Lilies forum: Polyploidy & Unreduced Gametes

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Name: Patrick
Midland, Michigan (Zone 6a)
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auratum
Mar 7, 2017 3:51 PM CST
Hoping to provide a “relatively simplistic” explanation of polyploidy & unreduced gametes. I am not a botanist so if someone sees something here that they know is not correct please jump in and offer a correction. I am sure polyploidy has been discussed on here previously so I hope this is not redundant. This came up in another thread but thought it would be good to make a new thread on just this topic. My knowledge/experience in this space is limited to Lilium and there are some differences on this topic for other plant groups - not all plant groups behave exactly the same way but the general principals apply for the most part. We are fortunate that with the commercial value of lilies, they support funding for lots of research of this nature.

Why do I care if a plant is a polyploid?
Unless you intend to breed lilies or grow them from seed the discussion of polyploidy is irrelevant. If your interest in lilies is to simply buy bulbs to grow and enjoy them then you really don’t need to worry about what brand of designer genes your lilies wear. On the other hand, if you fancy the idea of breeding lilies and growing your own hybrids then this concept is important to understand as it may have relevance on how you approach your breeding goals.

Polyploidy:
For Lilium, all species have 24 chromosomes in two sets of 12. Each plant gets a set of 12 chromosomes from the mother or pod parent and a set of 12 chromosomes from the father or pollen parent. The base state is called diploid (2 sets of chromosomes) and the parent cells (egg & pollen) are haploid (1 set of chromosomes) and when they fuse during fertilization create diploid plants. Errors happen in cell division that can lead to more than 2 sets of chromosomes in a plant and when a plant has more than two sets of chromosomes it is called a polyploid. The prefix indicates how many sets of chromosomes it has (triploid = 3 sets, tetraploid = 4 sets, pentaploid = 5 sets, hexaploid = 6 sets, and so on). Errors also cause incomplete/partial sets and extra chromosomes and a plant having an uneven number of chromosomes (something other than even sets of 12) is called an aneuploid.

Details of how we get polyploids:
Polyploids are formed by errors in cell division. There are two basic types of cell division – Mitosis & Meiosis. The process used in normal plant growth is Mitosis where one cell replicates itself to make two identical cells in the meristem tissues – for lilies one cell with 24 chromosomes produces 2 cells with 24 chromosomes. Meiosis is the cell division process used to produce reproductive cells (egg & pollen – also called gametes) and results in cells with half the number of parent cells chromosomes – for lilies one cell with 24 chromosomes produces many cells with 12 chromosomes.

Polyploids from Mitosis:
The process for cell division is complex and there are names for the different stages – if you want more details, pick up a biology book or do an internet search on plant cell division. A very simplistic explanation is that the cell makes a copy of itself inside the existing cell wall and then pulls the chromosomes and organs to the two sides of the cell and then a new cell wall forms to separate the one large double cell into two new small cells identical to the original cell that began the process. If the cell division process is interrupted after the cell has duplicated the chromosomes but before they are pulled to the opposite sides of the cell and the new cell wall doesn’t form this results in a cell with double the genetic information and is a polyploid cell. If that cell can be grown into a whole new plant then you would have a polyploid plant. In most cases the cell would be slower to grow (divide & make copies of itself) as a result of the abnormal cell content and would duplicate more slowly than the other cells around it. Researchers have found that you can artificially cause the formation of polyploid cells by treating plants with various chemicals that disrupt cell division. This is most often done by treating bulb scales to take advantage of the lily multiplication process of growing new bulbs from bulb scales where the callous (meristem) tissue at the base of the bulb scale is capable of regenerating a whole new plant. The chemicals typically used are colchicine & oryzalin (Surflan herbicide). There are many research papers written documenting the process of lily bulb scale treatment. The reason for trying to convert lilies is to use them in breeding – feedback from folks who have done a fair amount of conversions is that the resulting converted plants tend to be weaker than the unconverted plants and difficult to maintain long term. The plants either die off or convert back to diploid (plant was a mix of diploid & tetraploid cells and the diploid cells outgrow the tetraploid cells over time). Most polyploids that exist today were created by converting lilies during mitosis and subsequently used to breed plants that carry the extra sets of chromosomes. While the converted plants tend to be weaker growing, the seedlings are much stronger and perform better. Fertility can be an issue also in the converted plants but can be improved in subsequent generations.

