Do these pentaploids involve hybridization of different taxa? If so, at higher ploidy levels than triploid, there may be increased fertility through changes in meiotic behavior in pairing of homologous (from the same species) chromosomes vs. homeologous (from different species) chromosomes, and subsequent production of a higher fraction of gametes with a viable chromosome complement. 

Chromosome counts of fertile shoots may not be pentaploid. This needs to be examined.

hi John,

General, the fertility of triploid plants is because the distribution of chromosomes at metaphase. it is always trivalents. These mating of chromosomes is randomly distributed where group dives 1n and another group gives 2n. These ratio always have very little chance to go each pole. So, the most probability is 1n- and 2n- causing aneuploid plants. In triploids, you can notice: (1) ovules have more fertility than pollen grains, (2) the fertility is decreased in triploids increasing the number of (n). 

Seedlings resulting from crosses of 3x X 4x for deciduous azaleas have flow cytometry scores of 3x, 4x, 5x, and aneuploids from 3x to 4x.

Flow cytomtery of seed from crosses of 3x X 4x has shown male contributions of 2x but female contributions of 1x to 2x centered at 1.5x.

Do these pentaploids involve hybridization of different taxa? If so, at higher ploidy levels than triploid, there may be increased fertility through changes in meiotic behavior in pairing of homologous (from the same species) chromosomes vs. homeologous (from different species) chromosomes, and subsequent production of a higher fraction of gametes with a viable chromosome complement. 

As Ploidity increased chromosome unequally Separation of chromosomes at Anaphase to poles in meiosis division is occurred, which usually accompanied by chromosome aberration  

Patrick McIntyre's answer sounds good if the triploids and pentaploids are interspecific hybrids, but this an important point that you gave no information about. However, as you specified the genus Rhododendron and did not specify a species then I assume that you are referring to hybrids. Perhaps you could let us know some more details. 

Patrick and Keith 

There are15 closely relate species of east coast deciduous azaleas. 10 species are diploid and 5 species are tetraploid. All the diploids have similar flow cytometry scores. All the tetraploids have similar flow cytometry scores which are approximately twice the scores for the diploids. Flow cytometry of seed indicates that meiosis of tetraploid species is more regular than the meiosis for the diploid species. 

The pentaploid came from a cross of a triploid X tetraploid. Most of the seedlings from this same cross were aneuploids between triploid and tetraploid. The triploid seed parent resulted from a cross of diploid X tetraploid where the diploid was a cross of several diploid species and the tetraploid was a species. The tetraploid pollen parent was the result of crossing 2 different tetraploid species. 

This pentaploid proved fertile in both attempts at applying pollen from different tetraploid species. Other pentaploids from this cross have also been fertile but flowering on these has been limited. The seedlings from the triploid X tetraploid are only 3 years old. Deciduous azalea often do not flower when grown outside here in Salem, NH until they are 6 or more years old.

We have not done flow cytometry on seedlings of the pentaploid X tetraploid but hope to fall of 2016.

In short the pentaploid contains several different deciduous azalea species. Moreover since the deciduous species species are often in contact zones with other deciduous azalea species of the same and/or differing ploidy levels it is not always clear when one is using a "pure" species in a cross.

Worth mentioning is Rhododendron vaseyi is not closely related to the other east coast deciduous azalea. Rhododendron canadense although able to reluctantly cross with some east coast deciduous azaleas has a flow cytometry  score much different from the other diploid east coast deciduous azaleas.

Also worth mentioning is natural contact zones between diploid and tetraploid east coast deciduous azaleas often contain triploids, tetraploids showing introgression from the diploids, diploids seeming to have introgression from the tetraploids, and some fertile triploids. Diploids and tetraploids exhibiting introgression are often in close proximity with the fertile triploids indicating that the triploid bridge may be a factor in bidirectional interploidy introgression in east coast deciduous azaleas. How much these fertile triploids are a factor in specialization of the east coast deciduous azaleas is an open question but interaction between the diploids, triploids, and tetraploids does seem to occur naturally.

Hand crossing indicates that diploid deciduous azaleas are much more likely to set seed after accepting pollen from the tetraploid deciduous azaleas than the tetraploids are to set seed after accepting pollen from the diploids. Nearly all seedlings from hand crosses of 2x X 4x deciduous azaleas are triploids but rarely tetraploids and even more rarely diploids do occur from such crosses.

I can only think of the same answer as Patrick. Intriguing question. We are working on fertile pentaploids of interspecific blueberry crosses. Unfortunately we don't have diploid or triploid Vaccinium here in NZ.

When we have done flow cytometry on seed collected on open pollinated triploid deciduous azaleas, the ratio of the embryo to the endosperm is very close to 2/3 indicating equal contribution from the 2 parents yet the flow cytometry score for the embryo does vary from seed to seed.

