Orb web of an araneoid spider, Argiope trifasciata, from Palo Alto, CA. [Image courtesy of Mark Chappell]
据美联社6月22日报道，科学家的最新研究结果称，最为经典的蜘蛛网只发明过一次，年代可追溯到大约1.36亿年前的白垩纪时期。
这一经典蜘蛛网称为“圆网”，一般是由两个重要蜘蛛家族织出的圆形结构，这也提高了这两个蜘蛛家族分别进化这种织网形式的可能性。据最新一期《科学》杂志刊登的一篇论文称，通过对与结网有关的蜘蛛基因进行比较，研究人员发现车轮装圆网仅进化过一次。
由美国加州大学里弗赛德分校的杰西卡·戈尔布教授领导的研究小组对比了鬼面蛛(Deinopoidea)和金蛛(Araneoidea)织出的蜘蛛圆网。这两种蜘蛛织网均是为了捕捉猎物，鬼面蛛包括“撒网蛛”，这种蜘蛛见到猎物时，会将改进过的圆网扔向它们。金蛛包括诸如金丝蛛和皿网蛛等圆蛛，金丝蛛能结成螺旋形蜘蛛网。
戈尔布在一份声明中表示，他们的研究发现“确实不支持有关圆网具有两个来源的理论，而且还表明蜘蛛家族这种独特的结网设计只进化过一次。”尽管两个蜘蛛家族可能都在进化来自同一个“祖先”的圆网，但它们结成的捕食蜘蛛网的途径各不相同。
戈尔布表示，金蛛网具有粘丝，会将猎物粘在蜘蛛网上，鬼面蛛则可利用另一种丝纤维将其蛛丝包起来，直到在显微镜下观察它外观像维可牢尼龙一样，它们就可利用类似的方式捕食猎物。但并非所有的蜘蛛都具备织出圆网的本领，例如，一种称为“黑寡妇”的蜘蛛就只能织出乱糟糟缠成一团的蜘蛛网，并未呈现出圆形状。
同一期Science杂志刊登的另外一篇论文中，由美国自然历史博物馆戴维·格里马尔蒂领导的一个研究小组报告说，他们发现了一种白垩纪时期的蜘蛛网，这个蜘蛛网连同一些被捕捉到的昆虫一同包在琥珀中。这块在西班牙发现的琥珀保存有26股丝，其中许多彼此相连。蜘蛛网上的粘丝清晰可见，捕捉到的猎物有苍蝇、甲虫、黄蜂等。
据格里马尔蒂介绍，这块琥珀的年代可追溯到距今大约1.1亿年前，是已知最古老的蜘蛛网捕捉昆虫的例子。研究人员表示，这项发现证实，蜘蛛和结构复杂、粘性蜘蛛网的年代可追溯到一亿多年前，这足以影响一些最为多样的飞行昆虫家族的进化。戈尔布的研究受到美国国家科学基金会和美陆军研究办公室的资助，而格里马尔蒂的研究经费则来自西班牙-法国科学研究项目和西班牙教育科学部。
相关报道: 1.2亿年前琥珀中发现蜘蛛 可能生在恐龙时代
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
