I found many papers insisted that prepollination isolation barriers (i.e. eco-geographic, phenology, pollinator) is stronger than post pollination barriers (i.e. pollen-pistil interaction, failure to form hybrid seeds and sterility of hybrid offspring). However, my impression in the species-rich mountains of Himalayas, there are so many congeners co-occurring in same communities, especially these species pairs from large genera, i.e. Rhododendron. These species are super generalists sharing also super generalist pollinators (i.e. Apis cerana and Bumblebees). Logically, post-pollination barriers should be important in these species. Pedicularis may be different but we also detected strong post-pollination isolation in species pairs of this genus.
I have not a complete answer to your interesting question. One point to consider is the level of ploidy within a species, which may cause post pollination failure to the reproduction. For example, you may check the following publications:
- https://www.researchgate.net/publication/12234716_Polypoid_incidence_and_evolution
- https://www.researchgate.net/publication/11493587_Fecundity_and_offspring_ploidy_in_matings_among_diploid_triploid_and_tetraploid_Chamerion_angustifolium_Onagraceae_Consequences_for_tetraploid_establishment
Article Polyploid Incidence and Evolution
Article Fecundity and offspring ploidy in matings among diploid, tri...
In the eastern United States there are natural contact zones between diploid and tetraploid deciduous azaleas. These contact zones often contain triploids and some of the triploids are fertile. There is visual evidence of bidirectional introgression between the diploid and tetraploid populations. There is strong evidence for generic flow from the diploid to tetraploid populations and weaker evidence for the generic flow from tetraploid to diploid populations. Hand crossing has shown that diploid X tetraploid commonly produces triploids for deciduous azaleas. Hand crosses also indicates that some of these triploid offspring are fertile. In some instances, such a triploid will be very fertile. The offspring of these fertile triploids are triploids, tetraploids, pentaploids, and aneuploids between triploid and tetraploid. Hand crosses of fertile triploids with diploid pollen has produced seedpods but to date we have not done flow cytometer to confirm diploid offspring from such crosses.
Pentaploid deciduous azalea appear to be more fertile than triploid deciduous azaleas.
We have confirmed, using flow cytometry that for elepidote Rhododendron, fertile triploids can produce diploid, triploid, tetraploid, and pentaploid offspring.
Experiment Findings More Weighings: Exploring the Ploidy of Hybrid Elepidote Rhododendrons
Presentation Strategies for Deciduous Azalea Hybridization Success
Yes, pre pollination isolation barrier is stronger, than post pollination barriers.
Generally, the more important for isolation is a morphological pre-pollination barrier (reflected in their flower structure), and it depends of eco-geography. The first reaction on climate changes - is not good development of flowers (metamorphose, monstrously and terratology), and more later - polyploidy or hybridization.
see
In the case of mountains very old genera as you study, Rhododendron, having many generalist pollinators, the fruits is composed from tree types of seeds: with one nucleus, - with two nucleus & - without the nucleus.
M.Telepova Metamorphoses in researchgate
For North American deciduous azaleas, seed is much more likely to develop for 2x X 4x than for 4x X 2x. 2x x 4x almost always produces seedpods. 4x X 2x almost never produces seedpods.
Article Rules of Engagement: Have Pollen—Will Travel
Yes, I agreed with Marpha Telepova-Texier in certain extent. Pre pollination isolation barrier is stronger, than post pollination barriers. But we need to consider about strength of pre-zygotic self incompatibility and post pollination self-incompatibility barriers. Some times post pollination incompatibility is more stronger than morphological prezygotic barriers.
In relation to reproductive biology of sympatric polyalthia species, you can refer following articles. They share common pollinator and common floral syndromes.
I have always assumed that post-pollination barriers are stronger, for the reasons you suggest: closely related congeners that share pollinators are relatively common. However I don't know if the question has been addressed often enough to really answer it properly across a wide range of taxa.
Could be a very interesting topic for a quantitative review or meta-analysis to address this question in relation to regional plant diversity, if enough studies were available.
Dear Prof. Ollerton, I agree, this is what we should discuss when we are in Yunnan this coming summer. Reproductive isolation of congeners without pollination syndrome shifting is a really interesting topic.
Yes, let's do that. In the past I've framed this as a question about "pollination niche" overlap - see for example our 2003 Annals of Botany paper on grassland asclepiads in South Africa. Lots of pollinator sharing in these taxa.
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