| | http://only-perception.blogspot.com/2008/01/blog-post_9151.html
2008-01-20
科學家發現大樺斑蝶遷徙的分子基礎
Scientists discover molecular basis of monarch butterfly migration
http://www.physorg.com/news119003419.html
January 08, 2008
自從被發現之後，北美洲東部大樺斑蝶（monarch butterfly，帝王蝶、王斑蝶）的年度遷徙擄獲了人類的想像力與心靈。每年有數百萬隻蝴蝶飛行了幾千英里到中墨西哥松樹林叢生的中墨西哥，涵括範圍約 70 平方英里，在許多方面來說，是個令人敬畏且神祕的事件。然而，過去 20 年來，科學家開始揭開這趟旅程背後的緣由：那一種生物學壯觀的結果，由蝴蝶腦部一小群細胞中，一種錯綜複雜的分子機制所驅使。
麻州大學醫學院教授以及神經生物學系主任 Steven M. Reppert, MD，成為了致力揭開大樺斑蝶遷徙神祕面紗的一股先導力量。他的先前研究證明，蝴蝶利用一種具時間補償的太陽羅盤（time-compensated sun compass）與日光提示協助引導牠們抵達松樹林。他的研究證明時間補償是由蝴蝶的晝夜節律時鐘（circadian clock）所提供，即使太陽橫跨天空，那仍允許帝王蝶持續校正其飛行方位以維持一個固定的方向。
現在，有二篇論文將於本週發表在開放進入之出版者 Public Library of Science (PLoS) 旗下的二份期刊上。 Dr. Reppert 等人首度詳述大樺斑蝶的晝夜節律時鐘，並確認與描繪一種全新時鐘基因的特性，其所提供的深刻理解不只包含了蝴蝶的生物學與牠的遷移，還包括一般晝夜節律時鐘的演化。
在「Cryptochromes Define a Novel Circadian Clock Mechanism in Monarch Butterflies That May Underlie Sun Compass Navigation」這篇文章中，發表在 PLoS Biology 上，Reppert 等人揭露大樺斑蝶的晝夜節律時鐘利用一種新穎的分子機制，那迄今為未曾在其他昆蟲或哺乳類動物身上發現。
藉由研究其他二種生物 -- 果蠅與老鼠 -- 科學家認為牠們分別是理解昆蟲時鐘與哺乳類動物時鐘的優良模型。透過這些研究，科學家描述了一種時鐘機制，那本質上是種循環（loop），蛋白質在一個大約需要 24 小時來完成的週期內被製造與摧毀。此外，研究者確認出這些一同作用，驅動此過程的因子。
Reppert 等人對於其中一種因子格外感到有興趣：CRY，一種隱花色素（cryptochrome）蛋白，那最初在植物當中發現，隨後也在果蠅與老鼠身上發現。在果蠅中，CRY 作用如同一種藍光受體（blue light photoreceptor），讓光線能進入（access）包含時鐘的細胞。在老鼠身上，CRY 並不會吸收光線，相反地，它是一種不可或缺的元件，為中央的發條（clockwork）提供動力，讓回饋圈（feedback loop）能持續下去。（在老鼠身上，光透過動物的眼睛進入此時鐘。）
考慮到 CRY 在果蠅身上的功能以及光在遷移當中的角色，科學家假設大樺斑蝶的時鐘與果蠅相似。Reppert 等人因發現蝴蝶的時鐘如同其遷徙一樣壯麗而感到吃驚與興高采烈。遺傳研究揭露，大樺斑蝶不只具有像果蠅一樣的 CRY，而且還有另一種隱花色素，那進一步測試後確認為蝴蝶體內，一種新的時鐘分子。令人驚訝地，這種隱花色素，封為 CRY2，在結構上反而與脊椎動物的 CRY 更為相似。
特別是，科學家們也發現大樺斑蝶時鐘的核心成份與那些哺乳類動物時鐘相似。如同在老鼠身上，CRY2 在蝴蝶體內作用以維持此回饋圈，而 CRY1 則如同果蠅那樣，讓光線仍能進入細胞。
"這是個相當有趣的，關於一人如何考慮昆蟲時鐘模型的重新組合。在此沒有理由懷疑蝴蝶的時鐘與果蠅（drosophila）會相異。而這些差異已經告訴我們一些關於晝夜節律時鐘如何演化的事，" Reppert 解釋道。"我們在蝴蝶體內所看見的是種令人震驚的時鐘機制，那與我們自己的晝夜節律時鐘更加相似，而與果蠅反而不太相同！出現二種功能迥異的 CRYs 指出大樺斑蝶的（時鐘）是一種祖傳的（ancestral）時鐘；這種時鐘，在整個演化路線中，已經改變成不同於其他昆蟲與哺乳類動物。"
Reppert 等人不只發現 CRY2 在帝王蝶時鐘中的功能，他們也發現 CRY2 也具有標記一種關鍵神經路徑，從晝夜節律時鐘到太陽羅盤，的功能。這種時鐘到羅盤的路徑在時鐘與太陽羅盤之間提供一種基本的連結，這兩種都對於成功的定位與導航不可或缺。如同 Reppert 所解釋的，"CRY2 顯然具有雙重功能 -- 身為一種核心時鐘元件與一種輸出模組，將時鐘與羅盤連結在一起。"
在他們大樺斑蝶時鐘的研究與關於 CRY2 的鑑別之外，Reppert 等人也同時合作創造出一個蝴蝶基因組資源。
在「Chasing Migration Genes: A Brain Expressed Sequence Tag Resource for Summer and Migratory Monarch Butterflies (Danaus plexippus)」這篇發表於 PLoS ONE 的文章中，Reppert 等人描述一種腦部表現順序標籤（brain expressed sequence tag，EST），藉由比較基因在遷徙性與非遷徙性蝴蝶腦中的基因表現以確認涉及遷徙行為的基因。
他們已確認約 10000 種 ESTs，那佔大樺斑蝶基因組 50% 以上的基因。ESTs，那代表基因在蝴蝶腦中的表現單位（expression units），目前正在分析與歸類，而且 Reppert 希望此遺傳資訊將能被世界各地的科學家所廣泛利用（譯註：詳見相關報導）。
"這資訊，以及在此研究中所確認的遺傳標記，將協助我們區別族群之間，或甚至是遷徙性與非遷徙性蝴蝶之間的遺傳差異，" Reppert 說，並補充道："此資訊為蝴蝶基因組的複製設置了舞台。"
事實上，Reppert 以及他的研究員最近與 SymBio Corporation (www.sym-bio.com) 開始了一項合作協議，要將整個蝴蝶基因組定序。根據 Robert A. Feldman，SymBio 的總裁與 CEO，表示："我們勘查大樺斑蝶基因組的序列感到相當興奮。所取得的資訊不僅能幫助闡明蝴蝶遷徙的分子基礎，還能夠為比較基因組的研究增添大量知識。" SymBio 專精各種生物的基因組定序，從細菌到哺乳類動物。
