Orb web of an araneoid spider, Argiope trifasciata, from Palo Alto, CA. [Image courtesy of Mark Chappell]
據(jù)美聯(lián)社6月22日?qǐng)?bào)道，科學(xué)家的最新研究結(jié)果稱,，最為經(jīng)典的蜘蛛網(wǎng)只發(fā)明過一次,，年代可追溯到大約1.36億年前的白堊紀(jì)時(shí)期。
這一經(jīng)典蜘蛛網(wǎng)稱為“圓網(wǎng)”,，一般是由兩個(gè)重要蜘蛛家族織出的圓形結(jié)構(gòu),，這也提高了這兩個(gè)蜘蛛家族分別進(jìn)化這種織網(wǎng)形式的可能性。據(jù)最新一期《科學(xué)》雜志刊登的一篇論文稱,，通過對(duì)與結(jié)網(wǎng)有關(guān)的蜘蛛基因進(jìn)行比較,，研究人員發(fā)現(xiàn)車輪裝圓網(wǎng)僅進(jìn)化過一次。
由美國加州大學(xué)里弗賽德分校的杰西卡·戈?duì)柌冀淌陬I(lǐng)導(dǎo)的研究小組對(duì)比了鬼面蛛(Deinopoidea)和金蛛(Araneoidea)織出的蜘蛛圓網(wǎng)。這兩種蜘蛛織網(wǎng)均是為了捕捉獵物,，鬼面蛛包括“撒網(wǎng)蛛”,，這種蜘蛛見到獵物時(shí)，會(huì)將改進(jìn)過的圓網(wǎng)扔向它們,。金蛛包括諸如金絲蛛和皿網(wǎng)蛛等圓蛛,，金絲蛛能結(jié)成螺旋形蜘蛛網(wǎng)。
戈?duì)柌荚谝环萋暶髦斜硎?，他們的研究發(fā)現(xiàn)“確實(shí)不支持有關(guān)圓網(wǎng)具有兩個(gè)來源的理論,，而且還表明蜘蛛家族這種獨(dú)特的結(jié)網(wǎng)設(shè)計(jì)只進(jìn)化過一次。”盡管兩個(gè)蜘蛛家族可能都在進(jìn)化來自同一個(gè)“祖先”的圓網(wǎng),，但它們結(jié)成的捕食蜘蛛網(wǎng)的途徑各不相同,。
戈?duì)柌急硎荆鹬刖W(wǎng)具有粘絲,，會(huì)將獵物粘在蜘蛛網(wǎng)上,，鬼面蛛則可利用另一種絲纖維將其蛛絲包起來，直到在顯微鏡下觀察它外觀像維可牢尼龍一樣,，它們就可利用類似的方式捕食獵物,。但并非所有的蜘蛛都具備織出圓網(wǎng)的本領(lǐng)，例如,，一種稱為“黑寡婦”的蜘蛛就只能織出亂糟糟纏成一團(tuán)的蜘蛛網(wǎng),，并未呈現(xiàn)出圓形狀。
同一期Science雜志刊登的另外一篇論文中,，由美國自然歷史博物館戴維·格里馬爾蒂領(lǐng)導(dǎo)的一個(gè)研究小組報(bào)告說,，他們發(fā)現(xiàn)了一種白堊紀(jì)時(shí)期的蜘蛛網(wǎng)，這個(gè)蜘蛛網(wǎng)連同一些被捕捉到的昆蟲一同包在琥珀中,。這塊在西班牙發(fā)現(xiàn)的琥珀保存有26股絲,，其中許多彼此相連。蜘蛛網(wǎng)上的粘絲清晰可見,，捕捉到的獵物有蒼蠅,、甲蟲、黃蜂等,。
據(jù)格里馬爾蒂介紹,，這塊琥珀的年代可追溯到距今大約1.1億年前，是已知最古老的蜘蛛網(wǎng)捕捉昆蟲的例子,。研究人員表示，這項(xiàng)發(fā)現(xiàn)證實(shí),，蜘蛛和結(jié)構(gòu)復(fù)雜,、粘性蜘蛛網(wǎng)的年代可追溯到一億多年前，這足以影響一些最為多樣的飛行昆蟲家族的進(jìn)化,。戈?duì)柌嫉难芯渴艿矫绹鴩铱茖W(xué)基金會(huì)和美陸軍研究辦公室的資助,，而格里馬爾蒂的研究經(jīng)費(fèi)則來自西班牙-法國科學(xué)研究項(xiàng)目和西班牙教育科學(xué)部,。
相關(guān)報(bào)道: 1.2億年前琥珀中發(fā)現(xiàn)蜘蛛 可能生在恐龍時(shí)代
The orb web, the classical wheel-shaped net that made Charlotte and countless of other spiders famous, is spun by two different groups of spiders. For a long time, scientists debated whether these two groups had evolved this marvel of engineering independently. Now, new evidence reported in the 23 June issue of Science suggests the orb web had a single evolutionary origin and may have been snagging flying insects as early as 136 million years ago.