Polyploids from Meiosis:
The process for producing reproductive cells is called Meiosis. The process of meiosis is more complicated than mitosis and details of the process can be found in biology books and through an internet search. Similar to mitosis, the process of meiosis includes the replication of the chromosomes but the replication actually occurs two different times and there are two opportunities for errors to produce polyploid reproductive cells. Meiosis normally starts with a diploid cell and ends with multiple haploid cells. If an error occurs resulting in doubling up of chromosomes as the cells go through the process, the result is diploid egg or pollen cell. In meiosis, unless these cells are involved in fertilization to make an embryo, the cells die through the normal life cycle of the lily and nothing comes of it. If the cells are involved in fertilization then polyploid embryos are formed which can result in polyploid seed. The ploidy level of the seed depends on the ploidy level of the parent cells (egg/pollen). In diploid lilies, the egg & pollen are both haploid n + n = 2n resulting in a diploid where the “n” is meant to represent a full set of chromosomes. If either the egg or the pollen is diploid then the result is n + 2n or 2n + n = 3n & we call this a triploid. To get tetraploids it is most often 2n + 2n = 4n but it is also possible to have n + 3n = 4n. As with mitosis, in meiosis there are different ways the errors can occur to produce the polyploid egg or pollen cells – these cells produced from these errors are called unreduced gametes because the normal reduction in chromosome content did not occur. Unreduced gametes can form as a result of high temperatures, errors in cell division, or due to chemical treatments that interrupt the normal process. There have been documented cases where plants were subjected to high temperatures (with the intent to increase fertility) that produced polyploid offspring (usually triploid). Some hybrids resulting from wide crosses (crosses of lilies from different natural breeding groups) have shown to produce unreduced gametes at high enough frequency to be used in polyploid breeding and I believe this is due to the chromosomes from each parent being too different to allow them to pair normally causing errors in meiosis. Some wide crosses are completely sterile and others have some normal fertility and still others produce unreduced gametes and it is not clear why these different hybrids from different and similar backgrounds behave differently. Lastly, chemicals have proven effective at producing unreduced gametes. Research studies have demonstrated by treating lily buds at the correct stage of development with certain chemicals will disrupt normal meiosis in some of the reproductive cells causing a portion of the gametes to have unreduced chromosome numbers. Chemicals that have been used for this are colchicine, caffeine, and nitrous oxide. I have been experimenting with treating buds with a mixture containing oryzalin (Surflan herbicide) with the intent to produce unreduced gametes but it is too early to be certain of any success.

Value of Polyploids from unreduced gametes:
Research has shown that wide crosses that are converted from diploid to tetraploid tend to have very limited variability in their seedlings (sometimes referred to as permanent hybrids). Additionally, through the research with unreduced gametes, it has shown that by using unreduced gametes vs. polyploid conversion of the original plant that seedlings produced have significantly more variation. Through very complex techniques (GISH) researchers have shown that unreduced gametes allow for significantly more crossing over during the gamete formation process which is the process in meiosis that creates additional variability of seedlings by mixing the portions of the chromosomes from the mom & dad differently for each egg/pollen cell. In many cases, breeders are looking for large variability in seedlings with the goal of finding something new that they have not seen before.

