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Evolution and Culture: May 2009 Archives

COLD SPRING HARBOR, NEW YORK—Going from one cell to many all in one organism was no easy feat. Multicellularity required molecular machinery that made it possible for cells to stick and work together. They needed to be able to talk to one another and to recognize and deter intruder cells.

But the last unicellular ancestor of animals was ready to make that leap. And even the most ancient multicellular animals were equipped with a skeleton crew of the genes that made the diversity of animal forms seen today possible. That was the news from the Cold Spring Harbor Symposium “Evolution: The Molecular Landscape,” held from 27 May to 1 June. Two teams have reached down to the base of the animal tree of life to learn how this key evolutionary transition occurred.

Nicole King of the University of California, Berkeley, and her colleagues use choanoflagellates as stand-ins for the last unicellular ancestor of animals. These single-celled creatures ColonyClose look and function a lot like cells called choanocytes in sponges. A flagellum sticks out of one end, whipping up water currents to circulate bacteria back toward a "collar" that slurps up the microbes. Some of the 150 species form colonies (left), approximating multicellular life.

A choanoflagellate genome, published last year, revealed genes for proteins that animals use for cell adhesion and signaling, and King has been looking into what those genes do in single-celled organisms. From those studies, she says, "we can learn some basic mechanisms of what was in place in the common ancestor."

One surprise in the genome were two dozen genes for cadherins, proteins that are critical for holding cells together in all animals. If cadherin genes are disabled during development, the embryo falls apart. Such genes had never been found outside a multicellular animal before. "We think [cadherins] are distinguishing different prey," King reported at the meeting. She and her colleagues have been examining these cadherin genes one by one. They have determined that some cadherin genes are active in the collar. They also find the amount of a particular cadherin protein depends on what bacteria are present in the surrounding water. For example, three cadherins are upregulated in the presence of flavobacter microbes but not when enterobacter bacteria are present, whereas the concentrations of other cadherins are unaffected.

King suspects that cadherin senses and binds to the bacteria, as some pathogenic microbes dock at cadherins to invade human cells. "[Choanoflagellates] have systems in place that allowed cell-cell recognition. [We] can see how one could evolve into a multicellular organism by using [these proteins] in a different way," says Bruce Stillman, president of Cold Spring Harbor Laboratory in New York.

Choanoflagellates were a prelude to sponges, which evolved 600 million years ago and as such are the oldest extant animals. Sponges split off from the animal tree of life early on and maintainadult4ed their simple body plan—sans muscles and a nervous system—while the eumetazoans evolved wings, fins, feet, heads, and tails to create the myriad of shapes and sizes seen in the animal kingdom today. By looking at the newly sequenced genome of one sponge, Amphimedon queenslandica (left), and choanoflagellates, "we can appreciate how multicellular organisms came about," says Stillman.

The protoanimal genome was quite busy during the more than 100 million years between choanoflagellates and the evolution of sponges. Bernard Degnan of the University of Queensland, Brisbane, Australia, and his colleagues have looked for what's common to all the animal genomes, concluding that they share about 5000 ancestral gene families, 1300 of which must have evolved during that time because they have no representatives in the choanoflagellates yet are found in sponges. For example, most of the genes needed to make the epithelium, which separates an organism from the outside world, appeared first in the sponge. In other gene families, in which choanoflagellates have a single gene, the families have expanded in sponges and sometimes exploded in more complex animals.

The sponges have precursors to some of the key development genes. One called hedgling codes for a protein that's like the signaling molecule called hedgehog, but it's locked to the cell membrane and cannot travel from cell to cell as does hedgehog. Hedgling "is still probably a signaling molecule but plays a different role," Degnan reported.

What sponges seem to lack is complex regulation of all these genes. In other animals, genes are often used in multiple contexts, but this is not the case for the sponge, says Degnan. Its genome is compact, with little room for regulatory DNA in between genes. This regulatory simplicity, he adds, "may be the key to why the [sponge] body plan has not changed" for 600 million years.

—Elizabeth Pennisi

Credits: Choanoflagellate Colony, Mark Dayel; Sea Sponge, Gemma Richards

After receiving the red-carpet treatment in New York City and London (Science, 29 May, p. 1124), the fossil primate known as Ida returned home to Oslo this week, where she will appear at the University of Oslo Natural History Museum starting 5 June. Meanwhile, life-size casts of Ida are already on display at the American Museum of Natural History in New York City and the Natural History Museum in London—and others will soon appear at museums in Germany, not far from the Messel pit where the 47-million-year-old fossil was found, according to paleontologist Jørn Hurum of the Oslo museum.

