The Year of the Worm
A special section in the December 11, 1998 issue of the journal Science was devoted to a celebration of the roundworm Caenorabditis elegans. Although a lengthy name has been conferred on this creature, the organism itself is actually at most about 1 mm long. Of the 20,000 or so known species of roundworm or nematode, most are parasites. This species, however, (generally called by the more catchy title of C. elegans), lives free in the soil. Hardly visible to the naked eye, an individual worm nevertheless appears very large when viewed through an ordinary microscope. Except for cells destined to become eggs or sperm, this animal consists of only 959 cells. The whole creature is quite transparent so that, through the microscope, one can easily view the various cells and organs. Development of an individual is fast too, requiring only three days in a life span of two or three weeks. Such an organism seemed ideal for laboratory studies. Thus in 1963 Cambridge biologist Sydney Brenner began a research program which continues to the present. And what fascinating results have been obtained!
Dr. Brenner’s interest was in the genetic controls of development. A colleague, John Sulston, initiated a study of the full developmental history of every one of the worm’s 959 adult cells. This program was completed by the late 1980s. The next stage, initiated in 1990, was the effort to describe the entire order of base pairs (chemical letters) in the DNA molecules within the worm. It was as the completion of this program approached that the scientific community celebrated the results of the C. elegans study. But what had these studies revealed? The answers depend upon whom you consult. Everyone agreed however that many surprises had been uncovered.
As everyone knows, a fertilized egg soon divides into 2 cells which soon divide again to yield four cells. Further divisions result in an embryo with more and more cells. Dr. Sulston and colleagues painstakingly followed what happened to each cell after each division. Like a family tree in human families, they constructed a lineage tree for every single cell of the developing organism. What they expected to find was that certain blocks of related daughter cells would all develop into the same kind of tissue. However these scientists were in for a surprise. They discovered instead that development in the roundworm was “remarkably nonmodular”. As Michael J. Denton reports on the topic in his recent book “It might have been predicted that each organ would form separately, perhaps from a separate set of genetic instructions or a separate clone of cells.” (M. J. Denton. 1998. Nature’s Destiny: how the laws of biology reveal purpose in the universe. The Free Press. New York p. 335). What they found however was that component cells in a given organ came from a wide variety of cell lines. The cells making up the pharynx for example (part of the digestive system), came “from eight completely different cell lines scattered throughout the lineage tree.” The same thing was found with all the organ systems in this roundworm. Another “curious aspect” which they discovered and indeed “one that would never have been predicted” was that identical organs on the left and right sides of the body came from completely different types of cells in the embryo. As Dr. Denton put it “This is like making the right and left headlight on an automobile in completely different ways and utilizing completely different processes.” (p. 335)
Not only did the development of C. elegans turn out to be much more complicated than anticipated, but the results make evolutionary explanations much more difficult. The fact that there was not the slightest trace of logical hierarchy in development of the worm Dr. Denton termed “astonishing and completely unpredicted.” We should note that this situation, one of closely integrated development, is more easily explained as design or purpose and planning. Modules, Dr. Denton points out, would allow for a much more orderly evolutionary change of one part of the organism at a time without wrecking what might be advantageous features in other parts of the body plan. Moreover it is an interesting fact, says Dr. Denton, that the same complex pattern of development (although generally less extreme) is found throughout the animal kingdom. None of this lends itself to evolutionary speculation.
The developmental pattern in the worm is however old news. The present excitement came from the publication of almost the entire genetic code of C. elegans. This is the first multicellular organism whose entire genome (order of ‘chemical letters’ in the DNA molecule) has now been documented. Apparently C. elegans has 19,099 protein-coding genes and 800 other genes which code for other functions. This information was discovered through collaboration over eight years of an American team from the Genome Sequencing Center at Washington University in St. Louis and the British team at Cambridge. One of the first surprises was the sheer size of the C. elegans genome. Apparently this seemingly humble little worm has more genes than the fruit fly — even although the entire worm boasts fewer cells than there are in the fruit fly’s compound eye. Scientists had expected several times fewer genes than were discovered. Nobody knows the reason for the discrepancy. Certainly it is hard to explain in evolutionary terms.
Now the fun part began. Scientists could hardly wait to compare their results from C. elegans with gene sequences in other organisms. About half the genes in the worm were identified as similar in base pair sequence and function to genes in such organisms as yeast and a tiny bacterium called Mycoplasma genitalium. Scientists however have no idea about the function of the other fifty percent of the genes in C. elegans. This situation seems to be typical of these studies. A CNN story described the work of Dr. J. Craig Venter of Rockville, Maryland-based Celera Genomics Corporation (January 24, 1999 Science Life) concerning the genes of Mycoplasma. Dr. Venter confided that life appears to be more complex than previously expected. Scientists had hoped that DNA sequencing would demonstrate that living creatures make use of many similar genes. Unfortunately however while some genes have been found to be common to most organisms, many basic functions are in fact organized by different genes in different creatures. Dr. Venter bewails the fact that in every organism studied thus far, fifty percent of the genes are new to science and we don’t know what they do. “It’s very humbling,” he admits.
While Dr. Venter admits dismay, other specialists put a more positive spin on the situation. Julie Ahringer of Cambridge declares that “We now know that all animals are put together in very similar ways – humans, flies, worms and everything.” (quoted in article by Georgina Ferry. 1998. The Human Worm. New Scientist. December 5 pp. 33-35). Drs. Gary Ruvkun and Oliver Hobert (The Taxonomy of Developmental Control in Caenorhabditis elegans. Science December 11, 1998) opines “Although animals have much in common, worms are not puppies and humans are not sea cucumbers.” (p. 2040) They further muse about whether the obvious differences in animal form and function are caused by differences in universal genes which regulate how other information is expressed, or whether the differences are caused by new regulatory genes or both? Further expressions of an almost ridiculous nature came from Bruce Alberts, President of the National Academy of Sciences “In the last 10 years we have come to realize humans are more like worms than we ever imagined.” (quoted in the New York Times front page story by Elizabeth Pennisi, December 11, 1998.) Similarly on the Dallas Morning News front page, a human geneticist at the University of Texas Southwestern Medical Center Dr. Geln Evens was quoted as remarking “The worm represents a very simple human, genetically… It’s a very simple model for how people work.”
Once we proceed beyond the excitement of interesting new data however, we discover that we still know very little about genetic controls of development in C. elegans and even less about such details in other animals or even people. The details that we do know do not fit evolutionary predictions. Hence all the surprises. Thus if I were an evolutionist I would not be rhapsodizing about C. elegans. After all these years of work their hopes and predictions have yet to give them much cause for celebration.
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