צ'רלס דארווין פרסם את ספרו המהפכני "מוצא המינים" לפני למעלה ממאה וחמישים שנה, אולם העדויות הגנטיות המחזקות את את תורת האבולוציה התגלו הרבה לאחר מכן. כיום בזכות עדויות גנטיות אנו מסוגלים לחשב קרבה בין מינים ועל סמך אותה קרבה להעריך את מיקומו של כל מין בעץ האבולוציוני. הסרטון שלפנינו מציג את התפתחות החיים מהתא הראשון ועד לאדם בהתבסס על מידע גנטי הקיים היום.

הסרטון הופק בידי 'קרן Welcome trust ותורגם בידי צוות אתר דוידסון אונליין
 
הרעיון העומד בבסיסו של עץ החיים מאוד פשוט. החיים דינמיים ושונות גנטית נוצרת ומצטברת כל הזמן. אותה שונות שמאפשרת התפתחות והתמיינות גם משאירה את חותמה על ה-DNA של היצורים החיים. על מנת להבין את הרעיון בואו נסתכל על משפחה. לאדם מסויים נולדו ילדים. הילדים שלו דומים זה לזה גנטית ודומים לאביהם. עברו השנים ולילדיו נולדו ילדים משלהם. הנכדים דומים לאביהם ואביהם דומה לסבם, אך אם נשווה את הדמיון בין הנכד לסב נגלה שהוא נמוך יותר.  אם ניקח את רצף ה-DNA של שלושת המינים (נכד, אב וסב) ונשווה אותם, נמצא שהנכד קרוב לאב והאב קרוב לסב, אבל הנכד רחוק יותר מהסב וכך נוכל לשבץ אותם במקומם הנכון באילן היחסין הגנטי.

כאשר נסתכל על בני הדודים נראה שיש בינהם קרבה גנטית אבל פחותה מאחים או מאב ובן. עקרון זה נכון גם לגבי מינים של בע"ח. כאשר מין מתפצל למין חדש, המין הבן דומה גנטית למין האב, אך פחות דומה למין הסב. אם ניקח רצפי DNA ממינים שונים נוכל לבדוק עד כמה הם דומים זה לזה ולתת להם ציון על פי מידת הדמיון. בעזרת הציונים שנתנו לקטעי ה-DNA מהמינים השונים ניתן לדעת מי התפצל ממי, ואפילו להעריך לפני כמה זמן זה קרה. באופן לא מפתיע העדויות הגנטיות הללו מתאימות באופן מאוד מרשים לתיארוך מאובנים. בניית עצי  חיים או בשמם המדעי עצים פילוגנטים, ממחיש לנו שלכל שני מינים יש מין אב משותף, או במלים אחרות שלכולנו מוצא משותף.

10 תגובות

  • שרון

    שאלה

    האם יתכן מצב שבו מבני אדם התחיל תהליך של נסיגה אחורנית לכיון של קופים?
    כמו שמוצאים רטרו וירוס שפועל הפוך מהדוגמא הקלאסית של ביולוגיה מולקולרית.
    ומ RNA הוא מיצר DNA?

  • קמילה

    אפשרי תיאורטית אך מאוד לא סביר וודאי שאין סיבה לחשוב שקרה

    מצב כזה ייתכן תיאורטית, אבל הוא לא נתמך על ידי הממצאים. במקרה כזה היינו צריכים לראות עדויות (גנטיות, אנאטומיות, מאובנים וכד') מהן עולה הסדר: בני-אדם -> דמויי קוף. אין בידנו עדויות לסדר אירועים שכזה ואילו ישנן עדויות רבות לסדר: אב קדמון דמוי קוף -> שלבי ביניים דמויי קוף שהולכים ומציגים דמייון הולך וגדל לבני-אדם -> בני-אדם. אפשר כמובן להציע ששני התהליכים התקיימו, כלומר שמאב קדמון משותף התפצלו קופים ודמויי בני-אדם, ובהמשך התפצלו מהאחרונים בני-אדם וקופים אחרים. אפשרי תיאורטית אבל כאמור לא נתמך על ידי אף אחד מסוגי הראיות שקיימים בנושא למעט אולי במספר מקרים שאירעו לאחרונה בקרב אוהדי הכדורגל בישראל... :-)