The fossil of an ancient adapid primate formally named Darwinius masillae was unveiled by Mayor Michael Bloomberg in New York City on 19 May. In one remarkable week, Ida was the subject of a media blitz worthy of the winner of American Idol. First, she starred on 25 May in The Link, a documentary film shown by The History Channel in the United States. The next night, Ida appeared in London with the filmmaker Richard Attenborough, who narrated the version of the film shown on BBC One in England on 26 May. By the end of the week, she was the first fossil to be featured on Google’s home page and the subject of a companion book to the documentary and a Web site.

But one group was conspicuously absent in the toasts to Ida’s success: the scientists who are the world’s experts in primate evolution. Even though they were impressed by how complete and well-preserved the fossil primate was, many were troubled by the branding of Ida as a “missing link” to humans (ScienceNOW, 19 May). Most complained that Ida was the ancestor of lemurs, not humans, and that the hype was not justified by the analysis provided in the scientific paper published last week in the online journal PLoS ONE. The uncovering of Ida’s true identity, however, failed to rain on her parade: At least 2 million viewers tuned in at 9:00 p.m. EDT on 25 May to watch Ida on The History Channel, according to Nielsen Fast Cable ratings. That is up 67% compared with The History Channel’s prime-time average.

—Ann Gibbons

Now a Ph.D. student in evolutionary biology, Nicholas J. Matzke was a public information officer at the National Center for Science Education (NCSE) back in 2005. As such, he played a key role in NCSE’s participation in the Kitzmiller v. Dover Area School District trial that pitted intelligent design (ID) proponents against supporters of evolution. In particular, Matzke was central to the trial’s focus on the evolution of the immune system and the cross-examination of ID proponent Michael Behe. He recalls that episode, described in this month’s Origins essay looking at the evolution of the immune system, in an e-mail interview (edited for clarity) with Science's John Travis.

Q: Was it obvious to make the origin of the immune system a focal point of the case? I read that previous online debates with ID proponents led to the choice.

N.M.: Yeah, partially. The fuller story is that for several years, 2001-2004 or so, a number of us "Internet creationism fighters," of which I was one (as a hobby, before I worked at NCSE), would get on various UBB bulletin boards and newsgroups (and blogs starting in about 2004) and debate the ID guys. We were the people associated with TalkOrigins.org, TalkDesign.org, etc. (Later, this group became the Panda's Thumb bloggers.) Anyway, these debates were long and covered just about every topic in more detail than almost anyone could want. After doing this for years, we got a sense of not just where and how the IDists were wrong (since they are wrong on just about everything), but areas where they are spectacularly, obviously, blatantly, embarrassingly wrong. E.g., Behe's irreducible complexity (IC) argument is the favorite ID argument. And it is true that in 1996, some of the biochemical systems Behe used as examples had not received much attention in terms of their evolution. However, the immune system had received lots of attention even in 1996, and much more by 2005, primarily because (1) it is medically crucial, so there are many more researchers/funding/studies on it, and (2) much of immunology going back to the beginning has relied on comparative studies in animals, so there has been an explicit evolutionary context for 100 years in that field.

The amount of work is relevant because the IC argument always goes like this: 1. ID guy: Natural selection can't explain an IC structure because all of the parts would have to come together at once. 2. Evo guy: Here are some systems with only some of the parts but they still have some function, so your argument doesn't work. 3. ID guy: That doesn't explain how these systems arose, we need to see peer-reviewed publications giving detailed, testable explanations. 4. Evo guy: Here is a peer-reviewed publication on the topic. 5. ID guy: It's not detailed enough, I need to see every single mutation and selection event detailed or I will still say that ID was responsible, not evolution.

At this point, the ID guys have (a) given up on their original IC argument and (b) demanded an impossible, ridiculous amount of detail for the evolutionary explanation, while providing no details or tests of their own explanation. It looks ridiculous from the outside, but ID guys, including Behe, made these moves so regularly that it was pretty predictable.

So, in 2002 this began to become obvious when Matt Inlay wrote it up in an essay for talkdesign.org ("Evolving Immunity"). We then jumped Dembski with it in 2002 or 2003 on his own Internet forum at iscid.org and observed the above progression. Then we posted a bunch of references to articles on the topic and challenged Dembski to provide as much detail for the ID explanation. Here was Dembski's response:

"ID is not a mechanistic theory, and it's not ID's task to match your pathetic level of detail in telling mechanistic stories."