How do I know if I have a polyploid?
There are several ways used to estimate the ploidy level of a lily but the only certain way to know for sure is to prepare a sample of the plant and count the chromosomes under a microscope. Even counting chromosomes can lead to some errors if the plant has different numbers of chromosomes in different tissues (chimera). The typical method for counting chromosomes is from a root tip preparation where the root tip is treated with chemicals to soften the tissue and then stain the chromosomes and the small piece of root is flattened on a microscope slide for inspection by a microscope. Under the microscope you will see individual cells and you literally count the individual chromosomes in the cell.
Leaf stomata & pollen size have been correlated with the number of chromosomes a plant has. Stomates are the small openings on the bottom of the leaves that can be observed using 50x magnification. The size of these openings formed by two guard cells has been well correlated to chromosome count with larger stomates indicating higher numbers of chromosomes. The same with pollen – the size & shape of the pollen have been correlated generally with chromosome number but if the plant does not produce viable pollen then this is less useful. A challenge with these techniques is that there is an overlapping ranges meaning some diploids are bigger than some triploids and some triploids are bigger than some tetraploids.
Another more recent technique for estimating ploidy is Flow Cytometry. This is an analytical technique where a piece of the plant is prepared and analyzed to determine the relative amount genetic content. It is easy to be confused by data from this technique and that is why it is very important to understand what it is actually telling you. The key point to understand is that it is a measure of genetic content but not an absolute count of chromosomes. If you look at pictures of chromosomes you see there are some that are larger than others and with this technique they all get scrambled up together. It is like looking at a quantity of shredded carrots and trying to figure out how many carrots were used to get that quantity of shredded carrots (did it take 24, 36, or 48 carrots to get that quantity or is it somewhere in between?) An absolutely critical part of using flow cytometry is to have reference material with a known chromosome count to compare with. Also it is important that the reference material have a similar genetic background as different lily species have different chromosome shapes & sizes and different amounts of genetic material. Without the proper standard to compare to, any conclusion drawn from the test data could be significantly skewed. I have seen published flow cytometry data for some lilies that makes no sense. For instance – there was one report showing that the OT hybrid Anastasia is a tetraploid. It is not a tetraploid – it was confirmed by chromosome count to be a triploid. Flow cytometry is not more precise than counting chromosomes and may not be any more precise than measuring leaf stomates. It is a useful tool that can be used to estimate whether a lily is a diploid, triploid, or tetraploid but it is not sensitive enough to indicate if a particular cultivar is an aneuploidy (having extra or missing some chromosomes from the normal sets of 12). With any published list of polyploid lilies, it is very important to understand what methods they used to make that determination as sometimes they used a more precise method and other times it is just a “best guess”.

I hope this write-up is helpful and I wouldn't be surprised if it raises a few questions. I did not include specific references to research articles but can do so if there is a particular aspect someone would like to read more about directly from the article.
Name: Joshua
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Australis
Mar 7, 2017 5:06 PM CST

Plants Admin

Thanks for the detailed post - very informative.

I'd just like to add that while all Lilium species are diploid, triploid plants do occur in nature - the most well-known being Lilium lancifolium (the Tiger Lily). I have also read that a triploid form of Lilium bulbiferum is known to exist.
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Name: Patrick
Midland, Michigan (Zone 6a)
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auratum
Mar 7, 2017 6:37 PM CST
Australis - thanks for the addition. I was aware of the triploid L. lancifolium but was not aware of the triploid L. bulbiferum.
Name: Dave
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Nhra_20
Mar 7, 2017 6:44 PM CST
Nice write up Patrick.

One question for now though after reading this. Regarding fertility and crosses. Diploids, are generally fertile, but limited in making the wider crosses? Triploids tend to be sterile as pod and pollen parent?Tetraploids tend to be more fertile and more accepting of wider crosses?

Okay, so technically 3 questions, but this is the general rule of thumb with ploidy concerning crossing and hybridizing?
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magnolialover
Mar 7, 2017 7:39 PM CST

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Triploids can be pod parents and often match up when fertilized with tetraploid pollen of similar genetics (example: triploid OT as pod parent, pollen parent tetraploid trumpet/tetraploid OT/or tetraploid oriental.
Tracey
Name: Patrick
Midland, Michigan (Zone 6a)
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auratum
Mar 7, 2017 8:15 PM CST
Thanks Dave.

Nhra_20 said:Diploids, are generally fertile, but limited in making the wider crosses?

Diploids are generally the most fertile (make the most seeds per pod). Not sure I understand the comment about wider crosses? Need to clarify when talking about wide crosses on whether it is the F1 or first wide cross or if you are trying to breed with the F1 seedling. In general, when trying to make the F1 wide cross, it is recommended to do this at the diploid level. When trying to breed with the F1 wide cross, it is often sterile (Black Beauty is an example OT like this) but in some cases is fertile (Tall Boy is an example OT like this) or even produces unreduced gametes (the L. auratum x L. henryi cross 82111 is an example OT like this).

