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I am receiving conflicting information while researching polyploid speciation. On the one hand, some sources state that a polyploid can only reproduce sexually with polyploids having the same number of chromosomes. But I am also finding information claiming tetraploid x diploid reproduction in orchids, or tetraploid x triploid reproduction of orchids (for example,see American Orchid Society, AOS.org).

Is polyploid sexual isolation always the case, sometimes the case, or never the case? Can someone clarify this?

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  • $\begingroup$ Welcome to Biology.SE. That would help if you could link to all your sources. $\endgroup$ – Remi.b Aug 18 '15 at 17:13
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Ploidy and reproductive isolation in plants

Speaking generally, there is simply no question that for flowering plants there can be a sexual pathway between diploid and polyploidy levels. The question is how wide is the bridge and how much flow is there in either direction. In other words the questions become "How much flow?", "How meaningful the flow?", "How much flow back to the original diploid and polyploid populations?", and "How often over the last 200 million years has such interaction been an important factor in the evolution of flowering plants?".

Examples from Rhododendron

In the following, I will give a number of examples from the plant genus Rhododendron to show how ploidy level can function in relation to sexual isolation. In this taxa, diploid X tetraploid normally produces triploids although occasionally a tetraploid or even a diploid offspring is produced. The triploid offspring of diploid deciduous azalea species X tetraploid deciduous azalea species often look very similar to the tetraploid pollen parent. 'Margaret Abbott', which is a seed fertile tetraploid deciduous azalea, has a fragrant white flower produced by crossing a fragrant pink flowered diploid seed parent with a non-fragrant yellow flowered pollen parent.

Our research on hybrid elepidote Rhododendron indicates that triploids, although reluctant to reproduce sexually, can produce diploid, triploid, tetraploid, and pentaploid offspring when crossed with pollen from diploids, triploids, and tetraploids. 'Phyllis Korn' is an example of a fourth generation triploid. 'Graf Zeppelin', which exhibits polyploid characteristics, is an example of a diploid offspring of a triploid. 'Countess of Derby' is an example of a tetraploid offspring where both parents are triploids. 'White Ginger' is a an example of a pentaploid offspring of a triploid. 'Pink Pearl' is an example of a triploid that has produced diploid, triploid, and tetraploid offspring from interaction with diploids.

In deciduous azaleas triploid X tetraploid crosses often produce aneuploids between triploid and tetraploid. Such crosses can on occasion produce pentaploids.

Our limited experience working with triploids and pentaploids as seed parents indicates that in general pentaploids are better seed producers than triploids.

An interesting finding is neotetraploids can produce diploid offspring in the first generation. In general tetraploids can produce diploids, triploids, tetraploids, and pentaploids when crossed with pollen from diploids, triploids, and tetraploids.

Triploids are common in natural contact zones where diploid deciduous azalea populations overlap with tetraploid deciduous azalea populations. There is strong evidence indicating there is gene flow from the diploid population to the tetraploid population in these contact zones. There is some evidence of gene flow from the tetraploid population to the diploid population. There is some evidence to suggest multi-generational triploids exists in these contact zones.

Worth noting is that the meiosis for triploid deciduous azaleas that do set seed is distributed around 1.5x gametes ranging from 1x to 2x with an occasional 3x gamete. This uneven distributed splitting of triploids is what produces aneuploid offspring between 3x and 4x when semifertile triploids are crossed with pollen from stable tetraploids. Cyril Dean Darlington in the late 1920's and early 1930's documented this uneven distributed splitting of triploids during meiosis. Evidence suggests that neotetraploids can also undergo uneven distributed splitting during meiosis.

Darlington's history of the garden hyacinths showing a triploid pathway from diploid species to tetraploid hybrids parallels the history of the hybridization of the tetraploid elepidote Rhododendron 'Countess of Derby'.

We have some evidence for deciduous azaleas that 3x X 4x can produce aneuploids close to 5x indicating that some of the "3x gametes" are coming from first an uneven splitting of the triploid followed by a failure to reduce in a later phase of meiosis. However this result of aneuploidy near pentaploid has not occurred often enough to be certain that is what is happening.

In general for deciduous azalea species;

  • diploid X diploid normally produce seedpods and high percentage of viable seed.

  • tetraploid X tetraploid normally produce seedpods and high percentage of viable seed.

  • diploid X tetraploid normally produce seedpods but a highly variable percentage of viable seed.

  • tetraploid X diploid almost never set seedpods.

  • triploid X tetraploid sometimes produce seedpods and a low percentage of viable seed.

For hybrid elepidote Rhododendron, 2x X 2x, 2x X 3x, 2x X 4x, 3x X 2x, 3x X 3x, 3x X 4x, 4x X 2x, 4x X 3x, and 4x X 4x are all documented to have occurred.

For deciduous azaleas, we have produced viable seed from 2x X 2x, 2x X 4x, 3x X 4x, 4x X 4x, 4x X 6x, 4x X 8x, 5x X 4x, 6x X 4x, and 6x X 6x.

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.

The ancestry for 'Phyllis Korn' illustrates that triploids can be multi-generational.

Four Generations of Triploids

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.

Offspring of the Triploid 'Pink Pearl'

The family tree for the Rhododendron elepidote hybrid 'The Duchess' illustrates that interploidy interaction can be multi-generational.

The family tree for 'The Duchess' illustrates that interploidy interaction can be multi-generational\

Picture of 'Margaret Abbott' with a quote from Frank Abbott.

'Margaret Abbott'

Picture showing how much the offspring of a diploid crossed with a tetraploid looks like the tetraploid pollen parent.

Triploid Offspring Looks Like Tetraploid Pollen Parent

Picture showing the successful development of seedpods from crossing a diploid by a tetraploid.

Diploid X Tetraploid Produces Seedpods

Picture showing the failure in developing seedpods from crossing a diploid by a tetraploid.

Tetraploid X Diploid Fails to Produce Seedpods

Picture showing interploidy hybrid swarm at Audra State Park, WV.

Interploidy Hybrid Swarm at Audra State park

References

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

Rules of Engagement: Have Pollen—Will Travel by John Perkins · Sally Perkins in Azalean 2010

Frank Abbott's Village of Azaleas by John Perkins · Sally Perkins in Journal American Rhododendron Society 2011

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

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  • $\begingroup$ Very comprehensive and well referenced +1. I edited your answer to bring the general conclusion (for plants) to the forefront, and added a couple of headings. I hope you are fine with my suggestions - otherwise feel free to edit or rollback. $\endgroup$ – fileunderwater Aug 24 '15 at 23:05
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Depending on the species the hability of generating hybrids will be different.

Some can, others don't. However generating a hybrid of a diploid and a tetraploid will give rise to a triploid, usually sterile plants (as it happens with seedless whatermelons, which are a cross between tetraploid and diploid).

This is where most of the isolation comes from, as these triploid hybrids are sterile (they can't undergo meiosis because of the control systems that require an even amount of chromosomes) they are not a viable species, and thus a genetic barrier begins.

I don't know specifically on the Orchids though.

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