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Go Back   Orchid Board - Most Complete Orchid Forum on the web ! > ORCHID DISCUSSIONS > Scientific Matters
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  #11  
Unread 01-09-2013, 03:01 PM
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VJO:
ZWUM is offline right now - I hope he will go ahead and respond with his answer when he comes back on, but in my opinion, you only would have this done if you are breeding and want to make sure you have the correct ploidy to cross [2(N) X 2(N), 4(N) X 4(N), etc], or perhaps if you suspect you have purchased a seedling that has mutated into a 4(N); it should look considerably different than it's 2(N) sibs - same coloring and such, but much larger or more vigorous overall. The average home grower would only want to know in order to correctly label their collection - if they gave a start to someone that does breed they may want to know the ploidy of the plant, if there is an inkling that it is not 2(N). I am sure there are a multitude of reasons out there for testing, but these were the only ones that pops into my head right now. Basically, you are correct - the ploidy is only of main concern to the breeding aspect of the plant. Again, if you think you purchased a seedlign that had developed into 4(N), you may want to know in order to understand it's development better, but this would not occur to everyone.
Hope this further assists!
Steve
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  #12  
Unread 01-09-2013, 03:39 PM
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4N plants are without a doubt more expensive. Look at the prices of the 4N Soph. coccinae from Japan that J&L sells! They are about double to even triple the size, both plant and flower.

I like to know if the plant is 4N but if your not selling or breeding I guess you don't have to find out, but why not! I import a lot of plants and see a good amount of diversity within a single species. I have a bunch of Bulb. contortisepalum's that I imported and one is exceptionally large in both flower and plant size. I'll be bringing a sample of the live root tip to the lab soon to see if it is a 4N. But it's up to you if you'd like to know. There's always potential for a more valuable plant.
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  #13  
Unread 01-10-2013, 12:36 AM
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I think Stray covered most of what you asked, but I wanted to clear something up just a bit. Let's take humans as an example, our n (called a haploid number) equals 23. This means that there are a total of 23 unique chromosomes in a human gamete. Normal, human, non-reproductive cells (anything but sperm and ovum) are 2n, or diploids. This means each of the 23 unique chromosomes has a duplicate attached to it. This is the whole reason why reproduction is possible. The cells responsible for making sperm or ovum, which are 2n (46 chromosomes, or 23 matching pairs), split into haploid cells (gametes), each containing one of the 23 chromosomes. When the male and female gametes meet, the haploid chromosomes combine and you get a fertilized ovum with the full diploid number.

Tetraploids are possible be treating small, germinating seeds with chemicals that inhibit mitotic spindles. During a certain phase in cell division, all the chromosomes make a duplicate of themselves, so for a moment it is in a sense a tetraploid, as there is four copies of each chromosome. Normally, shortly after the doubling, little threads in the cells start pulling the chromosomes away from eachother, which results in two daugher cells with a diploid chromosome number. What these chemicals, such as colchicine or oryzalin, do is in some form dissolve those little spindles that pull the chromosomes apart. This action causes the cell to stop, and reset the process, leaving one cell with a tetraploid number of chromosomes, which then restarts the process and eventually splits into two, tetraploid daughter cells.

Now I'm positive that any alteration to the human chromosomes is usally fatal, and I think its the same case in the animal world, but for some reason plants can handle it much better. In fact, most of the times tetraploids are more vigorous than their diploid counterparts, but in a good percentage of the cases, triploids may actually be the most vigorous of all. The huge downfall of triploids is that they are very difficult to get to breed. The reason is that during the point where the cells producing gametes start dividing into their haploid numbers, you have three pairs of chromosomes to go into two cells, which often case doesn't go nice and evenly and you have plants that are genetic messes. In some very rare cases, certain gametes of a triploid may luckily end up with all three pairs of chromosomes, giving you a triploid, or 3n, gamete. This gamete, when paired with a haploid (1n, or normally called n) gamete, gives you a tetraploid (4n) cell, which will then be able to breed in the future. I do really want to stress that the chances of getting even tetraploids out of a 3n x 2n cross are very small, and in the Phalaenopsis world, there is only one triploid hybrid that is known to make triploid gametes more often than the nearly never of most other triploids, although there are still many cases where you get aneuploid (not being a whole number) seeds from it if you get anything at all.

