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Are guinea pigs REALLY from the rodent family?? Read on to find out

Reprinted from a Scholastic News Ranger Vol. 48 No. 2 October 1991

    from UMCBA Newsletter Spring 1992

Houston, Texas - Scientists are working hard to find a family for the guinea pig. An animal family, that is.

The guinea pig is officially listed as a rodent. That would make it a member of the rat and mouse family. But some scientists are not sure guinea pigs belong in that family.

Scientists already know that proteins in guinea pig blood are not like other rodents'. Some scientists decided to find out more. After doing research, they decided that it was possible that guinea pigs are a family all by themselves.

Dividing animals into families makes researching animals easier. That way, scientists can study one family member to learn about the other. That is why it is important to know exactly which family an animal comes from.

So far, the guinea pig researchers don't have any real answers. They just want to be sure that guinea pigs end up in the right family tree.

June 13, 1996 Guinea Pigs Not Rodents? Scientific Panel Says DNA Is the Key By NATALIE ANGIER Copyright 1996 The New York Times -------------------------------------------------------------------------------- Consider it the case of the guinea pigs that roared. One cried, "I am Rodent!" and the other, "You're nuts -- Rodentia is dead!" In a bluntly worded new report, researchers from Italy and Sweden have declared that the guinea pig, famed as a child's pet and medicine's sacrificial lamb, is not now and never has been a rodent. That conclusion may sound like a rather narrow scientific matter, but in fact the implications of the guinea pig's unmoored status are profound, calling into question the entire concept of rodenthood. People may think they recognize a rodent when they see it scurry by in the park, gnaw through apartment plumbing or jump merrily over a glue trap. Yet the new analysis suggests that guinea pigs, rats, mice, squirrels, porcupines and hundreds of other species, long classified together under the order Rodentia, may not warrant assemblage into a single distinctive order. Instead, the new work indicates, the creatures may be better thought of as a ragtag band of only vaguely related animals displaying generic and ancient mammalian features. The report, a detailed molecular analysis of genetic relatedness among various species of rodents and other mammals, appears on Thursday in the journal Nature. Among the hallmarks of a biological order is that members of the group are all thought to descend from a single common ancestor -- that is, they are said to be monophyletic. But the latest report presents data indicating that rodents stem from at least two distinct ancestors, and possibly more. "The main achievement of this paper is to say that rodents are polyphyletic," said Dr. Cecilia Saccone of the University of Bari in Italy. "There are at least two, and maybe other, branches in the group." Dr. Saccone wrote the report with Dr. Ulfur Arnason of the University of Lund in Sweden and their colleagues. Not surprisingly for a report with such radical ramifications, other scientists attacked it as uncredible, naive and full of holes. "It's the most ridiculous thing I've ever heard of," said Dr. Rodney Honeycutt, who studies the molecular evolution of rodents and other mammals at Texas A&M University. "There's a huge amount of data showing that rodents are unequivocally monophyletic." Beyond upsetting traditional notions of what a rodent is, the report threatens biology's understanding of mammalian evolution as a whole. Dr. Michael Novacek, an expert in evolution and taxonomy at the American Museum of Natural History in New York, explained that rodents had long been viewed as a sort of exemplary group of mammals. They are extremely successful and diverse, with more species to their credit than any other mammalian group. By current reckoning, half of all 4,000 known mammals are rodents. "Looking at a model system like rodents can tell us a lot about how evolution works," Novacek said. "Understanding whether rodents come from one or two or more points of origin has a lot of bearing on how we view the world's most successful mammalian group." Regarding the group's importance in understanding the evolution of mammals, he said, "it's like asking whether life on earth originated once or multiple times." Novacek said that while he was not yet ready to "dissolve the entire order Rodentia," he was impressed by the strength of the new molecular data. The current report is not the first to question the sanctity of the rodent order, or the classification of the guinea pig. In 1991, Dr. Dan Graur of Tel Aviv University and his colleagues fired the first shot by writing a paper for Nature titled, "Is the Guinea Pig a Rodent?" and proceeded to cast doubt on the matter. The latest study does not twitch in its bold declaration, again in the headline, "The Guinea Pig Is Not a Rodent." Previous molecular studies of rodents were based on more limited comparisons of a handful of genes or proteins. Dr. Saccone and her colleagues took the burdensome route of spelling out, or sequencing, all 16,000 subunits in the rings of genetic material found in the structures that power the cell. After completing the sequencing, they compared the genetic pattern in guinea pigs to that found in rats and mice, as well as in 13 other mammalian species, including chimpanzees, humans, gray seals, cows, opossums and others. Through complex statistical and computational manipulations, the scientists constructed phylogenetic "trees," linking animals that were genetically most closely related. Those calculations allowed them to conclude that while rats and mice are close cousins, as any city dweller can attest, guinea pigs are off on a distant branch and deserve their own order. And with the guinea pig, in theory, would go 17 other types of South American rodents thought to be its close relatives. Other recent studies have also cast doubt on the presumed kinship between rabbits and rodents, while still others have questioned the relationships between porcupines and other rodents of the Americas. All told, a bit of disorder has shaken the great rodent burrow. Opponents of the notion of multiple lineages for rodents criticize the strictly molecular approach on many fronts. Dr. Patrick Luckett of the University of Puerto Rico, an expert in rodent anatomy and embryology, said it was ludicrous to reach so many sweeping conclusions about rodent taxonomy based on a sampling of three rodent species, guinea pigs, rats and mice. "There are 2,021 living species and probably that many extinct ones," Luckett said. "There are 29 families in the rodent order. The authors of the Nature paper have looked at three species taken from only two rodent families. They say they have a 'comprehensive' data set. Well, 3 out of 2,000 is not comprehensive to me. It's scanty." Luckett, Honeycutt and others pointed out that the integrity of the rodent order was buttressed by vast amounts of data from morphology -- the study of body structure and form -- and paleontology. They said rodents were distinguished by their entire head region. They have specialized incisors with enamel only on the front of the teeth, allowing them to be self-sharpening and ever-growing. The jaw musculature permits them to gnaw with their incisors at the same time that they are chewing with their molars. The fetal membranes found in rodents are unique among mammals, as is the pattern of embryonic development. "I could show a guinea pig to my 10-year-old daughter," Honeycutt said, "and she could tell me it's a rodent." But Graur insists that molecular results are stronger and more objective than anything to be gleaned by studying anatomy or paleontology. "I am one of those people who believe that DNA is the ultimate way to answer questions," he said. "I don't believe morphological data; they're defined so vaguely. People talk about something being 'slightly slanted' or 'moderately curved.' I come from a mathematical background, and I don't like definitions like that." But when it comes to a subunit of DNA, Graur said, there is no arguing which is which. Graur admits that the debate now is polarized beyond immediate resolution. "People call it the Big Divorce," he said. "There really is no point in talking to each other. We just quarrel." Yet he cannot help but knock the morphologically minded as being, in his view, scientific fossils. "We people who do taxonomy have a 300-year history of being nasty to each other," he said. "I like to keep up with tradition." Copyright 1996 The New York Times


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