Beriberi

Just as Lewis and Clark are celebrated in the United States, so too, in Australia, are Robert O'Hara Burke and William John Wills, leaders of the first European expedition to cross that island continent. In 1860, Burke and Wills, along with two other expedition members, John King and Charles Gray, made the entire journey from Melbourne in the south to the Gulf of Carpentaria on the north coast. On the way back, however, through a combination of bad planning and bad luck, they ran out of food. Gray died, but the others turned to a wild resource that they had learned about from some Aborigines: the sporocarps--the hard, bean-like reproductive bodies of a small fern. The fern, Marsilea drummondii, called nardoo by the Aborigines, is more commonly known elsewhere as water clover because of its four leaflets. In a time of need, here, it seemed, was a fern friend indeed.

Burke and Wills prepared the sporocarps the most sensible way they knew how: they ground them into a powder, added a little water, and molded the mixture into small cakes. These they dried and baked in the hot ashes from their campfire. The food satisfied their hunger, but, mysteriously, they still became weaker with each passing day. In the end, Burke and Wills both died of malnutrition; King was rescued, but he suffered permanent nerve damage in both legs.

For many years, it was assumed that the sporocarps simply lacked food value. But about ten years ago, nutritionists provided a new explanation. The sporocarps, they discovered, are loaded with thiaminase, an enzyme that destroys thiamine, or vitamin [B.sub.1]. When they examined the explorers' journals, they found recorded a classic progression of the symptoms of thiamine deficiency, or the disease known as beriberi.

So much, it would seem, for Aborginal knowledge! But why didn't the Aborigines die from eating the sporocarps of nardoo? The secret lies in the preparation. Unlike Burke and Wills, they mixed the ground-up sporocarps with enough water to make a kind of drink or paste, which they spooned into their mouths with a mussel shell. Diluting the thiaminase, it turns out, decreases its harmful effects to the point that the plant is safe to eat. The mussel shell was also a smart move. If, for example, they had rolled up a eucalyptus leaf to make a spoon (a common Aboriginal technique), the enzyme could have latched onto organic molecules in the leaf that would have increased its potency.

It seems likely that some Aborigines in the distant past, through trial and error coupled with astute observation, had hit upon the right combination of procedures to unlock a resource in their environment. Perhaps those procedures had become so ingrained that they were taken for granted by the people who met Burke and Wills. Or perhaps the explorers failed to pay enough attention to what they were told.

One moral of the story is, surely, that a little knowledge can prove a dangerous thing. But another lesson is the extraordinary power of modern biology to offer unexpected insights--practical as well as theoretical--about organisms as familiar and commonplace as ferns. Recent investigations can explain far more than the basis for such practices as the Aboriginal preparation of nardoo, or the true cause of death of two national heroes. The study of fern biology is a vast enterprise in itself, encompassing some 12,000 species of ferns, in about forty families, that grow throughout the world. The species range from tropical tree ferns with leaves measured in yards, to small free-floating aquatics with leaves less than a sixteenth of an inch long. The new tools of molecular analysis--along with painstaking field observations--are changing the botanical view of these plants. Among the latest advances is the use of genetic information to help establish the place of ferns in the family tree of plant life. In some cases DNA analysis has overturned some long-accepted conclusions.

As a group the ferns have an ancient pedigree among the species of the Earth. Some living families have fossil records that date back to the Carboniferous Period, between 359 million and 299 million years ago, a time long before the rise of the dinosaurs [see diagram on next page]. Later, during the Late Triassic, Jurassic, and Cretaceous periods, when dinosaurs dominated the Earth (between 225 million and 65 million years ago), the bellies of these animals were tickled by the lush growth of fern families such as the forked ferns, the twin-leaf ferns, the tree ferns, and the ferns of the family Matoniaceae. The very study of ferns is called pteridology, which to some calls to mind the pterosaurs, flying reptiles that were contemporaries of the dinosaurs. The name is not purely coincidental: the root pterido-, from the Greek for "fern," is akin to pteron-, the Greek for "wing" or "feather" (think featherlike fern).

Before the advent of molecular phylogeny, biologists constructed evolutionary trees largely by studying the morphology and anatomy of living species. They also went to the fossil record, however incomplete, for additional clues that could broaden the contemporary picture. When had various groups of plants first appeared? What kinds of extinct species had once belonged to the groups? The DNA-based evidence developed in the past few years has reinforced some of these traditional classifications and shown where others were mistaken.

