora subfertilis, mentioned hybridization/ pollen fertility and used that lactophenol-cotton blue test for pollen viability.

Most of the work, by far, with hybrid pollen has been done with the light microscope and was done in the 60s, 70s, and 80s I think, and did not study changes in aperture development or exine sculpturing that can be seen with SEM. No studies have been published about passiflora. The development of the grain wall is an interplay of the mother and the baby, mostly the baby, so probably this "hybrid-distortion" provides clues to how the pollen grain wall actually develops. If one knows the parents, can one predict the kind or degree of distortion? Who knows? But remember, if even 1% of the grains are functional, you might be able to use it to create further hybrids... you just might have to pollinate many many flowers to get one to take.

Text © 2005 Dr John MacDougal