Genome duplication responsible for more plant species than previously thought

Aug 12, 2009
Polyploidy is important in the origin of new plant species and has caused some species to have an astoundingly large number of chromosomes. This photograph is a "chromosome squash" of a hexaploid (six genome copies) fern species of the genus Cystopteris. This species has 126 pairs of chromosomes. Credit: Michael Windham

Extra genomes appear, on average, to offer no benefit or disadvantage to plants, but still play a key role in the origin of new species, say scientists from Indiana University Bloomington and three other institutions in this week's Proceedings of the National Academy of Sciences.

Plant biologists have long suspected polyploidy -- the heritable acquisition of extra chromosome sets -- was a gateway to speciation. But the consensus was that polyploidy is a minor force, a mere anomaly that accounts for 3 or 4 percent of the world's flowers and ferns.

The first direct, comprehensive survey of polyploid speciation in plant evolution severely challenges that notion.

"In the present paper, we make it clear that it is a common process," said evolutionary biologist and lead author Troy Wood, who began the research during graduate training at IU Bloomington. "Fifteen percent of flowering plant species and almost a third of fern species are directly derived from polyploidy."

Wood is now a research scientist at University of Muenster in Germany.

Could polyploidy provide plants with a powerful advantage over their chromosome-challenged peers? Not necessarily. The scientists' exhaustive survey of published phylogenetic and genomic data also shows that plant lineages starting with a polyploid ancestor appear to be no more successful at spawning species than diploid plants, which have two sets of chromosomes.

"The fact that polyploidy seems to have no effect on diversification rates should reduce the number of enthusiastic commentaries about the 'advantages of polyploidy,'" said IU Bloomington and paper coauthor Loren Rieseberg, who supervised the research. "However, our diversification rate analyses only examined recent polyploids. A future area of research should be to ask whether more ancient polyploidy events have increased diversification rates."

Rieseberg holds joint appointments at the University of British Columbia and IU Bloomington.

"The present study developed out of an ongoing project to write a book about plant speciation," Rieseberg said. "I felt that recent estimates of the polyploid speciation rate were too conservative because they did not take genealogical history into account. Troy began compiling chromosome number data and phylogenetic trees so that we could generate a more accurate estimate of the frequency of polyploid speciation."

While the variation that leads to new species is usually a glorious accident, evolutionary biologists are beginning to identify the biological properties of organisms that make those accidents stick around long enough for new species to become established. If whatever separates the new breed from its original population is tenuous, it's possible the new and old populations will comingle, negating the possibility of a new species. Geographic separation or "reproductive isolation" is crucial.

Mechanisms of reproductive isolation are almost as vast and varied as the species they make possible.

In some animals, sudden, heritable changes in the size and shape of genitalia have the potential to prevent some individuals of a population from mating with most of the others. Even though sexually reproducing plants do not rely on this sort of "lock and key"-type of sex matching, they have equivalent, more subtle systems for preventing the wrong pollen from fertilizing their eggs.

Polyploidy can also result in speciation, as polyploid individuals often cannot produce viable offspring with their diploid (two sets of chromosomes) relatives. While the polyploid and diploid individuals may appear more-or-less identical to one another, their genetics make sexual reproduction unlikely or impossible.

Some animals can handle polyploidy, but for most vertebrate species, an extra chromosome set is a death sentence. Humans, for example, can barely tolerate the presence of even one extra chromosome out of the total set of 23. Most human "trisomies," as these are called, result in natural abortion, or miscarriage. Non-lethal human trisomies result in developmental disorders, such as Down Syndrome. Human zygotes with three full sets of chromosomes do not develop.

Plants are pretty special. Not only can many species tolerate extra chromosome sets, but polyploidy appears to be a recurring theme throughout plant evolution. The question is why.

"Recent data reveal evidence of polyploidy in an array of plants, like grapes, poplar trees, corn, and many others," Wood said. "In most of these cases the evidence points to ancient polyploid events. Some species of flowering plants have more than 400 and some fern species more than 1,000 due to repeated instances of polyploidy during their evolution. While these examples might seem remarkable, given the high frequency of polyploidy speciation documented here, the bigger surprise would be if plant lineages were found in which polyploidy was absent."

One implication of the PNAS paper is that Wood, Rieseberg, and their coauthors may be getting close to solving the mystery. If extra genomes provide no special advantage over relatives, the ubiquity of polyploidy in plants could simply be because polyploid mutants are commonly produced. Evolutionary change that doesn't involve the plus-or-minus forces of natural selection is called "neutral" in evolutionary biology parlance.

"I really thought we would find evidence that polyploids have an advantage," Wood said. "The idea that the large number of polyploid species and the attending high chromosome numbers might be simply due to a neutral process is intriguing."

Source: Indiana University (news : web)

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zilqarneyn
not rated yet Aug 13, 2009
But the consensus was that polyploidy is a minor force, a mere anomaly that accounts for 3 or 4 percent of the world's flowers and ferns.


"Fifteen percent of flowering plant species and almost a third of fern species are directly derived from polyploidy."


Now, looking up the word "polyploid" (whether that has anything to do with grafts of fruits trees), I first took the old biology textbook "View Of Life" (by Luria, Gould, Singer (LGS), 1981), the keyword took to the evolutionism portion (by S.J.Gould), and his guess is a lot more than that consensus, even more than triple the new estimate. LGS (1981) wrote, p.621,
Among species of flowering plants, 50% may be polyploids.





