Good informatiion ,I printed it out to study . Gin
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This is a discussion on what does 3N or 4N mean in Phrag. names. within the Breeding & Hybridization forums, part of the Orchid Propagation category; ...
Where to start…
I guess I’ll start with the definitions:
N (Haploid)- Describes a nucleus, cell or organism possessing a single set of unpaired chromosomes. Gametes are haploid.
Polyploidy- cells or organisms contain more than one copy (ploidy) of their chromosomes. Polyploidy occurs in animals but is especially common among flowering plants, including both wild and cultivated species.
Gametes-Specialized haploid cells produced by meiosis and involved in sexual reproduction.
Male gametes are usually small and motile (spermatozoa), whereas female gametes (oocytes) are larger and nonmotile.
2N (Diploid)- A cell with a full set of genetic material, consisting of chromosomes in homologous-(corresponding in structure, position, origin, etc.) pairs and thus having two copies of each autosomal genetic locus- (The position of a gene or chromosome segment on a chromosome. Alleles are located at identical loci on homologous chromosomes). A diploid cell has one chromosome from each parental set.
Most animal cells have a diploid set of chromosomes. The diploid human genome has 46 chromosomes. The gametes (eggs and sperm) contain a single set of chromosomes (haploid).
4N (Tetraploid)- Having four times the haploid number of chromosomes in the cell nucleus.
3N (Triploid)- Having three times the haploid number of chromosomes in the cell nucleus.
Colchicine- A poisonous, pale-yellow alkaloid, C<sub>22</sub>H<sub>25</sub>NO<sub>6</sub><sub></sub> (N-[(7S)-5,6,7,9-tetrahydro-1,2,3,10-tetramethoxy-9-oxobenzo[a]heptalen-7-yl)acetamide]), is obtained from the autumn crocus and used in plant breeding to induce chromosome doubling and also in medicine to treat gout.
The alkaloid extracted from plants of the genus Colchicum and especially from the corms of the autumn crocus, Colchicum autumnale (meadow saffron). The metabolic effect of colchicine is not known, but it is thought that it may decrease production of lactic acid and prevent accumulation of uric acid crystals in the body, making it useful in the treatment of gout. Colchicine and derivatives such as demecolcine inhibit mitosis, or cell division. As a mitotic poison, it inhibits rapidly proliferating cells and has been used in cancer therapy and as an immunosuppressive drug. Colchicine has also been used to visualize chromosomes photomicrographically and to induce mutations experimentally. In laboratory setting colchicine is also used for inducing polyploidy in plant cells during cellular division.
There are several mechanisms by which polyploids arise. Three examples are somatic doubling, gametic nonreduction and triploid bridges. Somatic doubling occurs at the zygotic, embryonic, or meristematic stages of a plant’s life cycle. Polyploid offspring can be generated from the production of polyploid tissues. Although many examples of polyploidy via somatic doubling have been reported, this mechanism now seems less common than gametic nonreduction, or the production of unreduced gametes, as a means of polyploid formation in natural populations (Harlan and deWet, 1975). Unreduced gametes have been reported in a number of species, most notably those that also produce polyploids (Ramsey and Schemske, 1998). Both auto- and allopolyploids can arise in one step after unreduced gamete formation by the union of two unreduced gametes from the same plant.
Alternatively, the production of either an auto- or allotetraploid may involve a ‘triploid bridges”. Tetraploids are formed by triploid intermediates formed within a diploid population by backcrossing to diploids or by self fertilization of the triploid. This two-step method has been considered a significant pathway to polyploid formation (Harlan and deWet, 1975), but some (Bretagnolle and Thompson, 1995) suggest that the one-step process involving the union of two unreduced gametes may be more common. Ramsey and Schemske (1998) concluded that the triploid intermediates may be more significant in the formation of tetraploids since triploids could be more reproductively viable than originally thought.
The Advantages of Polyploidy
The main advantage of polyploidy is the production of heterozygotes. Typically the diploid Mendelian cross of AA and A’A’ would produce a 1:2:1 genotypic ratio (1 homozygous AA, 2 heterozygous AA’ and, 1 homozygous A’) of progeny. However, in a polyploid species the frequency of heterozygotes are much greater. A tetraploid, for example, cross of AAAA and A’A’A’A’ would result in a genotypic ratio of 1:32:1 (Stebbins, 1947). Heterozygotes offer many advantages including buffering effects, protection from inbreeding depression, and the unidirectional introgression phenomenon (Soltis and Soltis, 1995).
