H: Friday, October 21, 2011

Weighing In: Discovering the Ploidy of Hybrid Large Leaf Elepidote Rhododendrons

Weighing In (Published in RHS Rhododendrons, Camellias, and Magnolias Yearbook 2012)

When we mention the word ploidy most gardeners' eyes glaze over. What does ploidy have to do with their beautiful garden specimens of rhododendrons?

Yet if we mention 'Cynthia', 'Gomer Waterer', 'Grace Seabrook', 'Horizon Monarch', 'Marinus Koster', 'Pink Pearl', 'Phyllis Korn', 'Point Defiance', 'Taurus', or 'Trude Webster', gardeners quickly add that these are among their favorite rhododendrons or high on their wish list. Yes, these Rhododendrons display "something different" and "highly desirable."

As dozens more Rhododendrons with larger than normal ploidy levels are revealed below, we hope gardeners see the connection with characteristics of thickness in the leaf and firmness in flower substance. Indeed, remarkable vigor and substance overall, coupled with outstanding floral performance at a young age, starts to make sense. Even gardeners, who do not want to talk about ploidy, love talking about polyploids.

We are not geneticists. We do have science backgrounds with a passion for knowledge. Our ploidy journey began as simple curiosity combined with a willingness to coordinate with others, scour the Rhododendron literature and the web, and do some field work leading to more discoveries than we ever imagined.

First, when we refer to ploidy we mean the size of the genetic material of the plant. Genetic material is, in seed bearing plants, the genetic material found in the nucleus of the cell packed into structures called chromosomes.

There are 2 different techniques used to determine how much genetic material is in a cell and therefore, an estimate of the number of chromosomes that are present in the cell. One can count the ways or weigh the counts.

Count the ways: The classic way to determine the number of chromosomes in a plant is to visualize the chromosomes with stain when they are actively growing, as in a root tip, and then count the different pairs under the microscope. Reports are that this is very tedious, more so in rhododendrons, prone to error, and even eager graduate students are reluctant to cooperate. There are very few studies, mostly older, and even less duplication of results.

Weigh the counts: With the technique of flow cytometry it is possible to weigh the genetic material by taking healthy plant tissue and measuring the weight of the genetic material. This technique is much less time consuming and therefore easier to verify by duplicating results. Flow cytometry was developed to detect mutations in tumors and cancer cells. If the cells are normal and growing there would be a small number of cells with double the weight of their chromosomes as they would be in the phase prior to division. Any cells with less than or more than that weight would be an indication of mutations of the amount of genetic material in the cell (i.e. cancer). This valuable technique can also be used to detect the normal weight of genetic material in different species and hybrids of rhododendrons.

Polyploidy: Beginning with 1, 2, 3, 4, 5

In most plant cells, i.e. leaves, stems, roots and some parts of the flower; the chromosomes are paired with a matching chromosome to form the diploid state. We say most cells because when it comes time to reproduce, the unfertilized seed and the pollen are formed by the splinting apart of the paired chromosomes forming a nucleus with a single set of chromosomes; the unpaired or haploid state. And just to make things complicated true seeds have extra diploid tissue from the seed parent which merges with a haploid pollen nucleus to form the endosperm of a seed. The fertilized endosperm therefore has 3 sets of chromosome (2 from the seed mother and one from the pollen father) and is triploid. This extra genetic material nourishes the germinating seedling.

Most Rhododendrons get one set of chromosomes, denoted as 1x, from each parent (female and male) resulting in two sets of chromosomes, are commonly referred to as diploids, and denoted as 2x (1x + 1x = 2x). However some Rhododendrons have four sets of chromosomes, are commonly referred to as tetraploids, and denoted as 4x (2x + 2x = 4x). Triploids have three sets of chromosomes and are denoted 3x. Pentaploids have five sets of chromosomes and are denoted 5x. Rhododendrons having more than two sets of chromosomes are referred to as polyploids. [reference]

Although most Rhododendron species are diploids, tetraploid Rhododendron species exist [reference]. Individual triploid Rhododendrons, appearing to be hybrids, sometimes occur naturally where diploid and tetraploid species of Rhododendron are co-located. [reference]

Discovering: the Journey

In the fall of 1989, our Rhododendron polyploidy journey unknowingly started when we overheard at a local Rhododendron meeting a statement Frank Mossman wrote in 1972 concerning his hybridization efforts with Rhododendron occidentale: [reference]

"We have found that Rhododendron occidentale will cross with many other rhododendrons or azaleas if Rhododendron occidentale is the seed parent, but Rhododendron occidentale as a pollen parent produces few seed."