  • מומחה מצוות מכון דוידסוןארז גרטי

    תשובה

    שלום שרון
    בהחלט ישנם מקרים של רגרסיה, התנוונות ומצבים בהם אנו רואים חזרה אחורה לכאורה, בהנחה שתנאי הסביבה נותנים העדפה למצב זה. העניין הוא שחזרה זו לעולם איננה זהה למקור, כמו שהתקדמות בנויה על שינויים אקראיים, כך גם נסיגה וניתן לראות זאת בצורה ברורה בעקבות הגנטיים.
    מקווה שעזרתי
    ארז

  • שרון

    מבקש להרחיב את ההסבר על זה

    Kevin Peterson grabs a pen and starts to scribble an evolutionary tree on the paper tablecloth of a bar in Hanover, New Hampshire. Drawing upside down to make it easier for me to see, he maps out the standard phylogenetic tale for placental mammals. First, Peterson scratches a line leading to elephants, which branched away from the rest of the placentals around 90 million years ago. Then came dogs, followed by primates (including humans) and finally rodents — all within a frenetic 20 million years. This family tree is backed up by reams of genomic and morphological data, and is well accepted by the palaeontological community. Yet, says Peterson, the tree is all wrong.

    A molecular palaeobiologist at nearby Dartmouth College, Peterson has been reshaping phylogenetic trees for the past few years, ever since he pioneered a technique that uses short molecules called microRNAs to work out evolutionary branchings. He has now sketched out a radically different diagram for mammals: one that aligns humans more closely with elephants than with rodents.

    “I've looked at thousands of microRNA genes, and I can't find a single example that would support the traditional tree,” he says. The technique “just changes everything about our understanding of mammal evolution”.

    Related stories
    Turtles emerge from their evolutionary shell
    Evolution: A can of worms
    Leggy creatures and long branches
    More related stories
    Peterson didn't set out to rewrite textbooks. A mild-mannered but straight-talking Montanan, Peterson had made a quiet career studying how bilateral body plans originated more than 500 million years ago. He has a particular interest in marine invertebrates and had intended to stick with that relatively obscure branch of the animal tree. But a chance investigation of microRNAs in microscopic creatures called rotifers led him to examine these regulatory molecules in everything from insects to sea urchins. And as he continues to look, he keeps uncovering problems, from the base of the animal tree all the way up to its crown.

    That has won him many critics, but also some strong supporters. “Peterson and his colleagues have demonstrated that microRNAs are a powerful tool in determining the relationships of major animal groups,” says Derek Briggs, director of the Yale Peabody Museum of Natural History in New Haven, Connecticut.

    Now, together with his colleagues around the world, Peterson is putting it all on the line with mammals. “If we get this wrong, all faith that anyone has in microRNAs [for phylogenetics] will be lost,” says Philip Donoghue, a palaeobiologist at the University of Bristol, UK, who has teamed up with Peterson. And there is more at stake than just the technique. “It could well be the end of all our careers,” he says.

    Fossil find
    If Peterson does end up switching careers, it won't be the first time. In the early 1990s, he was working the night shift unloading trucks at a freight company in his hometown of Helena, Montana, trying to figure out what to do with his life. He had recently graduated with a pre-medical degree from a local liberal arts college, but he knew he didn't want to become a doctor. Then, rummaging in his parents' barn, he happened on the first fossil he had ever collected, as a four-year-old: a crinoid, or sea lily, about the size of a button. “After I found it, I knew right away that this was what I wanted to do,” he says. “I applied to graduate school the next week.”

    He soon enrolled in a PhD programme in the Department of Earth and Space Sciences at the University of California, Los Angeles. There, he teamed up with developmental geneticists Eric Davidson and Andrew Cameron at the California Institute of Technology in Pasadena, and over the course of his graduate and postdoctoral work the three men developed a provocative idea, dubbed the set-aside cell hypothesis1. They posited that the ancestor of modern-day animals was a larva-like creature containing a group of undifferentiated cells that retained the capacity to give rise to the spectrum of adult body types seen during the Cambrian explosion. The idea subsequently came under fire from the evolutionary and developmental-biology communities.

    A few years after moving to Dartmouth in 2000 to start his own lab, Peterson was looking for a way to test the hypothesis when he became intrigued with microRNAs. First discovered in 1993 by Victor Ambros, now at the University of Massachusetts Medical School in Worcester, these short, hairpin-shaped molecules bind to messenger RNAs and stop them from making proteins. A team that included Davidson had shown that a microRNA called let-7 was present in animal lineages that had bilateral body plans but not in simpler organisms such as jellyfish and sponges2, hinting that microRNAs could hold the secret to morphological complexity.