最終，Reppert 實驗室將繼續研究以了解大樺斑蝶時鐘如何以 CRY2 的力量與太陽羅盤「交談」。研究者的研究目標是要了解時鐘--羅盤交互作用的分子與解剖學基礎，當太陽在一日之內越過天際時，那能夠讓遷徙維持在固定的飛行方向上。
Dr. Reppert 也說明，"大樺斑蝶為神經生物學提供一種迷人的動物模型。藉由了解更多關於晝夜節律時鐘與太陽羅盤交互作用的方式，讓大樺斑蝶能夠達到其生物學密度，我們將獲得關於「腦部如何作用以包含關於時間及空間資訊」的有用洞見，那已遠超過與蝴蝶之間的關連。"
http://www.physorg.com/news119003419.html
Scientists discover molecular basis of monarch butterfly migration
Since its discovery, the annual migration of eastern North American monarch butterflies has captivated the human imagination and spirit. That millions of butterflies annually fly a few thousand miles to reach a cluster of pine groves in central Mexico comprising just 70 square miles is, for many, an awesome and mysterious occurrence. However, over the past two decades, scientists have begun to unveil the journey for what it is: a spectacular result of biology, driven by an intricate molecular mechanism in a tiny cluster of cells in the butterfly brain.
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University of Massachusetts Medical School Professor and Chair of Neurobiology Steven M. Reppert, MD, has been a pioneering force in the effort to demystify the migration of the monarch. His previous research has demonstrated that the butterflies use a time-compensated sun compass and daylight cues to help them navigate to the pine groves. His studies have shown that time compensation is provided by the butterfly’s circadian clock, which allows the monarch to continually correct its flight direction to maintain a fixed flight bearing even as the sun moves across the sky.
Now, in two papers that will be published this week in two journals of the open-access publisher Public Library of Science (PLoS), Dr. Reppert and colleagues describe in detail the monarch butterfly circadian clock for the first time, and identify and characterize an entirely new clock gene that provides insight into not only the biology of the butterfly and its migration, but also the evolution of circadian clocks in general.
In “Cryptochromes Define a Novel Circadian Clock Mechanism in Monarch Butterflies That May Underlie Sun Compass Navigation,” published in PLoS Biology, Reppert and colleagues reveal that the circadian clock of the monarch uses a novel molecular mechanism, heretofore not found in any other insect or mammal.
By studying the clock in two other organisms—the fruit fly and the mouse—scientists thought that they had very good models for an understanding of the insect clock and the mammalian clock, respectively. Through these studies, scientists had described a clock mechanism that is essentially a loop where proteins are made and destroyed over a cycle that takes approximately 24 hours to complete. Further, investigators identified those factors that work together to drive this process.