We wondered why.

In the fall of 2011, we uncovered that in 1972 Harold Greer wrote the following concerning his hybridization with 'Countess of Derby' to produce 'Trude Webster': [reference]

"If you are one of those who feels that there could be nothing outstanding produced in a pink rhododendron I would have been the first to agree with you. That was until I saw the first bud unfold on the original seedling of R. 'Countess of Derby' selfed."

Both Mossman and Greer had encountered the wonder accompanying the many puzzles presented by polyploid Rhododendrons, so we were in good company.

Starting in the early 1990's, we unknowingly crossed deciduous azalea involving different ploidy levels leading in 2010 to collecting samples of diploid, triploid, and tetraploid Rhododendrons for ploidy testing at the University of Coimbra in Portugal. Each step on this pathway revealed more about the wonderful world of ploidy in our own Rhododendron garden.

Below is a summary of what we discovered, often based on the research, observations, and documentation of many others, about the ploidy of hybrid large leaf elepidote Rhododendrons and the people encountered on our slow but wondrous journey.

Figure 1: Generational Breeding of Polyploid Hybrid Elepidotes

The ploidy of named hybrid large leaf elepidote Rhododendrons as determined using flow cytometry by

Dr. João Loureiro, Dr. Silvia Castro, José Cerca Oliveira, 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

unless otherwise indicated.

Summary of Ploidy

2x Diploids

1000 Butterflies
Anna (Lem, 1952) U
Betty Hume
Cheyenne *
Colonel Coen
Countess of Athlone
Duke Of York
Everlasting *
Fantastica *
Furnivall's Daughter
Gill's Triumph
Goldflimmer *
Goldsworth Orange #
Graf Zeppelin (van Nes, 1934) P
Horizon Lakeside
Hurricane (Whitney, 1960) P
Isabel Pierce
Janet Blair *
J.G. Millais (Waterer, 1915) P
Jingle Bells #
Kathy Van Veen
Kristian's Pink
Kupferberg #
Lady Bligh
Lady de Rothchild
Lady Eleanor Cathcart
Lem's Cameo
Loderi Venus
Madame Carvalho
Maxicat *
maximum Kalamity
Mindy’s Love
Mother of Pearl (sport, 1925) P
Mrs A. T. De la Mare
Mrs Lindsay Smith
Mrs. Furnival
Nancy Evans
Norman Gill
Nova Zembla *
Olin O. Dobbs
Orange Leopard (Brack, 1988) P S
Peach Charm
Peach Recital (Barlup, 1996) P
Phipps Yellow
Pink Prelude
Polar Bear *
Puget Sound *
Red Olympia
Rendezvous (Hachmann, 1968) P S
Stony Brook (Brack, 1988) P S
Summer Peach (Barlup, 1988) P
Summer Wind (Barlup, 1996) P
The Honourable Jean Marie de Montague (van Nes, 1901) U
Viscy #
Vulcan *
Vulcan’s Flame *
White Pearl syn Halopeanum
Wild Affair