    “It could well be the end of all our careers.”
    Peterson teamed up with Lorenzo Sempere, then a graduate student working with Ambros at Dartmouth, and the pair began to search for let-7 and a handful of other microRNAs in relatively simple invertebrates, including rotifers, and in more complex creatures. As they added more microRNAs, they found a clear pattern: the farther away from the trunk of the evolutionary tree the animals were, the more microRNAs they had accumulated3. The pair started to realize that the molecules provided “a brand new way to do phylogeny, using a set of rare genomic characters that no one had ever considered before”, Peterson says.

    MicroRNAs, Peterson and Sempere discovered, are unlike any of the other molecular metrics that biologists typically use to tease apart evolutionary relationships. DNA binding sites, for example, continuously mutate; microRNAs, by contrast, are either there or they aren't, so their interpretation doesn't require such complex sequence and alignment analyses. And once gained, microRNAs usually remain functional, which means that their signal stays intact for hundreds of millions of years. “No gene family was known to evolve in this way,” Peterson says. In addition, these small molecules are often expressed in specific tissues and help to regulate the development of certain organs, so they could explain the origin of morphological innovations over geological time4.

    According to Peterson's latest tally, 778 microRNA families have arisen during the 600 million or so years of animal evolution, and only 48 have been lost. This pattern of inheritance leaves an easy-to-follow evolutionary trail for phylogenetic sleuths. Eugene Berezikov, a geneticist who studies microRNAs at the Hubrecht Institute in Utrecht, the Netherlands, says that microRNAs give a clearer answer than other molecular markers of evolution “because the analysis is much simpler”.

    Out of obscurity
    At first, Peterson and Sempere had a tough time publishing their results suggesting that animals had accumulated regulatory microRNAs. “One of the reviewers said it was impossible, what we were describing,” says Peterson. In the end, the work was published in a specialized zoology journal3. Subsequent papers, however, won over some sceptics and Peterson was soon publishing in Nature and Science, and using his growing microRNA library to resolve relationships within and between an assortment of evolutionary lineages, from jawless fishes5 and reptiles6 to fruitflies7 and acoelomorph worms8.

    “It is a really clever and fresh approach to phylogeny,” says Peter Stadler, an evolutionary bioinformatician at the University of Leipzig in Germany. “I don't quite know why presence/absence of microRNAs is not used more frequently in deep phylogeny approaches.”

    Still, not everyone is convinced that microRNA genes trump other types of phylogenetic data. A key point of contention is whether microRNAs only rarely drop out of the genome, as Peterson contends. Andreas Hejnol, who studies invertebrate evolution at the Sars International Centre for Marine Molecular Biology in Bergen, Norway, is sceptical. “MicroRNAs behave like other genes — namely, they can be lost,” he says. “There's no special mystery about them.” Travis Glenn, an evolutionary biologist at the University of Georgia in Athens, agrees, saying that microRNA losses are probably underestimated. In May, he and his colleagues published a retort9 to a paper6 in which Peterson had argued that turtles are more closely related to lizards than to birds and crocodiles — the opposite of what most genomic data sets had indicated. Glenn argued that ultraconserved DNA elements — ones that evolution has kept intact over a long time — show that the conventional view is correct.

    The critics have mostly been a vocal minority, but as Peterson climbs up the evolutionary ladder with his microRNA analyses, he will be reaching a much bigger audience — and the detractors are likely to become a lot louder. “We're mammals, so this matters,” he says.

    Up a tree
    When Peterson started his work on the placental phylogeny, he had originally intended to validate the traditional mammal tree, not chop it down. As he was experimenting with his growing microRNA library, he applied it to mammals because their tree was so well established that they seemed an ideal test. Alas, the data didn't cooperate. If the traditional tree was correct, then an unprecedented number of microRNA genes would have to have been lost, and Peterson considers that highly unlikely. “The microRNAs are totally unambiguous,” he says, “but they give a totally different tree from what everyone else wants.”