Tetrasomic inheritance has a buffering effect on intermediate genotypes, an effect of great adaptive value to a population (Stebbins, 1950). This production of nearly all heterozygous ferns is an advantage because it makes the appearance of deleterious homozygous alleles rare because of the masking effect of additional genomes (Soltis and Soltis, 2000).
Two closely related tetraploid species, for example, that hybridize have five possible genotypic progeny: (aaaa), (aaaa'), (aaa'a'), (aa'a'a'), and (a'a'a'a'). Therefore, it can be expected that those with more "a" alleles would be well suited for an area where that allele thrived. Likewise it can be expected that a greater composition of the a' allele would be best suited for areas where a' thrived. This combination of recombination, natural selection, and genetic segregation is capable of producing an entire spectrum of genotypic populations that would be compatible with any region of intermediate environmental conditions (Stebbins, 1971).
An isolated plant is forced to undergo self-fertilization or selfing. In this situation the frequency of deleterious homozygous alleles increases leading to an overall reduction in the colony’s overall fitness. This inbreeding depression is defined as a reduction in fitness and vigor of individuals as a result of increased homozygosity through inbreeding in a normally out-breeding population. For example, an isolated colony of diploid plants would experience an increase in the number of deleterious homozygous alleles and have increased progression towards fixation or even loss of certain alleles. Studies have found that ferns actually have increased production of normal sporophytes when they are isolated and forced into self fertilization (Soltis and Soltis, 2000).
When considering a single allele at a single locus, the effects of inbreeding depression do not appear to a problem of great importance. The problem is escalated when all the loci are taking into consideration (Lynch, 1995). For example, examinations of lower organisms and other plants estimate about 100 deleterious alleles to be present in individuals when all genetic loci are examined. Individually these alleles produce only a small reduction in fitness ( 2%) when homozygous. However, the combination of all 100 homozygous loci accumulated through inbreeding could potentially reduce the fitness of the organism on the order of 200%! This is enough to "kill" the individual two times over (Lynch, 1995)
Despite the disadvantages of inbreeding, polyploid ferns have grown to favor selfing as a form of reproduction since polyploid protections against inbreeding depression. Two studies of inbreeding depression in diploid and tetraploid ferns show this tendency. In Phegopteris 30-60% of all selfed gametophytes from the diploid race produced normal sporophytes. However 100% of the selfed gametophytes of the tetraploid race formed sporophytes. Another example of selfing preference is present in Lepiosorus. Here, only 4% of the selfed diploid race produced normal sporophytes when nearly 100% of the selfed tetraploid race produced normal sporophytes (Soltis and Soltis, 1995).
Unidirectional introgression is an important advantage in that the tendency occurs to assist chromosomal segregation in helping ferns(or orchids) colonize into new areas. A combination of hybridization and natural selection on the backcrossed and better suited progeny, produces a tendency for the diploids to form tetraploids or other higher number systems. When triploids occur from this diploid–tetraploid cross, many are incompatible as a triploid hybrids. Therefore there is a tendency for these variants to form tetraploids (Sebbins, 1971).
http://www.bedfordorchids.com/ploidy.htm(A simpler explanation found here.)
hope this helps.
Good informatiion ,I printed it out to study . Gin
polyploidy is rare in the animal kingdom because it tends to be incompatible with life, especially when you get to the more evolved organisms.
some practical advice to the orchid hobbyist when picking out a phrag:
polyploidy is exceptionally prevalent in this genus due in part to the late don wimber's work at the eric young orchid foundation. without polyploidy, phrag hybridization is largely dead-end after a generation or two. it has been hypothesized that this is due to the vastly different chromosome counts across the species, leading to new counts in the primary hybrids (which may also be odd numbers) and who knows.
4N (tetraploids) tend to grow slower on average then 3N (triploids). on average is the key; i'm sure some 4N grow like weeds. both are capable of producing large flowers with excellent substance; however, polyploidy does not guarantee outstanding flowers. your exceptional 2N will usually be far more impressive than an average 3N/4N but the exceptional 3N/4N are sights to behold.