Yes (for what I thought looking), at the bottom of that p.621, allopolyploidy is told of.
Plant breeders were quick to recognize the agricultural potential of allopolyploidy.

bore leaves of a radish and roots of a cabbage.

Actually, that comment from Gould, talking about "potential" for people, fits the creationist's point of view, as told by the Creator. That is, Allah (Yahweh) created the Universe for us. Gould's last sentence of that paragraph is starting with "unfortunately" but you see, that finishes with that second quote. Thus, not truly "unfortunate," but a flexibility that we people might like, while crossing the plants.




To express with terms how I think mostly ( [url]http://www.imame.org/4/gf/[/url] ), I might reflect that, the system of polyploids, favor plants to co-exist, with gene-expression code that is permissive in referring to the types of the macro blocks -- & presumably, distributed processing by several nodes.

In programming languages such as C or Smalltalk, permissiveness is through polymorphism.

Likewise, flexible data structures allow appending/inserting new bytes, without any need to modify the program that will interpret all of that data.

In formal-nets, the self-interpreting (frag) capability (how I think that relates to stem cell totipotency/pluripotency, too), is allowing a formal-net to make sense of the "new structure" (that is not out of the bound of sensibility for the "old" gene-expression).

For example, if to allow processing arbitrary structures, most trivially, you would not refer to a strict memory location, for accessing your data (or, first, you would copy the form, to that specific memory). Let data pass through the data-processing nodes, and nodes will think which to process and which not to process. A field (or, combination of fields) of incoming tuples/arrays/chromosomes might fit as a key to lock, and the fitting nodes would process that chromosome to express a few aspects. (That might resemble how Da80 ( http://www.mid80....hine.htm ) was utilizing a tag-field of the token-attributes/fields to reflect the structure.) In the case of both-plants having a key for that lock, we might only look which of the rival chromosomes, that node is expressing (first-come? last-come? or, in cases, a combo?)


In summary, that is not changing the point that, the (genome-specific) gene-expression has to make sense of the structure of the genetic-code, to express that. That is well within the boundary of a constant genome, with flexibilities ( [url]http://www.imame.org/4/gf/[/url] ).
Ethelred
not rated yet Aug 13, 2009
Actually, that comment from Gould, talking about "potential" for people, fits the creationist's point of view, as told by the Creator


What a surprise, a Creationist taking Gould out of context. Must be the first to do so.

No sorry I meant to say it was a sport amongst Judeo-Christian Creationists right up there with taking Dawkins and Darwin out of context. Another thing that Islam has taken from Christians. Not a good thing either.

That is, Allah (Yahweh) created the Universe for us.


And you are certain of this because Muhammad said so. And you are certain of Muhammad because he wrote the Quran as poetry and you think that no one has written better poetry. Taken from a previous post of yours.

I beg to differ with you on this. Clearly some Arab couldn't possibly write better poetry than the Heroic Alliteration of the great Norse Bards as written down by Snorri Sturleson. Since you think that poetry is a key to truth than surely Men exist to do battle with Giants in the Final Battle. It therefor follows that the Earth was created by a giant cow licking a block of ice.

In other word give PHYSICAL evidence. Poetry as proof is beyond doubt the worst alleged proof of any religion that I have come across. And Heroic Alliteration is better anyway, at least for Manly Men and Warriors of the Sea. Too bad I can't read Icelandic.

In programming languages such as C or Smalltalk, permissiveness is through polymorphism.


In plants its likely because the parts are bit more independent than in animals.

Likewise, flexible data structures allow appending/inserting new bytes, without any need to modify the program that will interpret all of that data.


Except that programs usually don't have as much garbage as DNA does. While the garbage content of DNA has been overstated it is definitely still there.

In summary, that is not changing the point that, the (genome-specific) gene-expression has to make sense of the structure of the genetic-code,


Yes it did have to evolve to that point eventually. Its a matter of competition. Those life forms that did a less efficient job of reading and transcribing its own DNA were at a severe disadvantage to lifeforms that did a better job. So mutations that improved things were conserved by survival and the rest died out.

That is well within the boundary of a constant genome, with flexibilities


Or more likely well within the boundary of a system that had to evolve ways to deal with change. Change is the only constant in life.

sigh, you still haven't learned how to post a link.

Here is a link you may have meant to post.

http://www.imame....-frz.htm

Or maybe not but it is where I get taken after I trim the garbage out of YOUR code.

Its not that hard. Just copy it from the address bar and paste into the comment box. But DO NOT put it inline with the rest of the post. Make a separate line for it to lower the odds of Physorg screwing it up. Long links will always be botched so don't try if they take up more than one line.

The genome is not able to self-modify to become the genome of a different species.


True. Genomes don't actually do anything except store information. However errors in copying the genome can and do lead to speciation over long periods of time.

Allah is able to publish new species -- how Adam (a.s.) was the first human. New.


Since there was no Adam I have to doubt that claim. Seems like someone made it up. Not Muhammad in this case.

Perhaps you could clear something up. Are you a Young Earth Creationist or do you accept the measured age of the Earth or something in between?

Ethelred