Nhra_20 said:Triploids tend to be sterile as pod and pollen parent?

Correct, but there are more and more triploids available commercially that have some limited fertility as pod and/or pollen parent. There was a recent article where they studied plants produced from crosses onto triploids and they found the majority of the seedlings were aneuploids - a very small percentage had complete sets of chromosomes.

Nhra_20 said:Tetraploids tend to be more fertile and more accepting of wider crosses?

In general tetraploids are not more fertile. They can be quite fertile but it is not the norm. It is possible that tetraploids may be better than diploids for some wide crosses but it depends on the cross. For example crossing Starburst Sensation with Black Beauty - it would be very unlikely to make this cross at the diploid level because of the limited to no fertility of the diploid version of these plants. But take these to the tetraploid level and you are much more likely to get seedlings. Actually for this combination, Starburst Sensation has been used as a pod parent and two well known example seedlings are Silk Road & Anastasia and both are triploids resulting from crossing Starburst Sensation with diploid pollen so it is likely that Starburst Sensation produces some unreduced gametes. For a different example - say trying to make a Trumpet x Asiatic cross, it is not clear to me if crossing a diploid trumpet with a diploid Asiatic would be better than trying to cross a tetraploid trumpet with a tetraploid Asiatic? If you successfully make the cross, it would more likely be fertile at the tetraploid level than at the diploid level.

Not sure if I answered your questions?
Name: Joe
Long Island, NY (Zone 7a)
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Joebass
Mar 7, 2017 9:18 PM CST
Great stuff Patrick! So for the answer to my original question on the original thread, you are try to get unreduced gametes for more variability in seedings, correct?
Name: Patrick
Midland, Michigan (Zone 6a)
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auratum
Mar 7, 2017 9:36 PM CST
Joebass said:Great stuff Patrick! So for the answer to my original question on the original thread, you are try to get unreduced gametes for more variability in seedings, correct?


Three reasons I can think of:
1) as you connected here, better chance for greater variability in breeding
2) skip the time it takes to convert to tetraploid and grow to flowering size - I would estimate this takes at least two years
3) not have to manage the converted form - easier to grow the diploid plant and torture the flowers with chemical treatments than to try to maintain the tetraploid form for breeding.

The assumption is that I can pull this off and find the right treatment and apply it at the right time to get reliable production of unreduced gametes...
Name: Rick R.
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Leftwood
Mar 7, 2017 10:02 PM CST
Your deliberate and concise writing skills really show, Patrick. Thanks for condensing it all into such a readable format.
Sydney, Australia (Zone 10b)
Protoavis
Mar 8, 2017 1:06 AM CST
Nhra_20 said:Diploids, are generally fertile, but limited in making the wider crosses?


Limited in crossing no. Generally, across all types of plants, the wide crosses can be made its just common for the offspring to be sterile. At a tetraploid level though there's more likely to be fertility in those specific circumstances where the diploid level is infertile. There's probably more known examples of this in iris (ie Pacific coast iris with Sino Siberian, sterile as diploid but fertile as tertraploid.....less seed though) than lilium though.
Anyone with oryzalin (aka Surflan, Embargo), am looking for a small amount rather than 5litres from manufacturer (min size in Australia....)
Name: Joe
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Joebass
Mar 8, 2017 3:44 PM CST
Patrick do you believe that this surflan paste will lead to treated pod parents acting like tetras as well? I'm no biologist but that could really open doors.
Name: Patrick
Midland, Michigan (Zone 6a)
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auratum
Mar 8, 2017 4:52 PM CST
Joebass said:Patrick do you believe that this surflan paste will lead to treated pod parents acting like tetras as well? I'm no biologist but that could really open doors.


If the pod parent is a diploid. If the pod parent is a triploid it could breed like a hexaploid. That is, if it works...
Name: Joshua
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Australis
Mar 8, 2017 6:19 PM CST

Plants Admin

My understanding was that ploidies above 4N (tetraploid) weren't stable and the plants don't normally survive.
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Name: Lorn (Roosterlorn)
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Roosterlorn
Mar 8, 2017 7:58 PM CST
Auratum. I have a question. What percentage of conversion to 4n qualifies a plant to be called or defined as a tetraploid? Is there ever anything like 100% 4n conversion?. I suspect not because all the test results I've seen, whether by flow cytometry or leaf stomate count, the test results are always substantially less than 100%. Such as in a typical cyto analysis: tetraploid= 78%, triploid= 12%, diploid=10%.