4N are useful for breeding as they form complete sets of chromosomes in their gametes. 3N tends to be quite sterile, for intuitive reasons. you have 3 sets of chromosomes and you can't divide them during meiosis so that any particular gamete has one or two full sets of chromosomes. or at least very rarely so.
dishonest growers abound. 4N x 2N will presumably yield all 3N, assuming the parents ploidy is confirmed. i've seen 4N x 2N marketed as "producing tetraploids". (roll eyes here).
beware of hybrids supposedly using 3N parents. Paph Maudiae 'The Queen' is an outstanding 3N cultivar, and remains unsurpassed when it comes to the green-and-white maudiae types. there are crosses around that presumably feature it as a parent. 99.95% baloney; this clone is 99.95% sterile.
colchicine is a mitotic inhibitor (specifically the microtubules) used in gout and ploidy conversion in orchids. "colchicine treated" is a nice buzzword you hear about often. the dose of colchicine required to induce polyploidy is very near the lethal dose. often people will "treat with colchicine" just so they can charge more, and use so low a dose that there is no chance in high heaven of any polyploid conversion. and even when the appropriate dose is used, only a small percentage (anywhere from 10-35% or so) will actually convert to 3N/4N.
Here is the rest of that article Jmoney. i had posted a link to it under further reading (the forth one)
http://www.bedfordorchids.com/ploidy.htm(A simpler explannation found here.)
<table border="0" cellpadding="0" cellspacing="0" width="590"> <tbody><tr><td>An explanation of ploidy
by Robert G. Halgren, PhD<!--mstheme--></td> </tr> <tr> <td><!--mstheme-->The simplest way to explain polyploidy is that every cell in (virtually) every organism contains two of each kind of chromosome. So, I contain two chromosome 21s in each of my cells. This is the "normal" or diploid (2N) state. Di for two (of each), "ploid" for... ploid. (grin) Humans happen to have 22 pairs of chromosomes and a weirdo, XY or XX, which determines the sex of the human. Plants can have a wide range of chromosome numbers. Through the miracle of meiosis (a word to use in crossword puzzles meaning cell division), gametes have half of the chromosomes of the parent type (i.e. one of each kind). One would think that that would be called "ploid", but instead, it is "haploid" (half of the normal ploidy), or 1N. In the normal case, two gametes combine to form a zygote, which has, once again, a diploid (2N) set of chromosomes. Polyploid is just a fancy word for any multiple of the normal (diploid) ploidy. So 3N, 4N, 5N, 6N, etc. are all polyploid. Note that 3N is 1.5 times 2N, in other words, the multiplier can be fractional.
(with a few phal comments by Howard S. Ginsberg)
So, as Steve mentioned, in plants we can screw things up royally with chemicals (such as colchicine) and create tetraploid cells (tetra = four, or 4N), or octaploid cells (octo = eight, or 8N), etc. In animals, this doesn’t happen much in a normal case. I think fish can be polyploid. I’m certain mammals can’t be.
So far we have discussed even ploidy, since we can only double the chromosome number in a plant. So, diploid (normal), tetraploid (2x diploid), and octoploid (2x tetraploid) are possible.
What happens when we cross a tetraploid plant to a diploid plant? Each parent gives one half of their chromosomes in the gametes, which combine to form a zygote. 2+1 = 3, so the progeny are 3N, or triploid. So far so good, but what happens if we want to breed with a triploid? Half of three chromosomes is 1.5, that number doesn’t work too well! Triploids are generally not particularly good breeders because they are not particularly fertile. To complete the cycle, you can double the ploidy of a triploid with chemicals to make a hexaploid (6N). This is fertile, as half of 6 is three, a nice round number.
All this screwing around with chromosome number does have limits.... A cell can only contain so much DNA. So don’t go looking around for 20N plants any time soon. Also, crossing between species or genera of orchids isn’t exactly trivial. In paphs, for example, different species have different chromosome numbers. Which is why primary hybrids are easy to make, but these hybrids don’t always breed well (perhaps the plant is 2.2N or 1.9N - not ideal for dividing in half and getting a nice even number!). Phals, on the other hand, generally all have the same number of chromosomes but there are apparently other problems in breeding species (and hybrids) with large chromosomes to those with small chromosomes.
Why make polyploids at all? Well, in orchids at least, natural polyploids can occur. Mistakes happen sometimes. People have been selecting these out of the "el grande plant progeny pool" for a long time. A polyploid can be (but isn’t always), larger flowered, more intensely colored, etc. "Paph. Maudiae "The Queen" (perhaps the best Maudiae ever) is widely assumed to be a triploid. However, it isn’t particularly (if at all) fertile". It has more copies of the genes that make things good (also more of the ones that aren’t so good...). Often this works out spectacularly well. Sometimes you get a crippled, mutated mess. So, since we have seen over the years that polyploid plants can be superior, it is a natural instinct to use the technology we have developed and chemically alter the chromosome counts.