A similar episode happened with Behe in 2005.

In spring 2005, Eric Rothschild began preparing for Behe's deposition in the Kitzmiller case, which was happening in May. I gave him all this background and said if we wanted to pick one system to challenge Behe on, it should be the immune system. We poked him a bit on it at the deposition and got the expected replies.

So then, before the trial, I assembled the stack of books (from the UC Berkeley biosciences library) and articles on the evolution of the immune system and made a big exhibit for Eric to use. Eric asked the questions and got the expected replies, so when Behe started making noises about how the science "wasn't detailed enough," Eric started piling books and articles on the stand, and asking Behe if it was good enough for him. The rest is history...

Q.: You have called the Behe cross-examination on immune origins a "turning point" in the trial. Why do you say that?

N.M.: Well, it was kind of the ultimate Behe defeat amongst a long string of defeats during the Behe cross. I think Eric's whole cross was a "turning point" in that Behe's direct testimony was the one big chance the defense had to come back after the plaintiffs had been beating on ID for 3 weeks during the plaintiffs' case.

It was kind of a turning point for the whole ID argument over the last decade or two because it really exposed for all to see that ID was mostly boasting and dissembling, compared to the substance (physical substance, in the case of the immune system exhibit!) of the evolutionary science.

It was very gratifying to have my very obscure hobby turn into a key skill in an internationally recognized court case. It was kind of like the movie Galaxy Quest, where the Trekkie nerd gets told that the spaceship and aliens from the Star Trek-esque TV show are all real, and his nerdy knowledge saves the day.

Work being reported today in Biology Letters about pitcher plants would please Charles Darwin. This curious naturalist was so intrigued by carnivorous plants that he wrote a 400-page book, Insectivorous plants, detailing his thoughts and observations on these unusual species. He was originpitcherplantpixfascinated by the similarities in the insect traps of unrelated plants and demonstrated that these plants had the ability to digest and absorb the prey they caught. This week, researchers report on how one species, a pitcher plant found in Canada and the eastern United States (left), snags its dinner. At issue was whether a red color or a sweet treat is what makes this death trap so appealing.

In 2007, H. Martin Schaefer of the University of Freiburg in Germany evaluated the role of pitcher color in luring insects. Using 20 pitcher plants (Nepenthes ventricosa) from Southeast Asia, his team painted half the pitchers green and half of them red, then placed the plants outside, counting the number of trapped insects after 1 and 2 weeks. They found that red plants caught more prey, particularly flies. They assumed that the plants smelled alike—like paint—to the insects. Others had suggested that the UV marks or visual stripes on pitchers were needed to reel insects in, but the opaque red pitchers were quite effective without such decorations, they reported.

But Aaron Ellison of Harvard University wasn’t satisfied with this finding, given that the plants were not in their native environment, where typically they trap ants and termites, not flies. He wondered whether pitcher plants were taking advantage of a sweet tooth in their victims. Pitchers produce excessive amounts of nectar, separate from what the flowers make. And earlier this year, Ganesh P. Bhattarai and John D. Horner of Texas Christian University in Fort Worth had suggested that odors, including sweet ones, could be important attractants.

Ellison recruited Katherine Bennett, a local elementary school teacher who had become interested in science through a Harvard program for schoolchildren to help scientists gather data. Bennett suctioned prey carcasses from pitchers of 25 Sarracenia purpurea pitcher plants growing in a Massachusetts bog, then checked again 3 days later for new prey. After several rounds of suctioning and inventorying the plants, she and Ellison collected the plants and assessed their redness by measuring spectral reflectance. Next, they made 70 plastic centrifuge tubes into “pseudopitchers” painted either green, red, or green with red stripes. Ten were left unpainted. They streaked half the pitchers with thickened corn syrup and embedded all the tubes at angles in a bog close to the plants surveyed earlier. Several times, they counted the trapped insects, comparing the pseudopitchers’ body count with the real plants’.

They retrieved about 350 prey each from real and nectar-streaked fake pitchers, whereas the unsweetened pseudopitchers garnered only about 60, they report in Biology Letters. Ants were the most susceptible to the nectar lure. The amount of red didn’t seem to matter much at all. “We only manipulated the color of pitcher plants," comments Schaefer. "This study goes one step further and [shows that] the effects of sugar rewards outweigh those of color. This experimental support has been lacking.”

As for Bennett, she says, her experience has “transformed the way I teach science.”

—Elizabeth Pennisi

Photo: Katherine Bennett