Also, in you thinking, do you believe these numbers change over time as a plant ages and if so, might this have a similar affect as what's called clonal drift?
Sydney, Australia (Zone 10b)
Protoavis
Mar 8, 2017 9:39 PM CST
Australis said:My understanding was that ploidies above 4N (tetraploid) weren't stable and the plants don't normally survive.


Depends on type of plant. There's quit a few naturally occurring octoploid viola (ie Viola douglasii, populations north of San Francisco Bay are generally octoploid while those south are usually tetraploid). One of the Nordic (suvallii? Something like that) is an allo-octoploid, etc. There's a bunch of them.

A lot of commercial wheat is hexaploid.

The further from diploid the more energy required, often slower growth (as cell division is harder, requires more energy, it's why often newly partly converted plants often revert, the diploid cells grow faster), etc.

Most plants probably don't deal with it well but there are various plant families out there that don't seem affected by it that badly and it's part of their natural species state.
Anyone with oryzalin (aka Surflan, Embargo), am looking for a small amount rather than 5litres from manufacturer (min size in Australia....)
Name: Joshua
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Australis
Mar 8, 2017 10:53 PM CST

Plants Admin

Apologies for not being clear, @Protoavis. I understood Liliums with ploidy > 4N don't survive. I know other genera can survive at higher ploidies - Cymbidiums, for example, will survive up to 6N (8N just dies in the flask), although 4N seems to be optimal for them (and 2N is their natural state).
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[Last edited by Australis - Mar 8, 2017 10:54 PM (+)]
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Protoavis
Mar 9, 2017 12:28 AM CST
Understood. Just putting out information on the chance someone goes from here into the wider internet and starts seeing conflicting info, it can be confusing.

I think in general most of the usual garden plant types have been tinkered with to the point that their upper ploidy limit (barring unique flukes) has probably been reached in at least one cultivar (although whether commonly available or not is a different question), so if you don't come across anything higher than terra it's probably a decent indicator that's the limit.
Anyone with oryzalin (aka Surflan, Embargo), am looking for a small amount rather than 5litres from manufacturer (min size in Australia....)
Name: Patrick
Midland, Michigan (Zone 6a)
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auratum
Mar 9, 2017 11:52 AM CST
Roosterlorn said:Auratum. I have a question. What percentage of conversion to 4n qualifies a plant to be called or defined as a tetraploid? Is there ever anything like 100% 4n conversion?. I suspect not because all the test results I've seen, whether by flow cytometry or leaf stomate count, the test results are always substantially less than 100%. Such as in a typical cyto analysis: tetraploid= 78%, triploid= 12%, diploid=10%.

Also, in you thinking, do you believe these numbers change over time as a plant ages and if so, might this have a similar affect as what's called clonal drift?


Lorn – great questions! For me if it shows the traits of a tetraploid (stomata, pollen, leaves, flowers, etc) and breeds like a tetraploid then it is a tetraploid. The % thing is just confusing – let me explain more on where I think the % come from.

We need to be cautious as we try to interpret the data from Flow Cytometry analysis. This testing looks at relative amount of genetic material. I like the carrot analogy where we are trying to figure out how many carrots went into a certain amount of shredded carrots or carrot juice. If you look at the printout from a flow cytometry analysis, it looks like a bell curve (histogram). To really interpret the data, it is best to see the histogram. The tables I have seen published listing % for 2N, 3N, & 4N are intending to represent the data from the histogram but they need to be interpreted carefully. The histogram is a graphical display of the count of cells that are picked up by the detector on a given channel. The machine is supposed to be set-up with the 1N channel number for the reference material so if the 1N (haploid) for the reference hits channel 40, then diploid nuclei should center around channel 80 (double the channel number but some hits fall slightly lower and slightly higher) from this number. Triploids would hit centered on channel 120 and tetraploids would hit centered on 160. There is always some variation in the data and one thing they look at is the coefficient of variation (or spread) for a peak around a given channel. I have only seen a few histograms but there is also a noise level or baseline number of hits across all channels. It is possible to look at where a 3N peak should be and see nothing but baseline/noise signal hits and report the baseline in this area as a % using the standard coefficient of variation. Also, due to cell division there are likely some cells that are going through various stages of cell division so it would be expected on a 2N plant to see a small peak around 4N as some cells would have doubled their chromosomes prior to dividing and that would get picked up on the analysis. The bottom line is the data needs to be interpreted properly with appropriate reference material to draw correct conclusions.