The corrections are not limited to just a few matters of detail. Taxonomists can now say that the ferns' closest cousins are the seed plants--angiosperms (flowering plants) and gymnosperms (such as conifers). Both seed plants and ferns are vascular plants, having conducting tissue in their steins. The most obvious difference between them is that ferns do not form seeds; instead they disperse and reproduce by means of single-celled spores. Nevertheless, ferns share a more recent common ancestor with seed plants than they do with the lycophytes, a group of vascular plants that also reproduce via spores. Before DNA studies were made, the lycophytes were considered closely related to ferns and therefore termed "fern allies"--now a misnomer. By the same token, DNA analyses show that the whisk ferns and horsetails, two other groups also considered fern allies, now appear best classified as ferns.

One way the study of DNA can help reconstruct plant family trees is to interpret the genetic code as a kind of molecular clock. As species evolve, they may or may not diverge rapidly in outward appearance or various other major respects. At the molecular level, though, the accumulation of random mutations in DNA is thought to proceed at a fairly uniform pace within a given lineage, largely independently of natural selection. Measuring the accumulated divergence between two homologous, or corresponding, sequences of DNA in two different species can provide an estimate of how long ago the two species diverged. Although the method is controversial, it can be tested and calibrated against the fossil record, wherever independently dated fossils are available.

The molecular-clock technique can be particularly useful as a way to confirm hypotheses and corroborate other lines of evidence about what was happening when some particular group of plants arose. For example, toward the end of the Cretaceous (the period between 146 million and 65 million years ago), flowering plants rose to dominance in Earth's vegetation. Forests apparently became more deeply shaded than they had been earlier. This change may have helped cause the decline of some fern groups and the flourishing of others. For example, ferns in the families Polypodiaceae and Davalliaceae are epiphytes, or plants that grow on trees rather than in soil. The change in vegetation might have worked to their advantage. Consequently, you would expect to see epiphyte species radiating, or diversifying as they filled newly available niches. By working with molecular clocks in those two families, biologists can determine whether these ferns radiated at about the same time that the flowering plants began dominating the landscape.

Molecular biology has also focused attention on another aspect of fern evolution. When the second volume (Pteridophytes and Gymnosperms) of the Flora of North America was published in 1993, something appeared in its pages that had never been seen before in a book on plant identification: some novel and rather curious-looking, netlike diagrams called reticulograms.

The diagrams look quite different from the more familiar evolutionary family trees, in part because they do not attempt to show how recently two species shared a common ancestor. Instead, reticulograms depict the relationships between species and their hybrids, showing which species have come together to form which hybrids. They also indicate whether the hybrids are sterile (producing "aborted," or nonviable, spores) or fertile (producing viable spores). Nearly all hybrids are sterile when they first form, but if they double their number of chromosomes through "polyploidy," they automatically become fertile.

Reticulograms were included in the reference book because the processes they depict--hybridization and polyploidy--are important evolutionary mechanisms underlying the formation of new species of ferns and lycophytes. Of the 420 species of ferns and lycophytes described in the treatise, about a hundred originated as hybrids and later became fertile through polyploidy. What, then, are those two processes, and how do they work in concert to form new species?

The best way to explain polyploidy may be by example. Related species of ferns often have chromosome numbers that are multiples of a basic set. For example, some species of wood fern (of the genus Dryopteris) have 41 pairs of chromosomes in their somatic cells. Other species have 82, and still others 164. All those numbers, of course, are multiples of 41, the lowest-known number of chromosome pairs in the genus. Species with the lowest number in such a series are called "diploids" (two of each chromosome) whereas the ones with higher multiples are called "polyploids" (if you want to be more specific, you can use the terms "tetraploids," "hexaploids," "octaploids," and so on).

Polyploid formation is a process that typically starts with an abnormality in the cell division that produces spores. Normally a spore gets only one chromosome from each pair of chromosomes in the parent fern, but sometimes that fails to happen, and a spore gets a full complement of chromosomes--that is, two of each pair. When the abnormal spore germinates, the eggs and sperm that are ultimately produced also carry the doubled number of chromosomes. That sets the stage for polyploidy. If (for example, by self-fertilization) a "double" sperm then meets a "double" egg, the fern offspring will be tetraploid, and that genetic makeup will be perpetuated in future generations through normal cell divisions.