3x Triploids

Anita Gehnrich (Gehnrich) UM
Anna Rose Whitney (Van Veen, 1954) F NM
August Lamken (Hobbie, 1942)
Beauty of Littleworth (Mangles, 1884)
Betty Wormald (Koster, 1907) F
Broughtonii (Broughton, 1840) F
Cotton Candy (Henny and Wennekamp, 1958) F UM
Cynthia (Standish and Noble, 1856) F
Dame Nellie Melba (Loder, 1926)
Ebony Pearl (sport, 1966)
Django (Hachmann 1985)
El Camino (Whitney, 1976) NM
Gartendirektor Rieger (Hobbie, 1947)
Gomer Waterer (Waterer, 1900) F DM
Grace Seabrook (Seabrook, 1965) UM
Hallelujah (Greer, 1958)
Hank's Folly (Schannen) NM
Julia Caroline (Brockenbrough, 1990) NM
Lady of Spain (Lofthouse, 1966) NM
Lucky Strike (Van Veen, 1958) NM
Lydia (Greer, 1963) F NM
Markeeta's Flame (Markeeta, 1960) UM
Markeeta's Prize (Markeeta, 1970) UM
Opal Thornton (Thornton) NM
Pearce's American Beauty (Pearce, 1930) F
Phyllis Korn (Korn, 1969) F DM
Pink Pearl (Waterer, 1892) F DM
Platinum Pearl (Greer, 1983) F NM
Rothenburg (von Martin, 1944)
Rwain (Colombel, 1993) F NM
Solidarity (Schannen, 1969) F UM
Steredenn (Colombel) NM
Sugar Pink (Greer, 1960) NM
Super Dog (Bones) NM
Taurus (Mossman, 1962) F UM
Topsvoort Pearl (sport, 1935)
Val d'Aulnay (Croux and Fils, 1984) F
Van (Van Veen, 1930) NM

4x Tetraploids

Antoon van Welie (Endtz, 1930) 3X2
Brigg's Red Star (Briggs) T
Cherry Cheesecake (Briggs) T *
Countess of Derby (White, 1913) 3X3
diaprepes Gargantua (Stevenson, 1923)
Doreen Gale (Sanders) 4X4
Gentle Giant (Sanders, 1992) 4X3
Gorgeous George (Sanders) 4X4
Grand Slam (Greer, 1982) 4X3
Horizon Jubilee (Brockenbrough) *
Horizon Monarch (Brockenbrough, 1981) 2X4
Le Fouesnantais (Colombel, 1997) 4XQ
Legend (Barlup) 4X4
Lem's Monarch syn Pink Walloper (Lem, 1965) 2X4
L'Engin (de la Sablière) 4X2
Marinus Koster (Koster, 1937)
Point Defiance (Lem, 1970) 2X4
Summer Joy (Kehr) T
Super Nova (Briggs) T *
Trude Webster (Greer, 1960) 4x4
Very Berry (Greer, 1988) 4X2

F indicates a fertile triploid.
T indicates a chemically induced tetraploid.
P indicates a diploid with a polyploid ancestor.
S indicates a diploid with a tetraploid parent.
U indicates a diploid with a tendency to produce unreduced gametes.
DM indicates a triploid resulting from a triploid parent.
NM indicates a triploid resulting from a tetraploid parent.
UM indicates a triploid resulting from 2 diploid parents.
2X4 or 4X2 indicates a tetraploid resulting from a diploid and a tetraploid parent.
3X2 indicates a tetraploid resulting from a triploid and a diploid parent.
3X3 indicates a tetraploid resulting from 2 triploids parents.
4X4 indicates a tetraploid resulting from 2 tetraploid parents.
4X3 indicates a tetraploid resulting from a tetraploid and a triploid parent.
4XQ indicates a tetraploid resulting from a tetraploid parent.
* indicates flow cytometry ploidy testing was done by research team lead by Dr. Ranney.
# indicates flow cytometry ploidy testing was done by Tom Eeckhaut.
(,) indicates the name of the hybridizer and date of cross.

Noteworthy is the named form diaprepes 'Gargantua'. There is no evidence that the species diaprepes is tetraploid. ONLY the named form 'Gargantua' has tested as tetraploid. To date, no large leaf elepidote species as a population has tested as tetraploid, but this could change.

For a brief history on how a few of these named polyploid elepidotes evolved read It is a Beautiful Spring Day in 1913 so What Do You Do? See figure 1 above.

Historical evidence indicates that by 1910 the triploids 'Betty Wormald', 'Beauty of Littleworth', 'Broughtonii', 'Cynthia', 'Gomer Waterer', 'Pink Pearl' would have been on most peoples' lists of best large leaf Rhododendrons.

In 1958, George Grace's list of best large leaf Rhododendrons included all but one of these triploids plus the tetraploids 'Countess of Derby' and 'Marinus Koster'.

In 2008, the Siuslaw Chapter of American Rhododendron Society included on their list of best large leaf Rhododendrons the triploids 'Cynthia', 'Dame Nellie Melba', 'Grace Seabrook', and 'Taurus', and the tetraploids 'Grand Slam', 'Lem's Monarch', 'Horizon Monarch', 'Point Defiance', and 'Very Berry'.