There are a couple of articles in the 1990 NALS yearbook that go along with this discussion. The first is on doing chromosome counts by inspection under a microscope and the other is on flow cytometry and how it works. It is a good read on this topic and likely better explained than what I have done here. Also did some searching and found an article that may help show with a picture the different techniques - http://www.academicjournals.or.... Here is a picture from the article showing comparison of different cultivars and what the stomata, flow cytometry, & microscope count look like:
Thumb of 2017-03-09/auratum/d5ea0f
As for things changing over time, there is very little solid evidence to this for lilium. Can it happen – yes. Does it happen – maybe. How much does it happen – I don’t know. If you look at plants like hosta – it is a big deal and happens all the time but for lilies, they seem much more stable. The concept of clonal drift comes from changes in the plant over time due to errors or mutations in the cell division process. In most cases those errors don’t result in chromosome counts but more subtle things like flower color or leaf color. The only mutation I have heard of for this is with Judith’s Scheherazade producing Silver Scheherazade.

Australis said:My understanding was that ploidies above 4N (tetraploid) weren't stable and the plants don't normally survive.

Apologies for not being clear, @Protoavis. I understood Liliums with ploidy > 4N don't survive. I know other genera can survive at higher ploidies - Cymbidiums, for example, will survive up to 6N (8N just dies in the flask), although 4N seems to be optimal for them (and 2N is their natural state).


Australis - yes – ploidy levels about tetraploid are possible. There are some reports of hexaploids in lilium but they are not common. There are also reports of octoploids in tissue culture from chromosome doubling work but no evidence the plants were grown outside of tissue culture. A more recent research article from the NALS Yearbook talks about pentaploids.

Many people don’t realize the number of polyploids around us. Most of the apple cultivars we eat today are triploids. As already mentioned, bread wheat is a hexaploid and commercially grown strawberries are octoploids.

I had the opportunity to discuss this at length with Bob Griesbach. I had the same question as you – are tetraploids the limit? He shared that he did not feel that tetraploids were the limit but thought it would be possible to develop normal growing hexaploid (6N) lilies. He also shared that he did not think octoploid (8N) lilies would grow normally in the garden. He shared that the fewer number of base chromosomes a species has, the higher the level of polyploidy that would theoretically be possible to grow normally. He gave a very specific example from irises where they were playing with this and has done many conversions to tetraploid and then there was an attempt to further double to octoploid. They successfully produced and octoploid cultivar that was able to grow in the garden but it grew really slowly and was a dwarf when compared to the tetraploids. In lilium the concerns shared with polyploidy are more brittle stems and flower tepals along with difficulty for the flowers to open normally due to the increased substance of the tepals. Also tetraploids grow more slowly than triploids and diploids. The Dutch breeders aim for triploids as they have the balanced benefits of some increase in ploidy (somewhat larger flowers, thicker & more lush foliage) with not too excessive detractors (brittle stems & flowers, slow growth rate) from the higher genetic content.


Name: Joshua
Melbourne, Victoria, Australia (Zone 10b)
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Australis
Mar 9, 2017 5:46 PM CST

Plants Admin

Thanks Patrick; a very informative post.
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Name: Lorn (Roosterlorn)
S.E Wisconsin (Zone 5b)
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Roosterlorn
Mar 9, 2017 9:31 PM CST
Patrick, so really as far as any backyard hybridizer and breeder like myself is concerned, the good old fashioned stomata comparison/count and other tetraploid traits are all we really need to know, and forget about flow cytometry--at least initially.

You do such a good job of explaining things! Thanks.

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