Have not worked with Rhododendron species but in the Malvacea it is clear that the female parent calls the shots and even odd ball combinations can be made occasionally if the right female parent is chosen .  The variations on successful fertilization depend not only on chromosome level but on what the mother plant wants to accept , even if the chromosome levels are equal or at least balanced such as 4x x 6x , fertilization is not guaranteed and frequently failure is the most common occurrence .  In short chromosome matching only tells part of the story .  In the Amaryllidacea  plant A x plant B works well,  the reverse cross  plant B x plant A results in no fertilization .  There are layers upon layers of successful interaction that are not readily apparent .  I am thinking that what you are running into is that the 5x plants have some features that the 3x do not .  Keep trying , I have no idea on how to further elucidate what the inhibitory factors might be .

Ranjith

As the Vireya moved from the mainland of Asian to New Guinea, the flow cytometry scores for the 300 or so species linearly increase from 1.2 to 3.8 especially starting at 1.8 where the increase occurs in .1 increments.

Documented chromosome counts indicate the Vireya are all diploids but the flow cytometry scores indicate a marked increase in genome size as the Vireya migrated to the subtropics and tropics.

H. Barnes

We have found in deciduous azaleas 2x X 4x almost always takes and 4x x 2x almost never takes. We personally get about 95% takes when doing crosses of 2x X 4x and so far have never gotten a cross of  4x X 2x to take in 100 or so attempts.

this reminds me of a particular Hibiscus cross between two species ,  after about 500 failed crosses I have managed to get 2 fruit with a total of 11 plants that are hybrids . They have not bloomed yet  time will tell as to what the results are .  Dr. Tom Ranney at NCSU , Fletcher , NC has found that ploidy levels also affect the degree at which certain graft combinations take , a finding that seems obvious now but not one that those of us in the production world would automatically think of .

What William Barnes & John Perkins say is notworthy. A male with B female may work but (the reciprocal cross) A female with B male may not work, regardless of ploidy levels. This may be related to EBN (Endosperm Balance Number) or may be some other block to fertilisation by the female. Sometimes the pollen tube can grow all the way down the style, only to be rejected as incompatable.  These other factors need to be considered as well as ploidy level and these sort of things are often complex, unfortunately, and not easy to unravel.

I hope this helps

as to what Keith said , my point exactly , ploidy levels only tell part of the story , as per John's question, there might be a connection between the incompatibilities or compatibilities for that matter similar to that of the grafting work that T. Ranney uncovered . It is a really complex issue with several factors working at the same time

William and Keith

Have either of you seen any evidence for a female only accepting a partial contribution from the male?

We have some evidence that evergreen azalea X deciduous azalea only results in a partial contribution from the male. We have some evidence for polyploid ledum X polyploidy lepidote also resulting in only a partial contribution from the male. We speculate this is happening after fusion due to irregular mitosis. 

We know that neotetraploid Rhododendron seed parents can produce diploid offspring in the first generation. We have seen the same neotetraploid seed parent produce diploid, triploid, and tetraploid first generation offspring in controlled crosses.

from my friend , Tom Ranney at NCSU , Fletcher , NC , 

In some genera  4x x 2x (Citrus) seem to work better and in others 2x x 4x sometimes work better (Rhododendron).  Not sure why.  There is the endosperm balance theory that suggests the former should work better, but that doesn’t really hold all the time.  We generally try to go both directions and it can vary on a clone by clone basis as well. 

We definitely find that pentaploid Magnolias are more fertile than triploids, particularly when used them as males.  I always figured that plants with higher ploidies are more tolerant of aneuploidy as they have developed gene regulation and transcriptions systems to handle multiple/redundant  chromosomes, so plus or minus a chromosome here or there can be compensated for as long as there is one for each base.  Some transcription may still be gene dose dependant. I suspect the reason anisoploids are better male parents is just a matter of odds with a lot more opportunities for random segregation in meiosis to get a few pollen grains that actually work.

 Tom is on top of this a whole lot more than I am , what he says here makes sense to me . 

My understanding is that the EBN (Endosperm Balance Number) is more than a theory, but is an established fact. Am I wrong? It is a long time since I worked on hybrid or polyploid plants, but I am still interested. EBN can seemingly explain why an interploidy cross may work one way, e.g. diploid X tetraploid but fail to work in the opposite way e.g. tetraploid X diploid, if the two lineages have  different EBNs. Not all plants of the same species necessarily have the same EBN, although they usually do, but the EBN can sometimes be variable in a species, just as other characteristics can vary within a species.

I think the reason depends on the fact that in case of pentaploids there is a high chance for chromosome balances by forming frequent number of bivalents and tetrad gametes which are usually conserve fertlity, moreover there is great tolerance to effect of the deficient chromosome pairing or synapses.

simply not enough material to go around in a triploid ,  I do have a question about all of this .  If a diploid has  3x endosperm , what is the chromosome dosage in a 3x or a 4x , does it remain 3x or does it move on to become  4x and 5x respectively .  Has anyone looked at this?