    The results change the image of the proto-placental mammal. Because microRNAs place mice and rats at the base of the placental tree, they suggest that rodent-like traits, such as continuously growing incisor teeth, were common in the first placentals, then lost in the lineage that leads to primates, elephants, dogs and cows (see 'Duelling trees'). The findings also shift the geographical origin of placental mammals, suggesting that they started in the Northern Hemisphere, where the first rodent fossils are found, not in the Southern Hemisphere, as many researchers have assumed on the basis of fossil and DNA data.

    Expand
    At first, Peterson was shocked by his results, which still haven't been published. But he has spent the past year validating his tree with gene-expression libraries and genomic sequences, all of which he says support his findings.

    Many supporters of the traditional tree suspect that something peculiar is happening with the microRNAs — probably large losses in the mammalian lineage. “He's talking about the entire genome that has to be wrong,” says Robert Asher, a mammalian palaeontologist at the University of Cambridge, UK. “I don't give it any serious consideration,” says Mark Springer, a molecular phylogeneticist at the University of California, Riverside, who last year published the most comprehensive genomic data set so far in support of the traditional mammalian tree10. “There have to be other explanations,” he says.

    Peterson and his team are now going back to mammalian genomes to investigate why DNA and microRNAs give such different evolutionary trajectories. “What we know at this stage is that we do have a very serious incongruence,” says Davide Pisani, a phylogeneticist at the National University of Ireland in Maynooth, who is collaborating on the project. “It looks like either the mammal microRNAs evolved in a totally different way or the traditional topology is wrong. We don't know yet.”

    Hoping to resolve the issue, Donoghue and phylogeneticist Ziheng Yang at University College London have spent the past year amassing DNA sequences that span more than 14,600 genes from 36 mammalian species — a data set that dwarfs the one used by Springer. They are trying to determine whether the larger crop of DNA data produces the same tree as microRNAs yield. They have been able to date the origin and diversification of placental mammals11, but they are still working to resolve which lineages branched off first — a key test for the phylogenies.

    Peterson would like to put it all behind him. “What sucks about this mammal project is that it's all-consuming,” he says. “Ultimately, I don't really care how mammals are related to one another — it doesn't matter to me. But what does matter is the validity of the data set.”

    If it turns out that the traditional mammal tree is right, Peterson won't see that result as a defeat for microRNAs. It would just mean that something odd happened with mammalian microRNAs, he says. “That says something really interesting about the evolution of microRNAs and the construction of gene regulatory networks in mammalian evolution.”

    For now, he's trying to amass the best evidence he can before publishing the mammal study. Then he wants to return to the quiet life of an ancient-invertebrate biologist. But if Peterson's voyage upends the mammalian phylogeny, he'll have left a furry mess in his wake.

    Nature 486, 460–462 (28 June 2012) doi:10.1038/486460a
    References

    Davidson, E. H., Peterson, K. J. & Cameron, R. A. Science 270, 1319–1325 (1995).
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    Pasquinelli, A. E. et al. Nature 408, 86–89 (2000).
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    Sempere, L. F., Cole, C. N., McPeek, M. A. & Peterson, K. J. J. Exp. Zool. B Mol. Dev. Evol. 306, 575–588 (2006).
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    Peterson, K. J., Dietrich, M. R. & McPeek, M. A. Bioessays 31, 736–747 (2009).
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    Heimberg, A. M., Cowper-Sal·lari, R., Sémon, M., Donoghue, P. C. J. & Peterson K. J. Proc. Natl Acad. Sci. USA 107, 19379–19383 (2010).
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    Lyson, T. R. et al. Biol. Lett. 8, 104–107 (2012).
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    Wiegmann B. M. et al. Proc. Natl Acad. Sci. USA 108, 5690–5695 (2011).
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    Philippe, H. et al. Nature 470, 255–258 (2011).
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    Crawford, N. G. et al. Biol. Lett. http://dx.doi.org/10.1098/rsbl.2012.0331 (2012).
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    dos Reis, M. et al. Proc. Biol. Sci. http://dx.doi.org/10.1098/rspb.2012.0683 (2012).
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    Related stories and links

    From nature.com
    Turtles emerge from their evolutionary shell
    19 July 2011
    Evolution: A can of worms
    09 February 2011
    Leggy creatures and long branches
    10 August 2010
    Evolution: Mouth to mouth
    09 September 2009
    Evolution & ecology
    Evolution supplement
    From elsewhere
    Kevin Peterson
    MicroRNAs and metazoan phylogeny: big trees from little genes
    Author information

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    Elie Dolgin is a news editor with Nature Medicine in Cambridge, Massachusetts.
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    Comments
    7 commentsSubscribe to comments
    Avatar for Robert Vander VeldeRobert Vander Velde•2013-04-01 02:42 AM
    @James Dwyer, there is no higher and lower on a phylogenetic tree, for example the traditional tree here makes rodents look higher than humans, but if you rotate it (which doesn't actually change anything about the tree) you can make humans look higher.