Anyway, I hope this helps. If nothing else, when you are buying plants, and see a lovely tetraploid (4N) plant crossed with an equally lovely diploid (2N) plant, don’t buy that flask with the hopes of getting the world’s best breeding stock. We now know that the progeny should all be triploid (3N), which may be a glorious flower, but not a particularly good breeder.
</td> </tr> <tr> <td><!--mstheme-->You can visit the author's web site at http://www.msu.edu/~halgren/ or you can contact the author of this explanation, Robert G. Halgren, PhD, by e-mail at firstname.lastname@example.org . <!--mstheme-->
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I can't seem to get past this paragraph:Can somebody (Matt?) please explain what a "Triploid Bridge" is, and why, given what we know about the sterility of most triploid parents, this:Originally Posted by mattbrownwas the conclusion the authors arrived at?triploids could be more reproductively viable than originally thought.
I want to understand this, but I need some intermediate steps. Thanks...
One way that triploids can be formed is if a germ line cell fails to divide its chromosomal count through meiosis. Thus, one parent ends up contributing 2n and the other parent the standard 1n, and the offspring is 3n. This same process can result in the formation of tetraploids, as the 3n parent may fail to meiotically divide, making its contribution 3n, which, combined with the standard 1n from the other parent, results in 4n offspring. This is the two-step "triploid bridge" which, so to speak, spans the gap between 2n and 4n individuals; in other words, this is one means by which tetraploid plants can result from the breeding of diploid plants: through the intermediary step of triploidy.Originally Posted by lja
If this process is the means by which a significant number of 4n individuals within a species come to be by natural processes, then this suggests that the same failure in germ cells may occur when triploids are bred together artificially. So, if the triploid bridge is more important in the formation of 4n plants, this means that triploid plants must ipso facto be fertile to a statistically relevant extent. Sorry about the latin. That was just for my own amusement.
I read (a bit of) the following article to figure this out. Looks interesting, but very technical:
Thanks, Sue. Is there some way to artificially force a triploid parent to fail to meiotically divide, so that it gives its 3N every time? Or is it pretty much just hit or miss, which gives triploid parents their rep for sterility when they try to give 1.5?
No way that I know of, but of course I'm not a biologist. I would think that if anybody figured out how they could make a good amount of money; this would make it possible to put together viable breeding programs with famous triploids like 'The Queen,' and it would also make it possible to produce polyploids without the kind of plantlet losses necessary with colchicine.Originally Posted by lja
i am trying to find the 1998 Ramsey and Schemske studies papers. this article makes reference to it, but it is non-specific as to how they came up w/their hypothesis.
Ramsey and Schemske (1998) take a broad view of a species by using the biological species concept, and for this reason these authors suggest that autopolyploidy occurs more often than does allopolyploidy. This view disagrees with most of the work published to date (Grant, 1971; Soltis, 1993). Ramsey and Schemske propose triploids to be a major mechanism to facilitate the formation of autopolyploidy. They discuss the possible reasons for inviability of triploids (triploid block) and how these maybe overcome by plants to produce viable polyploids. One possible explanation for the triploid block may be the ratio of the ploidy level between the embryo and the endosperm. This may explain why plants in several families, which lack endosperm in their mature seeds, have a higher frequency of polyploidy than other taxa. Ramsey and Schemske (1998) use various estimates from the literature and convoluted arguments in an attempt to establish their hypothesis that triploid plants may act as a bridge to the formation of tetraploids and various other polyploid derivatives. In conclusion, the literature pertaining to the subject of polyploidy is voluminous, but our knowledge about the mechanisms involved in polyploid formation and establishment remains enigmatic.
full article here
So, to make sure I'm getting this triploid bridge: a diploid parent fails to divide meiotically and contributes its full 2N, which when combined with a normally dividing individual contributing 1N, yields a 3N zygote. This triploid then also needs to fail to divide properly, contributes 3N instead of 1.5N, and when combined with a normal 1N, yields a tetraploid, 4N individual.
Maybe I'm just being simple, but it seems to me that the likelihood of such meiotic failure happening twice in sequence in a natural population would be extremely low. If this mechanism has actually been tested--reproduced in a lab--the procedure probably costs a fortune, so it may not be economically viable in terms of orchid production.
Anyhow.... thanks for the explaining!