By 2011, examination of rhododendrons of the year, proven performers, [2], [3], [4], [5], [6], [10], [11], Award of Garden Merit and best in show trusses adds to the "bests" mentioned above the triploids 'Anita Gehnrich', 'Anna Rose Whitney', 'Cotton Candy', 'Ebony Pearl', 'Gartendirektor Rieger', 'Hallelujah', 'Markeeta’s Prize', 'Platinum Pearl', 'Solidarity', and 'Super Dog' and the tetraploids 'Gentle Giant' and 'Trude Webster'.

'Pink Pearl' won the first Award of Merit in 1897 and was selected Rhododendron of the Year in 2006 by the Southwestern Chapter of the American Rhododendron Society. In 1950, a large 'Cynthia', bred in 1858, was the first Rhododendron planted in the Crystal Springs Rhododendron Garden. 'Trude Webster' won American Rhododendron Society's first Superior Plant Award in 1971 and is still found on lists of proven performers for the west coast. 'Broughtonii', bred in 1840, is still considered to be among the best warm weather Rhododendrons according to Burke who gardens in Australia.

In other words, over twenty-five of the fifty or so confirmed polyploid large leaf Rhododendrons have appeared on lists of the best Rhododendrons and once these polyploids appear on such lists they tend to make future such "best" lists.

The following hybridizers have worked with or produced polyploid elepidotes hybrids:

Croux and Fils
de la Sablière
Felix and Dijkhuis
Henny and Wennekamp
Standish and Noble
van Nes
Van Veen
von Martin
Winberg and Smith

Noteworthy is that more hybridizers have worked with confirmed large leaf elepidote polyploids than there are such confirmed polyploids. More importantly, some of the hybridizers on this list are best known for the polyploid elepidotes they have created. In fact, in a few instances polyploid elepidotes have been named in honor of a wife, a mother, or a grandparent.

By the way, Mossman working with the diploid deciduous azalea species Rhododendron occidentale discovered what Barlup later discovered working with hybrid elepidotes: diploids are much more likely to accept pollen from tetraploids than tetraploids are to accept pollen from diploids. Our Rules of Engagement addresses this topic in detail.

Jim Barlup wrote the following about using polyploid large leaf elepidotes as parents: [reference]

"I continue to test the pollen and plants which I doubt for 3 or 4 years to determine their fertility or sterility. If you cross a diploid with tetraploid pollen you can achieve beautiful seedpods but their germination is very difficult. 3% seed germination for 'Point Defiance'. Obtained are both diploid or tetraploid offspring."

Breeding with polyploid large leaf elepidotes is not an easy task which explains why so few polyploids have been created to date despite so many hybridizers having attempted to use them as parents.

Ron Naylor wrote the following about his best plant, 'Francis Augustus Storey', from a cross involving 'Point Defiance': [reference]

"'Francis Augustus Storey' - Best of grex of four plants from weak germination. One died in 2000 and another in 2001."

Dick Murcott wrote the following about the plant he calls 'TT116': [reference]

"TT116 – [('Jean Marie' x R. yakushimanum) x 'Grand Slam']. Only one seed from this cross germinated. Looks like a tetraploid. Pink. Looks like 'Trude Webster' but is definitely a seedling."

Barlup, Murcott, and Naylor each encountered both the wonder and puzzles presented by polyploid Rhododendrons.

We have discovered for deciduous azaleas that seed produced from tetraploid X tetraploid normally has high rates of germination but germination from diploid X tetraploid crosses varies greatly but is normally low.

To read about Frank Abbott's encounter with the wonders of working with deciduous azaleas of different ploidy levels see Frank Abbott's Village of Azaleas or 'Margaret Abbott' is a Tetraploid.