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    Avatar for Jack KruseJack Kruse•2012-06-30 01:10 AM
    I think this scientist is following the science and not the dogma. As a neurosurgeon, I believe if we do not understand where we come from we have no hope of helping mankind from the diseases we face as a species today. We are mammals but modern healthcare seems to exempt us from their biology. I no longer do. It is like trying to repair a Ferrari engine without a manual. Here are my thoughts on the distal end of the mammal tree where we currently live. The answer to micro RNA problem is perspective, in my view. It is all from viral marketing. I am in the middle of a series that views this problem from the brain and gut evolution from primate to human. It fits beautifully with these findings. Forget about their jobs/careers......homo's healthcare depends upon this theory and I think we all need to know the results. Awesome article here from Nature!

    1. http://jackkruse.com/brain-gut-1-who-are-we-really/
    2. http://jackkruse.com/brain-gut-2-viral-marketing/

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    Avatar for yu wangyu wang•2012-06-29 09:55 AM
    I have found that the importance of miRNA has been over emphasised in some cases. For example, I was amused by “Small regulatory RNAs pitch in”, Nature, Vol 455, 1184, in which the number of miRNA genes in an animal is correlated to the number of neurons. This would imply that, us, the most studied animal on this planet, is the most complex organism.

    Each organism is a masterpiece of Natural Selection in its particular ecological niche. Michael Phelps can hardly compete with an Indo-Pacific sailfish, Istiophorus platypterus, which can cruise at a speed of 110 km/h. An African cheetah can easily outrun Usain Bolt. Chris Sharma, one of the best rock climber in the world, still admires the elegance of a spider’s navigation on a vertical surface. Admittedly, most of us don’t smell better than an ordinary Jasmine flower. So, why were we so arrogant to guess, at the beginning of the genomic era, that we should have the highest number of protein coding genes, and now, the highest number of miRNAs?

    The fact that we can debate about organismal complexity doesn’t automatically qualify us the holy status which had been denied by Charles Darwin almost 150 years ago. Plants have evolved a very complicated (if not more) small RNA regulatory system since the last hundred millions of years. But they, as taciturn as they naturally are, don’t bother to argue with us who are more sophisticated.

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    Avatar for yu wangyu wang•2012-06-29 09:53 AM
    I have found that the importance of miRNA has been over emphasised in some cases. For example, I was amused by “Small regulatory RNAs pitch in”, Nature, Vol 455, 1184, in which the number of miRNA genes in an animal is correlated to the number of neurons. This would imply that, us, the most studied animal on this planet, is the most complex organism.

    Each organism is a masterpiece of Natural Selection in its particular ecological niche. Michael Phelps can hardly compete with an Indo-Pacific sailfish, Istiophorus platypterus, which can cruise at a speed of 110 km/h. An African cheetah can easily outrun Usain Bolt. Chris Sharma, one of the best rock climber in the world, still admires the elegance of a spider’s navigation on a vertical surface. Admittedly, most of us don’t smell better than an ordinary Jasmine flower. So, why were we so arrogant to guess, at the beginning of the genomic era, that we should have the highest number of protein coding genes, and now, the highest number of miRNAs?

    The fact that we can debate about organismal complexity doesn’t automatically qualify us the holy status which had been denied by Charles Darwin almost 150 years ago. Plants have evolved a very complicated (if not more) small RNA regulatory system since the last hundred millions of years. But they, as taciturn as they naturally are, don’t bother to argue with us who are more sophisticated.

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    Avatar for Michael LermanMichael Lerman•2012-06-28 09:54 PM
    It is clear that you cannot build phylogenetic trees based solely on one type of data sets.It is also possible that we have now on Earth a "forest of phylogenetic trees" not a single one. Michael Lerman, Ph.D., M.D.

