Fishing for
Enlightenment An invitation to eat poisonous fish would have most of us begging for a Big Mac.
But McDonald's cuisine was not an option for UNLV biological sciences professor Andrew Martin one night several years ago when his research took him to a remote Fijian island.
He had just helped some stranded islanders when they offered him fugu, also called pufferfish, for dinner. He explains that the fish, which contains a deadly toxin, is a unique culinary delight - with a twist.
"If prepared correctly, it leaves you euphoric," he says. "If prepared wrong, it leaves you dead."
Fortunately for Martin, it was a good night for pufferfish. The dish was prepared correctly, and he lived to tell us about it - and about the work that took him to that neck of the Pacific Ocean, as well as many other distant points on the map.
Martin is a marine biologist interested in the evolution of fish, and his research has taken him to such assorted locales as the Amazon Basin, Puget Sound, and Nevada's Devil's Hole, in addition to the South Pacific.
All of these spots offer Martin watery laboratories in which he can
study the evolutionary process that has resulted in such vast diversity
of life on our planet.
Referring to what some have called the "last frontier," Martin says that the seas are beginning to yield important clues about evolutionary change that could someday produce more than insights into the lives of fish. He believes these clues could eventually lead to breakthroughs in our understanding of the human body that might enable us to better control the aging process.
To this end Martin studies the DNA of fish; a few years ago, his work in this area led him to a discovery that forced people to take a new look at evolution.
Using DNA data collected from various species, Martin constructs family trees for fish. Among other things, these genealogical charts reveal the point at which two individuals or two species last shared a common ancestor and when they diverged along different branches.
But the point of his research is not to enable some shark to brag about being descended from the finny equivalent of Charlemagne. Martin's close study of these DNA family trees has shed new light on the pace of evolution. As one writer put it, he is helping "calibrate the evolutionary clock."
Evolution is the
result of mutation - changes in the genetic information coded in DNA -
and it is something that goes on in living things all the time, Martin
explains.
"In our bodies, for example, in the course of a single day, thousands of mutations occur, and our bodies have to deal with them. Our bodies either correct the mutations or they don't, and if the mutations aren't corrected, the DNA is damaged. The accumulation of damage is one of the reasons we age."
Mutations in DNA also occur from one generation to the next. Martin says it's a regular process, and until recently scientists thought that all species mutated at the same rate. This common mutation rate was known as the "universal molecular clock."
To develop a family tree, Martin says, "you count the changes that are revealed in a stretch of DNA and divide it by the clock." Scientists have used this method, for example, to try to determine when humans last shared a common ancestor with chimps and gorillas.
Only it turns out that the clock isn't quite so universal, after all.
In the 1980s, researchers began to realize that there is a lot more variation in the molecular clock than was originally thought. Among those questioning the conventional wisdom was Martin, then a University of Hawaii Ph.D. student.
"I tested the hypothesis [that there is a universal mutation rate or molecular clock] by comparing mutation rates in sharks and primates," Martin says.
Much was known about primates, such as humans and chimpanzees, and Martin selected sharks for comparison because, he says, "Amazingly, we have a lot in common.
"Sharks mature at a late age, and we mature late. They are live bearers, so are we. They are basically a lot like us in ways that potentially affect how fast mutations occur, but they are very different in physiology. In particular, they are cold-blooded."
Sharks have another attribute that made them good subjects for this
study: lots of teeth. Martin says a single shark may grow and lose as
many as 10,000 teeth in a lifetime. These teeth fall into sedimentary
layers of shoreline where they become part of the fossil record.
Using this fossil record, Martin and colleagues Stephen Palumbi of the University of Hawaii and Gavin Naylor of the American Museum of Natural History were able to estimate the mutation or evolutionary rate of several species of sharks. When they compared the shark DNA mutation rates with the carefully calibrated data that exists for primates, they discovered that sharks mutate much more slowly.
Sharks accumulate mutations, the raw material of evolution, at a rate about 10 times slower than primates, they calculated. Naturally, they wanted to know why.
"The only thing that could really explain it was the fact that they are living life at a slower rate," Martin says. "They respire [breathe] at a lower rate, and their metabolisms are much repressed in comparison with ours. We're really cranked up. Our cells process information really fast."
Martin explains that it is well established that metabolism is related to the size of a creature and whether it is warm-blooded or cold-blooded. Among warm-blooded animals, whales have a much slower metabolism than mice, and cold-blooded animals are slower than warm-blooded.
So, as Martin explains, "If you are cold-blooded and big, you are going really slowly."
No one had really considered that mutations might follow a pattern similar to that of metabolism.
"But it makes perfect sense," Martin says, "that the cells that govern what happens in us every day also influence the DNA and, thus, the mutation rate."
The discovery drew worldwide attention and opened new lines of thinking about evolution.
"It was neat," Martin modestly says of the reaction to the article they published in 1992 in the Proceedings of the National Academy of Sciences.
But like most scientific discoveries, this one raised as many questions as it answered, and led Martin deeper into the problem - as well as deeper into the ocean. He is now working with both deep- and shallow-water fishes; he compares the mutation rates of fish that dwell thousands of feet under the sea and those that live at around the 600-foot level.
His studies into the
deeper regions of the sea have brought him more than insight into his
research. He has also developed a kind of astonishment at the diversity
of life way down there. Far beneath the surface, he says, live all kinds
of marine creatures - worms, octopus, fish, and, yes, even monsters.
Monsters? Really?
Yes, says Martin.
"There are some really bizarre creatures down there, including some species that have changed little in the past 700 million years."
Martin is particularly intrigued by a shark he calls the "megamouth."