The following people and organizations donated samples for this research:

John Abbott,
Vivian Abney of East Fork Nursery,
Living Collection of Arnold Arboretum,
Natural Collection of Audra State Park,
Living Collection of Bartlett Arboretum,
Jim Barlup,
Bruce Clyburn,
Jane Brooks,
Joe Bruso,
Werner Brack,
Ned Brockenbrough,
Natural Collection of Canobie Lake, NH,
Dick Cavender,
Living Collection of Connecticut College Arboretum,
Marc Colombel,
Mike Creel,
Hans Eiberg,
Al Fitzburg,
Robert Fox,
Harold Greer of Greer Gardens,
George Hibben,
Living Collection of Highstead Arboretum,
Don Hyatt,
Lindy Johnson of Appalachian Native Plants,
Richard Jaynes of Broken Arrow Nursery,
Doug Jolley,
Fred Knippel,
Living Collection of Longwood Gardens,
Wayne Mezitt of Weston Nurseries
Ron Miller,
Dick Murcott,
Michael Medeiros of Planeview Nursery, Portsmouth, RI
Peter Norris,
Mike O'Hara,
John and Sally Perkins,
Ron Rabideau of RareFind Nursery,
Ellie Sather of Whitney Gardens,
Natural Collection of Stoddard Blog, NH,
Kristian Theqvist,
Patrick Thompson of Auburn University
John Thornton,
Hendrik Van Oostand of Azaleatuin,
Kathy Van Veen of Van Veen Nursery

We wish to thank everyone who donated samples for this research.

Named Elepidotes Suspected of being Polyploid:

Adriaan Koster
Annie E. Endtz
Arden Primrose
Ariel Sherman
Aristide Briand
Arnold Piper *
Arthur Ostler
Bernard Crisp
Bernard Shaw
Boskoop Concorde
Bruns Sirius
Canadian Beauty *
Cara Meg
Courtenay Duke
Diane Marie
Diane Titcomb
Direktor Siebert
Doctor A. Blok
Doctor Arnold W. Endtz
Doctor H.C. Dresselhuys
Don Juan
Donald Waterer
Doris Nolan
Double Drake
Dr. V.H. Rutgers
Edward Cornelius
Elizabeth Titcomb
Ester Grace
Eureka Maid
Fiona Wilson
Forever Violet
Fragrant Sensation *
Francis Augustus Storey *
George Hardy
Germania *
Gill's Gloriosa
Grab Ya *
Gwen Bell
Hachmann's Anastasia
Hachmann's Kristina
Halfdam Lem
Heat Wave
Heinje's Venezia
Helen Druecker
Horizon Serenity *
Ilam Apricot
Isobel Baillie
Jan Dekens
Janet Ward
Jean Lennon
Jean Marie Variegated
Jeanne Yvonne
Jenice Coffey
Johnny Bender
Julie Titcomb
Justa Pink
Kathe Heinje
Kathy Ann Pieries
Kay Too
Lady Longman
Leonardslee Giles
Loder's White
Lou-John Gem
Madah Jean
Margaret Mack
Maureen Ostler
Miss Kitty
Mrs E C Stirling (sister of Pink Pearl)
Newcomb's Sweetheart
Nicandra Newman
Orrie Dillie
Patricia Jacobs *
Peggy Bannier
Pink Goliath
President Kennedy
Pride of Roseburg *
Princess Debiann
Professor Hugo de Vries
Professor J. H. Zaayer
Qualicum's Pride
Queen Mary
Reverend Paul *
Red Walloper *
Rhododendron niveum
Robert Korn
Shari Laurel
Sheer Enjoyment *
Sierra Sunrise
Souvenier de Docotr S. Endtz
Titness Belle
Twins Candy
Virgo *
Vonnie Stockman
Walloper *
Whidbey Island
White Swan
William Avery *

Despite having created this suspected polyploid list, we believe that over 25% would not test as polyploid. 'Fragrant Sensation', 'Grab Ya', 'Pride of Roseburg', and 'Sheer Enjoyment' having both parents as tested tetraploids are almost certainly polyploids. We have marked using an * the dozen or so we think are the most likely (almost certainly) polyploids.

Most in this list are known to have at least one polyploid parent, be a sibling of a polyploid, or be a parent of one or more polyploids. However, both triploid and tetraploid hybrid large leaf elepidotes are known to be capable of producing diploid offspring when the other parent is a diploid. Many hybrids on our suspected polyploid list have one parent suspected of being a diploid. In other words, a diploid can have a polyploid parent or sibling. Moreover, two diploid parents can produce a polyploid offspring so having a polyploid offspring does not insure either parent is a polyploid.

