In the genus Rhododendron polyploid species are common especially in deciduous azaleas and lepidotes. Yet complete reproductive isolation of the poylpoid species from the closely related diploid species is the exception rather than the rule.
Closely related diploid and tetraploid species interact in natural contact zones to create triploids and some triploids are able to reproduce with both populations.
Hybrid triploid lepidotes seem to freely set small amounts of viable open pollinated seed.
Tetraploid and hexaploid species and hybrids appear to interact even more freely than diploids and tetraploids. Pentaploids are often semi-fertile.
For Rhododendron, ploidy level creates a barrier to sexual interaction but that barrier is not absolute and is sometimes surprising porous. In fact the triploids may act as a pathway in the transition from diploid to tetraploid in that 2x (reduced) X 2x (unreduced) can create 3x and 2x (reduced) X 3x (unreduced) can create 4x. Moreover, C D Darlington established that triploids can throw 1x and 2x gametes so 3x (reduced to 2x) X 3x (reduced to 2x) can produce 4x or 3x (reduced to 1x) X 3x (unreduced) can produce 4x. Darlington documents the historical development of garden hyacinths from diploid species to triploid and then to tetraploids using a triploid pathway.
The development of tetraploid hybrid elepidote Rhododendron 'Countess of Derby' also illustrates a triploid pathway.
One cannot rule out the possibility that the ability of the diploid species to interact with newly created polyploids actually contributes to the creation of the tetraploid species.
The diagram below illustrates how the triploid 'Pink Pearl' has produced diploid, triploid, and tetraploid offspring when interacting with diploids and has produced a tetraploid offspring by interacting with another triploid.
The flow cytometry for this research on the genus Rhododendon was performed by Dr. João Loureiro, Dr. Silvia Castro, José Cerca, and Mariana Castro Plant Ecology and Evolution Group, Centre for Functional Ecology, Department of Life Sciences, Faculty of Science and Technology, University of Coimbra, Portugal.
Meiosis in Polyploids
by W. C. F. Newton and C. D. Darlington
in Journal of Genetics 1929
The history of the garden hyacinths
by C D Darlington, J B Hair and R Hurcombe
in Heredity 1951
Ploidy level estimations in deciduous and elepidote hybrids of Rhododendron
by José Cerca de Oliveira, Mariana Castro, Francisco J. do Nascimento, Sílvia Castro, John Perkins,Sally Perkins, João Loureiro in Jornadas Portuguesas de Genética, Coimbra, Portugal; 05/2011
Weighing in: Discovering the ploidy of hybrid elepidote rhododendrons
by Sally Perkins, John Perkins, José Monteiro de Oliveira, Mariana Castro, Sílvia Castro, João Loureiro in Rhododendrons, Camellias and Magnolias, Royal Horticultural Society, Editors: Simon Maughan, pp.34-48 2012
Untersuchung des Ploidiegrades elepidoter Rhododendron-Hybriden by Sally Perkins, John Perkins, Mariana Castro, José Cerca De Oliveira, Silvia Castro, João Loureiro in Rhododendron und Immergrüne , Deutsche Rhododendron-Gesellschaft e.V. , Seite 21: pages 21-42; 11/2013