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    Avatar for James T. DwyerJames T. Dwyer•2012-06-28 02:17 PM
    Seeing that accumulated microRNA diversity indicates that cows and dogs are highest on the evolutionary tree, as an uninformed lay information systems analyst I have to wonder whether the accumulation of microRNA is associated with the diversity of traits allowed by humanity's influence on natural selection. Perhaps the breeding of dogs and cattle, and horses and cats (not shown), has produced greater diversity of microRNA within these species. In this case, the accumulation of microRNA would not indicate how long a species has been accumulating varieties but how many have been allowed to persist. Other selectively developed species, including crop plant varieties, would also have elevated levels of microRNA diversity.

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    Avatar for Michael ChisnallMichael Chisnall•2012-06-28 09:17 AM
    I've seen claims before about microRNAs being lost. I'm scpetical. Its been a while since I've read any microRNA papers in detail but as far as I know the software packages assume that microRNAs don't have introns. If this is still the case then any microRNA that gained an intron would be flagged as missing even though it was really still there.

  • מומחה מצוות מכון דוידסוןארז גרטי

    תשובה

    שלום שרון
    התייחסתי לנושא זה באחת התגובות למטה. אם יש לך שאלה ספציפית אתה מוזמן לשאול.
    כל טוב
    ארז

  • ברל לרנר

  • מומחה מצוות מכון דוידסוןארז גרטי

    בתחתית הנגן

    לחץ על הכפתור המלבני עם שני הפסים בתחתית הנגן ובחר באפשרות Hebrew

  • אבי

    עצים פילוגנטיים

    יש את המחקר הידוע של David Penny בשנות ה80 על רצפי חלבונים והעמדתם למבחן העץ הפילוגנטי.
    מצד שני יש את המחקר הזה:
    http://www.nature.com/news/phylogeny-rewriting-evolution-1.10885
    שכתוב בו:
    "Tiny molecules called microRNAs are tearing apart traditional ideas about the animal family tree."
    איך מיישבים בין 2 עצים פילוגנטיים סותרים (כשאחד בודק חלבון מסויים והשני את הmicroRNAs הנ''ל לדוגמא) ? איך בכלל יכולים להיות עצים פילוגנטיים סותרים? (שכל אחד מהם לעומת עצים אופציונליים אחרים, מובהק סטטיסטית ברמות מאוד גבוהות לפי מה שהבנתי)

  • מומחה מצוות מכון דוידסוןארז גרטי

    תשובה

    שלום אבי

    עצים פילוגנטים נבנים על סמך תכונה או גן מסוים. בודקים מידת קירבה של גנים אצל מינים שונים ועל סמך אותו גן או תכונה משליכים על הקרבה המשפחתית בין המינים. כאשר רוצים לוודא שהעץ הפילוגנטי אכן מייצג את הקרבה בצורה אמינה, לוקחים מספר עצים שנבנו על סמך מספר גנים או תכונות ומשקללים את הנתונים לעץ פילוגנטי שהכי מתאים לנתונים.

    מה שהחוקר מציג הוא עץ פילוגנטי המבוסס על קטעי מיקרו RNA, קטעים קצרים המשמשים לעיכוב ביטוי גנים (ראה מאמר בנושא). את האנליזה שלו הוא מבסס על כמה אלפי מיקרו RNA, כך שככל הנראה לא מדובר בשגיאה מקרית. עם זאת על פי המאמר בימים אלו מתבצע מחקר על כ-15אלף גנים מ36 יונקים שונים, דבר שאמור לבסס את העץ הפילוגנטי מבוסס ה-DNA. יתכן והעץ הזה יהיה דומה לקודם ויתכן שיהיה דומה לחדש. אך השאלה הנשאלת היא כיצד יכולים להיות שני עצים פילוגנטיים מבוססים אבל כל כך שונים?

    אפשרות אחת היא שהיות ולמיקרו RNA יש תפקיד בקרתי בעיקרו ופחות פונקציונאלי, לחץ הברירה הטבעית המופעל עליהם שונה מאשר על "גנים רגילים" ולכן הפרופיל השונה. אפשרות נוספת היא שיתכן ובחלקם הייתה רגרסיה חזרה למצב קדום יותר או קצב שינוי שונה מאשר של הגנים המרכיבים את העץ הפילוגנטי הקלאסי. אפשרות אחרת היא הולכה רוחבית בין זנים של מיקרו RNA, דבר שעשוי לשבש לחלוטין את העץ הפילוגנטי. יכולות להיות הרבה סיבות לשוני, וככה"נ שכולן נכונות במידה זו או אחרת.

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