If one excludes known or suspected polyploid hybrids listed above, creating a list of 100 suspected polyploid named elepidote hybrids, where more than 20% would test as polyploid, would be difficult. In fact, it is highly likely that most attempts at such a list of 100 named elepidote hybrids would include very few if any additional polyploids.

In other words, we speculate that nearly all (over 90%) named polyploid elepidotes hybrids named prior to this post appear in this single post. This is almost certainly the case for polyploid elepidotes hybrids named prior to 2000. The chances there were more than 200 polyploid elepidote hybrids named prior to 2000 are low. The chances there were more than 50 tetraploid elepidote hybrids named prior to 2000 are even lower.

Figure 2: Offspring from Triploids (Pink Pearl)

In short there are no rules of thumb for guessing the ploidy of the offspring for hybrid large leaf elepidotes if the parents are of mixed ploidy levels or either parent is a triploid or pentaploid.

Diploid X diploid will almost always (but not always) create diploid offspring.

Tetraploid X tetraploid will almost always (but not always) create tetraploid offspring.

However, diploid X tetraploid and tetraploid X diploid, which are normally associated with producing triploid offspring, are known to often produce a combination of diploids, triploids, and tetraploids when working with hybrid large leaf elepidotes.

Triploids, Fertile Triploids, and Triploids as the Progeny of Triploids

Triploids are normally believed to be produced by one of two mechanisms.

Two diploids can cross where one diploid parent instead of providing one set of chromosomes provides two resulting in an offspring that has 3 sets of chromosomes. This is commonly referred to as the unreduced mechanism for creating triploids.

Ploidy results suggest that triploids such as 'Anita Gehnrich', 'Grace Seabrook', 'Markeeta's Flame', 'Markeeta's Prize', 'Solaridity', and 'Taurus' were most likely created by this unreduced mechanism.

On the other hand, a diploid parent and a tetraploid parent can cross where the diploid parent provides one set of chromosomes and the tetraploid parent provides two sets of chromosomes resulting in an offspring with 3 sets of chromosomes. This is referred to as the normal meiosis interploidy mechanism for creating triploids.

Ploidy results suggest that triploids such as 'Anna Rose Whitney', 'Cotton Candy', 'El Camino', 'Hank's Folly', 'Julia Caroline, 'Lady of Spain', 'Lucky Strike', 'Lydia', 'Opal Thornton', 'Platinum Pearl', 'Rwain', 'Steredenn', 'Sugar Pink', 'Super Dog', and 'Van' were most likely created by this normal meiosis interploidy mechanism.

Triploids are commonly believed to always be sterile as both seed parents and pollen parents. Yet offspring are documented for triploids such as 'Anna Rose Whitney', 'Betty Wormald', 'Broughtonii', 'Cotton Candy', 'Cynthia', 'Gomer Waterer', 'Lydia', 'Pearce's American Beauty', 'Phyllis Korn', 'Pink Pearl', 'Platinum Pearl', 'Rwain', 'Solidarity', 'Taurus', and 'Val d'Aulnay'. See figure 1.

Triploids such as 'Pink Pearl', 'Phyllis Korn', 'Rwain', and 'Taurus' appear to be partially fertile as both seed and pollen parents. See figure 2.

In fact, triploids can be the progeny of triploids. Based on parental documentation, 'Broughtonii', 'Pink Pearl, 'Gomer Waterer', and 'Phyllis Korn' represent four consecutive generations of triploids. See figure 1.

Three sports of the triploid 'Pink Pearl' were ploidy tested. 'Ebony Pearl' and 'Topsvoort Pearl' tested as triploid whereas 'Mother of Pearl' tested as diploid. See figure 2.

Diploids can be the progeny of triploids. The diploids 'Graf Zeppelin', 'Hurricane', 'J.G. Millais', and 'Summer Peach' are documented to have a triploid parent. In the case of 'Graf Zeppelin', the triploid 'Pink Pearl' is documented as the seed parent. See figure 2. Although a diploid, 'Graf Zeppelin' exhibits characteristics often associated with named polyploids.