Reppert and colleagues were particularly interested in one of these factors: CRY, a cryptochrome protein that was initially discovered in plants and was subsequently found in the fly and the mouse. In the fly, CRY functions as a blue light photoreceptor, allowing light access to clock-containing cells. This enables the resetting of the clock by the light-dark cycle. In the mouse, CRY does not function to absorb light; rather, it is one of the essential components that power the central clockwork enabling the feedback loop to continue. (In the mouse, light enters the clock through the animal’s eyes.)
Given the function of CRY in flies and the role of light in migration, scientists presumed that the monarch’s clock would resemble that of the fly. Reppert and his collaborators were stunned and elated to find that the clock of the butterfly was as spectacular as its migration. Genetic studies revealed that the monarch had not only the fly-like CRY, but also another cryptochrome that further tests identified as a new clock molecule in the butterfly. Surprisingly, this cryptochrome, dubbed CRY2, is more similar in structure to vertebrate CRY than to that of the fruit fly.
Notably, the scientists also found that the core components of the monarch clock resembled those of the mammalian clock. As in the mouse, CRY2 functions in the butterfly to maintain the feedback loop, while CRY1 still allows light to access the cells, as in the fly.
“This is a very interesting realignment of how one thinks about insect clock models. There was no reason to suspect that the butterfly clock would be different from that of Drosophila. That it is different has already told us something about how circadian clocks have evolved,” explained Reppert. “What we have in the butterfly is an astounding clock mechanism, one that is more similar to our own circadian clock and less similar to the clock of the fly! The presence and function of two distinct CRYs suggest that the monarch’s is an ancestral clock; a clock that, over the course of evolution, has changed differently in other insects and mammals.”
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Reppert and colleagues not only discovered the function of CRY2 in the monarch clock, but they also found that CRY2 may function to mark a critical neural pathway from the circadian clock to the sun compass. This clock-to-compass pathway provides an essential link between the clock and the sun compass, as both are necessary for successful orientation and navigation. As Reppert explains, “CRY2 appears to have a dual function— as a core clock component and as an output molecule, linking the clock to the compass.”
Concurrent with their studies of the monarch clock and relevant to the identification of CRY2, Reppert and colleagues have been working to create a butterfly genomics resource.
In “Chasing Migration Genes: A Brain Expressed Sequence Tag Resource for Summer and Migratory Monarch Butterflies (Danaus plexippus),” published in PLoS ONE, Reppert and his collaborators describe a brain expressed sequence tag (EST) resource, used to identify genes involved in migratory behaviors by comparing the gene expression in the brains of migrating butterflies to those of non-migrating butterflies. They have already identified ~10,000 ESTs that likely represent over 50 percent of the genes that make up the monarch genome. The ESTs, which represent expression units of genes in the butterfly brain, are currently being analyzed and catalogued and Reppert hopes that the genetic information will be of wide use to scientists around the world.
“This information, along with genetic markers identified in the study, will help us distinguish genetic differences between populations or even between butterflies that are migratory and not migratory” Reppert said, adding, “This information sets the stage for the cloning of the butterfly genome.”
In fact, Reppert and his fellow investigators recently initiated a collaborative agreement with SymBio Corporation (www.sym-bio.com) of Menlo Park, CA to sequence the entire butterfly genome. According to Robert A. Feldman, President and CEO of SymBio, “We are very excited about the prospect of sequencing the monarch genome. The information gained will not only help elucidate the molecular basis of butterfly migration, but will also add substantial knowledge to comparative genomic studies.” SymBio specializes in sequencing the genomes of a wide range of organisms, from bacteria to mammals.
Ultimately, the Reppert laboratory will continue to work to understand how the monarch clock “talks” to the sun compass, with a focus on CRY2. The goal of the researchers’ studies is to understand the molecular mechanism and anatomical mechanisms for clock-compass interactions that enable migrants to maintain a set flight bearing as the sun moves across the sky during the day.
Dr. Reppert also states, “The monarch provides a fascinating animal model for the study of neurobiology. By understanding more about the way the circadian clock and the sun compass interact to allow the monarch to fulfill its biological destiny, we will gain valuable insights into how the brain functions to incorporate information about time and space, which has relevance far beyond the butterfly.”
Citation: Zhu H, Sauman I, Yuan Q, Casselman A, Emery-Le M, et al. (2008) Cryptochromes define a novel circadian clock mechanism in monarch butterflies that may underlie sun compass navigation. PLoS Biol 6(1): e4. doi:10.1371/journal.pbio.0060004
Citation: Zhu H, Casselman A, Reppert SM (2008) Chasing Migration Genes: A Brain Expressed Sequence Tag Resource for Summer and Migratory Monarch Butterflies (Danaus plexippus). PLoS ONE 3(1): e1345. doi:10.1371/journal.pone.0001345
Link: www.plosbiology.org
Source: Public Library of Science
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