New genetic evidence from one set of orb-web spinners called the Deinopoidea confirms that the group shares some key silk proteins with its fellow orb weavers the Araneoidea, according to postdoctoral researcher Jessica Garb of the University of California, Riverside and her colleagues. Based on fossil evidence, the ancestor of the two spider groups probably lived at least 136 million years ago, making the orb web an ancient adaptation.
In addition to spiders that construct typical orb-webs, deinopoids include the ogre-faced, net-casting spiders that throw a modified orb web stretched between their legs over their prey. Araneoids include the orb weavers such as golden silk spiders with their traditional spiraling web as well as sheet-web weaving spiders.
Garb says the finding “does not support a double origin for the orb web,” but indicates that the unique design evolved only once.
“A lot of people had said over the years that the orb web was a pinnacle of adaptive design. Our work confirms that not only is this web type very old, it was also lost in certain lineages of spiders,” Garb says, noting that some of today's descendants of the early orb weaver, such as the familiar black widow spider, weave a tangled web instead of the orb.
Piece of amber: Image showing one of the three small sections of amber that contains the fossilized spider web. [Image © Science]
Hymenopteran Evaniidae: Detail of a parasitic wasp trapped. [Image © Science]
In a second study, Enrique Peñalver of Universitat de Barcelona, Spain and colleagues provide a unique glimpse of an early weaver's handiwork. Encased in a 110-million year old piece of amber from a site in Spain , the researchers found pieces of a web with several insects still entangled.
The amber contains 26 web strands with a mite, a wasp leg and a beetle adhering to some sticky thread with visible droplets of web “glue.” The find is the oldest known example of a web with trapped insects, according to co-author David Grimaldi of the American Museum of Natural History.
Although Grimaldi and colleagues say there is not enough of the web preserved to know for sure, the parts that are preserved suggest an orb web design. In any case, Grimaldi said, the find indicates that the challenging silken traps of Early Cretaceous spiders may have put the pressure on many types of flying insects to evolve.
Although the insects in the ancient web belong to extinct groups, “their size and diversity are precisely what one would expect to see in modern webs—small wasps, flies and beetles, groups that are abundant and diverse today and that are important pollinators as well,” Grimaldi says. “Apparently, spiders have been fishing insects from the air for a very long time.”
But do orb weavers all use the same sort of fishing line? To find out, Garb and colleagues focused on the genetics of several key silk proteins that form the building blocks of an orb web, including the outside wheel, spokes and sticky capture threads of the orb's spiral.
The capture spirals of deinopoids and araneoids use different techniques to achieve stickiness. Araneoids produce glue droplets to make their capture spirals adhesive. Deinopoids, on the other hand, wrap their capture threads with a different type of silk fiber that “the spiders comb, until it almost has the appearance of Velcro under a microscope, and they snag insects that way,” Garb explains.
The sticky difference was part of what made scientists think deinopoids and araneoids had independently evolved the orb web instead of inheriting it from a common ancestor. But no one had looked at the underlying genetics of the deinopoid web silks. Working with assistant professor Cheryl Hayashi and two undergraduate students, Garb and the team now show that both spider groups use the same set of web-building silks.
The protein building blocks of spider silk are of great interest to industry, which would like to duplicate silk's amazing blend of strength, stretchiness and toughness for things like bandages, bulletproof fibers, aerospace tethers and nets. But for these applications to become a reality, researchers need to understand exactly how spider silks differ down to the genetic level.
“Because there's this diversity of spider silks and spiders use them for different functions, they have different mechanical properties,” Garb says. “And to understand where this variation in mechanical properties comes from, we have to start with looking at the proteins that make up these silks.”
Since most silk studies have been done on one or two species of araneoid spiders, Garb says the new look at deinopoid silk expands the list of “potential models for new materials” designed with the diversity of natural spider silks in mind.
The Garb study was supported by the National Science Foundation and the U.S. Army Research Office. The Peñalver study was supported by the Spanish Ministry of Education and Science.
Science is published by AAAS.
Becky Ham
22 June 2006