Tetraploids can be the progeny of triploids. 'Countess of Derby', a tetraploid, is documented to have 2 triploid parents, namely 'Pink Pearl' and 'Cynthia'. The tetraploids 'Antoon van Welie', 'Gentle Giant', and 'Grand Slam' are documented to have a triploid parent. In the case of 'Antoon van Welie', the triploid 'Pink Pearl' is documented as the seed parent. See figure 2.

Marc Colombel donated some of his suspected polyploid hybrid seedlings for testing. Noteworthy is that four seedlings of 'Rwain' X ‘L’Engin’ tested as tetraploid. 'Rwain' the seed parent, is a triploid. 'L'Engin', the pollen parent, is a tetraploid. Moreover, three seedlings of 'Horizon Monarch' X 'Rwain' tested as tetraploids but one seedling tested as triploid. 'Horizon Monarch' is a tetraploid.

Figure 2 suggests that a triploid parent, for instance 'Pink Pearl', can produce offspring that are diploids, triploids, and tetraploids. Figure 1 suggests that pentaploids are also possible from a triploid parent.

In the 1930's, C. J. Darlington showed that triploids could be fertile. Moreover Darlington confirmed a third mechanism for creating triploids. Darlington showed that during meiosis triploids chromosomes may split forming a bell-shaped curve distribution. This means that although there are a few cells formed with 1x and 2x chromosomes, most are closer to the midpoint of 1.5x. So in a few cases, a triploid parent can act as a diploid contributing 1 set of chromosomes or as a tetraploid contributing 2 sets of chromosomes.

Our ploidy results, when combined with the documentation of parentage, strongly suggest this third distributive meiosis mechanism does occur for fertile triploid large leaf elepidote Rhododendrons.

Hans Eiberg has determined in controlled lab experiments that for Rhododendrons hybrid triploid pollen is sometimes as viable as any hybrid diploid pollen.

Tetraploids and Diploids as the Progeny of Tetraploids

Tetraploids such as 'Doreen Gale', 'Gorgeous George', and 'Legend' have been created by the normal meiosis mechanism where both parents are tetraploids.

Tetraploids such as 'Horizon Monarch', 'Lem's Monarch', 'L'Engin', 'Point Defiance', and 'Very Berry' may have been created by the unreduced mechanism of a diploid parent with the other parent being a tetraploid.

Justin Ramsey's work with newly created neotetraploids suggests that such neotetraploids may experience irregular meiosis. Ramsey suggests that in some instances a neotetraploid may contribute only one set of chromosomes to the offspring. For the purposes of this article, we refer to this as the super-reduced mechanism.

Diploids such as 'Orange Leopard', 'Rendezvous', and 'Stony Brook' may have been created by this super-reduced mechanism. In the case of 'Rendezvous', the tetraploid 'Marinus Koster' is documented as the seed parent.

Noteworthy is that one seedlings of 'Horizon Monarch' open pollinated tested as diploid. The actual plant of 'Horizon Monarch' that was the parent of this particular diploid seedling tested as tetraploid. Other seedlings from the same seedpod tested as tetraploid.

Our ploidy results suggest that tetraploids may produce diploid, triploid, tetraploid, and pentaploid offspring.

To read more about the super-reduced mechanism visit Ploidy Reduction in Blackberries" by Kristine Naess or Cytogentics of Rhubus: Meiotic Instability in Some Higher Polyploids by Maxine Thompson.

Normal, Unreduced, Super-reduced, and Distributive Meiosis: By the Numbers

A diploid Rhododendron has 26 chromosomes. Normally a diploid Rhododendron as a parent splits in half contributing 13 chromosomes to the offspring.

A tetraploid Rhododendron has 52 chromosomes. Normally a tetraploid Rhododendron as a parent splits in half contributing 26 chromosomes to the offspring.

A triploid Rhododendron has 39 chromosomes. Half of 39 is between 19 and 20. Darlington showed that if a triploid having 39 chromosmes were to split it would split mainly 19/20 but, also to ever decreasing occurrences, 18/21, 17/22, 16/23, 15/24, 14/25, and 13/26 where the splitting as 13/26 occurs the least. This splitting would form a bell shaped curve between 13 and 26.