"This is a cool fish," Martin says. "It basically lives deep during the day and moves to the shallows at night. It's like a lot of ocean dwellers that go up and down with the light levels, following food sources."
The megamouth shark is related to white sharks and may get as big as 15 feet. It's fat, too. "Enormous," Martin calls it, "a really flabby fish." A particularly distinguishing characteristic is its huge lips that glow in the dark.
Scientists have obtained only a few megamouths for study, so they aren't sure what makes the lips glow.
"It's either got its own way of making light or it harbors bacteria that make light," says Martin. "It also has a structure in the back of its mouth that is a reflective surface, like a mirror."
The light helps the megamouth keep food on the table. "If you put a flashlight under water, you'll attract shrimp and other food. That's what these things are like - giant flashlights slowly moving through the water, sucking in whatever comes near and filtering the water out through their gills."
The megamouth studies and the comparisons between deep- and shallow-water fish are aimed at determining what makes the cells mutate at a particular rate.
"If we can figure out what controls mutation rates in a cell, we can potentially learn to control it ourselves and stop aging and some cancers," Martin speculates.
He adds that interest in shark mutation rates is heightened by the fact that "there's never been recorded a naturally occurring cancer in sharks. A lot of other fish get all kinds of cancer." Since mutation rates have a direct bearing on the development of cancer, the focus is once again on the issue of mutation, Martin says.
So the question becomes, "Is it just metabolism that affects the mutation rate or are there other factors involved?"
Martin works with bits of tissue taken from the sharks and other fish to continue analyzing the subject. The tissues he uses can be as small as a clipping of the fin or a piece of the gill.
"You extract the DNA from the tissue basically by just dissolving it in a detergent which makes the membranes fall apart. You're left with the DNA."
Then, says Martin, a process called a "polymerase chain reaction" creates billions of copies of a gene in a test tube in two or three hours.
"You need large numbers of copies of DNA in order to determine its sequence," Martin explains. Using radioactivity or dye, he then "labels" the DNA so that changes - mutations - will show up in his analysis.
Martin also applies this technique to the tiny Devil's Hole pupfish, an endangered species that lives in Ash Meadows between Las Vegas and Death Valley.
The entire pupfish population consists of about 200 individuals. "It's pretty much on the verge of extinction, but it has been that way probably for 50,000 years," Martin says.
Until recently all the pupfish were together. "If you have all one species in one place, something is going to happen and eventually they will be wiped out," he adds.
In an attempt to maintain the gene pool and ensure survival of the species, the U.S. Fish and Wildlife Service has created a new refuge and divided the population.
"They are reproducing," Martin says of the pupfish in the new location, "but we don't know if the gene pools are the same because mutations are constantly happening. They may actually be creating another species, not preserving the species we want to preserve."
When he examines the DNA patterns in the pupfish, he looks for "molecular markers," which are "highly variable pieces of DNA that allow you to identify individual fish, like fingerprints. By examining where these markers turn up in the DNA pattern, we can tell whether individuals in one place are more like each other than individuals in another place, and whether the preservation effort is succeeding."
The pupfish work has just begun so it is too early to know what is happening to one of the few fish species unique to the desert.
Ironically, it was the
variability of desert - not aquatic - life that first attracted Martin's
attention to the study of evolution. Growing up in the desert around
Tucson he collected snakes, lizards, spiders, scorpions, and other critters.
"I had tanks all over the house," he recalls, "and every once in a
while something would escape, and my parents would get upset."
These experiences led to a "nagging desire to find out why there is so much variability in nature," which in turn led to an interest in DNA. Martin wanted to continue his education after completing his bachelor of science degree at the University of Arizona, but he also wanted a change of scenery.
He decided Hawaii would be a "nice place" for graduate study, and the University of Hawaii fortunately had scientists doing path-breaking work with DNA. For his Ph.D. in biology he did the aforementioned study that led to the rethinking of the calibration of the molecular clock.
Obtaining a post-doctoral fellowship from the Smithsonian Institution, Martin next found himself on and in the waterways of the Amazon Basin in his continuing quest to understand diversity.
"We are interested in why there are so many species in the Amazon Basin," he says. "It looks like there were brief periods of time when the creation of new species was rampant, then everything pretty much stayed the same for awhile."
For example, he explains, five million years or so ago the world was cold with a lot of water tied up in glaciers. The Amazon Basin was then relatively dry, and evolutionary connections were broken. New species evolved, creating new branches on the evolutionary tree, and Martin has been tracking those changes.
He returns to the Amazon periodically to continue his studies of diversity, and there, of course, he encounters the Amazon's most infamous fish, the piranha.
The first time he went to the Amazon Basin, Martin recalls, he didn't know piranhas could be found everywhere.
"So I'm out there in the river, seining away, and the Venezuelan
ichthyologists I was with didn't say anything. Then, we pulled in the
net, and it was full of piranhas."
Once he recovered from his initial shock, Martin found he could largely ignore the fierce fish."They're no problem unless you have a sore, and then they clean it very nicely - maybe too nicely," he says.
As if to suggest turnabout is fair play, he notes that his expeditions have provided him with enough fish stories to fill a book - a cookbook, that is. He loves to eat fish and has sampled some unique species.
"Like a good biologist, I've sampled considerable diversity," he says.
He has tasted most types of coral-reef fish, and says the "weirdest looking has to be the bird wrasse," a small, blue fish with an elongated, beak-like snout, whose flesh is also bright blue.
Catfish abound in the Amazon, and Martin has sampled about two dozen varieties, including one specimen that was "as big as a boat and another that had whiskers at least five feet long," he says, swearing he isn't exaggerating.
Besides, as "fish stories" go, it would be hard to top the tale of the potentially poisonous pufferfish.