If you didn't notice by the end of the first paragraph, I absolutely LOVE anything to do with chromosomes, genetics, or genes! I really hope this helped, and if you have any further questions, ask away!
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  #14  
Unread 01-10-2013, 01:46 AM
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Humans do have trisomy conditions - a third chromosome in one of the 23 pairs. In most cases, this means mental retardation and physical impairment (e.g. Down Syndrome). In the case of the sex chromosomes X and Y, where there is an extra X or Y, we get a hermaphrodite. So while the effect is not necessarily fatal, it is usually negative (unless hermaphrodites disagree).
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  #15  
Unread 01-10-2013, 08:49 AM
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Well ZWUM there is probably no reason for me to check as there is probably no chance of me ever selling my orchids. I dont really know what will become of them when I die but I think most will end up in the trash as I dont know anyone who collects other than me. My kids REALLY arent into them and probably wont want to go to the trouble of finding good homes for them....Jean
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  #16  
Unread 01-10-2013, 01:59 PM
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Wow, TheBlazingAugust:
How detailed - Very well done. You have taken me back to High School with the chromosome-speak! I had to read through your response a couple of times as it has been years since I really looked inside DNA and strand separation.
ALToronto:
Thanks for mentioning the fact that humans / animals are definitely different in their tolerance of DNA aberrations - Good point. I worked in Hospice with a newborn that was diagnosed with Trisomy 13; basically the little guy was born with little more than a brain stem and his passing was so very difficult on the family.
VJO:
You asked a great question - we all have learned from this. Thank you for the Q's.
Steve
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  #17  
Unread 01-12-2013, 03:43 PM
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A couple of points:

Many hybrid lines and clones are 4n, but the chromosomes have never been counted. Hybridizers instinctively use these 4n plants because they have longer lasting flowers (heavier substance) with wider segments (rounder flowers). Flowers tend to be larger on tetraploid plants, but not always.

Triploid breeding (4n x 2n) can produce larger and more attractive (awardable) flowers than tetraploids, but the plants tend to be sterile. This is only a handicap if you wish to breed with the plant. You can, however, clone a triploid plant and make it available by the thousands.
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  #18  
Unread 08-05-2013, 12:02 AM
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I love all these posts, ahh genetics its an amazing subjects and plants are amazing even at that level
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  #19  
Unread 08-05-2013, 09:18 AM
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A couple of questions for the genetic experts: is a triploid truly sterile, or is this simply a case of haploid mismatch? If its the latter, wouldn't it be possible for a 3N x 3N cross?


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  #20  
Unread 08-05-2013, 03:53 PM
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Steve and Cody gave good explanation, but a couple things to add (some of my research is about evolutionary consequence of polyploidy).

Steve, in nature, actually, there are a lot of polyploid species. If we consider ancient polyploids, majority (47-100%) of flowering plants have experienced genome doubling. Even Arabidopsis, which has tiny genome, has experienced polyploidization. When we focus on the recent polyploids (i.e. looking at the chromosome numbers within genera), 34.5% of species are polyploids.

Polyploids are extremely important in plant speciation, but majority of advantage is believed to be from combining two genomes of different species (this is called allopolyploid). You can get a new "feature" by combining strengths of two species. When you simply double the chromosome number of a single species (autopolyploid), there isn't much of STRONG advantage, and the effect could vary among species. Generally, the cell size increases with larger nucleus (where DNA is stored), and this causes larger plants (this is the reason why lots of crops are polyploids). But this is not always the case. In some cases, polyploids grow slower, too.

With regard to synthetic polyploids, as Cody mentioned, they use pretty harsh, mutagenic process. Majority of seedlings are sick, and only a few may survive. The surviving ones may show deformed flowers/leaves etc. Also, internal balance of genes (e.g., epigenetics and dosage balance) are screwed up in newly created synthetic polyploids. After a couple generations, the synthetic polyploids can quickly sort out and re-establish the balance, but early generation synthetic polyploids are generally messed up. So I'm skeptical that they are "generally" more vigorous. This instability is less of an issues with naturally occurring variation in ploidy within species.

BTW, recently, most of us use flow cytometry to determine ploidy. It is much easier than chromosome counting, which requires lots of patience and skill. But the machine is a bit expensive, so it may not be accessible for orchid breeders.

Ray, triploids can produce offspring, but the fertility is very very low.
http://barleyworld.org/sites/default..._sterility.pdf
http://www.vitis-vea.de/admin/volltext/e046305.pdf

In the grape, both female and male fertility of 3x is extremely reduced. But if you do enough crossing, you can get a few offspring. During the meiosis, the chromosome pairing is wacky, and a pollen grain may have unbalanced number of chromosomes (e.g., 2 copies of chromosome 1 and 1 copy of chromosome 2, etc). So the dosage balance among genes are screwed up again, and I'm guessing this is a part of the reason why lots of pollen grains from 3x don't germinate or function well. So 3x X 3x is less likely to succeed than 3x X (even numbered ploidy) since both parents are crappy. The resulting offspring will have "aneuploidy" (abnormal number of chromosomes) in any case when 3x is used as a parent.

Finally, using 'N' in orchid world is a bit weird, since we, scientists, use 'x' to indicate ploidy. 'n' is used to report if your chromosome count is done in sporophyte (=adult plant; 2n) or gamete (= egg/sperm; n). For example, a chromosome count of 2n=4x=40 means that there are total of 40 chromosomes in an adult plant nucleus, and this count is done in sporophyte (2n). If we count it in gametes (e.g. pollen), this is same as n=20 (there are 20 chromosomes in the pollen). 4x part indicates the plant is a tetraploid, so there are 10 unique chromosomes, and there are 4 copies of each chromosome in a cell of an adult plant (40 = 4 x 10).

Last edited by naoki; 08-05-2013 at 03:57 PM..
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