Thus, in principle, for Rhododendrons

diploid X diploid usually results in a diploid since 13 + 13 = 26.

tetraploid X tetraploid usually results in a tetraploid since 26 + 26 = 52.

diploid X tetraploid usually results in a triploid since 13 + 26 = 39.

diploid X unreduced diploid can in a few instances result in a triploid since 13 + 26 = 39.

unreduced diploid X tetraploid can in a few instances result in a tetraploid since 26 + 26 = 52.

diploid X super-reduce tetraploid can in a few instances result in a diploid since 13 + 13 = 26

diploid X triploid can in a few instances result in diploid since 13 + 13 = 26 or in a triploid since 13 + 26 = 39.

triploid X tetraploid can in a few instances result in triploid since 13 + 26 = 39 or in a tetraploid since 26 + 26 = 52.

Noteworthy is other researchers found that the offspring of triploids are often aneuploids. For Rhododendrons, an aneuploid would have a number of chromosomes slightly more or less than 26 (2x), 39 (3x), 52 (4x), 65 (5x) or other multiples of 13 (x=13). The unstable meiosis associated with triploids and neotetraploids most likely means that some of the Rhododendrons listed above as diploids, triploids, or tetraploids do not have exactly 26, 39, or 52 chromosomes but instead may have a chromosome count close to these numbers. Flow cytometry being a method of weighting sets of chromosomes rather than counting the number of chromosomes is not well suited to separating euploids from aneuploids when the samples tested involve interactions between a wide range of species within the same genus.


Named hybrid large leaf elepidote polyploid Rhododendrons have played an important role in the garden for more than 150 years. The physical characteristics associated with polyploid Rhododendrons have proven to be highly desired by gardeners since their introduction by Broughton, Standish and Noble, and Waterer.

The ploidy of more than 100 named hybrid large leaf elepidote Rhododendrons is listed above.

Although to date all species of large leaf elepidote Rhododendrons have tested as diploid species more than 50 named hybrid large leaf elepidote Rhododendrons tested as polyploids.

Approximately 2/3 of the named named hybrid large leaf elepidote Rhododendrons which tested as polyploids tested as triploids with the remaining 1/3 testing as tetraploids.

Triploids can be fertile as both seed and pollen parents. Triploids are able to produce diploid, triploids, tetraploid, and pentaploid offpsring.

Tetraploids are able to produce diploid, triploid, tetraploid, and pentaploid offpsring.

The mechanisms of normal, distributive, unreduced, and super-reduced meiosis are discussed.

This research used as a foundation work done by the following:

Hybridization of Rhododendron Elepidote Polyploids by Jim Barlup

Rules of Engagement: Have Pollen - Will Travel by John and Sally Perkins

Ploidy Levels and Relative Genome Sizes of Diverse Species, Hybrids, and Cultivars of Rhododendron by Jeff R. Jones, Thomas G. Ranney, Nathan P. Lynch, and Stephen L. Krebs

Ploidy Breeding and Interspecific Hybridization in Spathiphyllum and Woody Ornanamentals by Tom Eeckhaut

Meiosis in Polyploids Part I. Triploid and Pentaploid Tulips by W. C. F. Newton and C. D. Darlington

Neopolyploidy in Flowering Plants by Justin Ramsey and Douglas W. Schemske

Posts for each sample ploidy tested are available on the Rosebay Blog. Posts have been grouped using tags to promote easy viewing of related posts. Please weigh in by exploring these posts to discover the wonderful world of ploidy in the Rhododendron garden.

On page 13 of 2001 Rhododendron Camellias and Magnolia Dr. Ray Thornton writes the following:

"I thought that these seedlings might be the record holders for rapid growth but this title instead may pass to 'Lem's Pink Walloper' X R. diaprepes 'Gargantua', from the RHS Seed Exchange, which is growing at a extraordinary rate this spring.

The 2010 Rhododendron, Camellias, and Magnolias references the following as exceptional plants of 2009

Grace Seabrook
Phyllis Korn
Horizon Monarch

Also mentioned are 2 new Award of Merit Winners:

'Brimble' has as a parent the famous R. griffthianum grown by Lionel Foretescue at the Garden House.

'Forest Sprite' is a augustinii X keiskei and the following is noted.

"The cross was tried several times without success, surprising as both parents are from the Triflora series. Eventually just one seedling was reared in 2002."

John and Sally Perkins
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