Polyploidy is often associated with hybridization. Hybrids form when the sperm from one species fertilizes the egg of another. The hybrid zygote grows into a plant with normal roots, stems, and leaves, but the plant turns out to be sterile. Its spores are misshapen, blackened, and nonviable, because during the cell division that produces the spores, the parents' chromosomes pair improperly, if at all, and are then distributed unequally to the daughter cells.

Here's where polyploidy enters the picture. If polyploidy leaves two copies of each chromosome in a hybrid's cells, each chromosome gets a partner that is an exact duplicate of itself. During spore formation in the hybrid, normal pairing of chromosomes can take place, and the chromosomes can be distributed equally to the spores. The new plant is now fertile, able to disperse and reproduce, sometimes beyond the ranges of its parents.

Hybridization and polyploidy have been well studied in Europe, Japan, and North America, but they have received little attention in the tropics, where most fern and lycophyte species occur. Future research will almost certainly show that the two phenomena are just as common there as they are in the temperate zones. They are evolutionary mechanisms that remain in action today, driving the development of new species of ferns and lycophytes for the future.

A Practical objective of the research on ferns is to combat what--from a human point of view--are noxious species. For example, one species, molesting salvinia (Salvinia molesta), is one of the world's most widespread and pernicious aquatic weeds. A free-floating aquatic native to southern Brazil, it was accidentally introduced into Sri Lanka in 1939, and has now leaped the continents to become a pest in Africa, Australia, India, and New Zealand. About thirty years ago it was also introduced into the southern United States, where it has spread primarily from Florida to Texas.

Under optimum conditions a colony of molesting salvinia can double in size in about three days, and given enough time it will carpet the water's surface with a thick, dense mat--a mat so dense that it can support the weight of a cinder block. By the 1970s, teams of entomologists had started searching for a biological control, an insect that would eat molesting salvinia into oblivion. They eventually found a small weevil native to the fern's home range in Brazil. The weevil feeds only on the fern, attacking it in two ways: the adults eat the leaves and the larvae tunnel through the stems and buds. The weevil has been spectacularly successful in controlling infestations in the Old World and is being investigated for use in the U.S.

Ferns are typified by leaves that unfold from coiled buds--the fiddleheads. The coil is, to be exact, a logarithmic spiral, a kind of curve that occurs widely in nature [see "The Golden Number," by Mario Livio, March 2003]. Fiddleheads have long been valued as a food item. Worldwide, the fiddleheads most commonly consumed are those of bracken (Pteridium). In Korea and Japan they are sold commercially and cooked as a spring vegetable. Many Korean American Families living in and around Los Angeles gather bracken fiddleheads in the spring, particularly in the nearby San Bernardino National Forest. Harvesting there is so popular that it is regulated by the U.S. Forest Service.

Those traditional dietary uses have also prompted basic research that is illuminating the potential--and risks--of ferns as food. Eating bracken fiddleheads over many years has been correlated with high rates of stomach cancer. Medical scientists in Japan have isolated a compound called ptaquiloside, which they think is the main carcinogen. Bracken fiddleheads also--like the sporocarps eaten by the ill-fated Burke and Wills--turn out to be loaded with thiaminase (though the mature leaves are not). Those high concentrations can be deadly for livestock grazing in early spring, when fiddleheads stand like beacons above the slower-sprouting grasses. The animals become stricken with severe thiamine deficiency. In Britain before the days of the automobile, bracken-induced thiamine deficiency was so apparent in horses that it earned the name "bracken staggers."

But don't be alarmed. The commonly eaten fiddleheads in eastern North America are those of ostrich fern (Matteuccia struthiopteris), a native woodland plant. Unlike bracken, its fiddleheads are safe to eat. And in tropical Asia and many Pacific Islands, the fiddleheads usually served are those of Diplazium esculentum, which tastes much like ostrich fern, and is also considered safe. So unless the nutritionists and plant biologists tell you otherwise, by all means enjoy the fiddleheads. They are delicious. Their taste has been likened to that of asparagus, but they have a flavor all their own, a flavor once described by the historic New York restaurateur George Rector as "simple and beautiful, like the soul of spring."

COPYRIGHT 2004 Natural History Magazine, Inc.
COPYRIGHT 2004 Gale Group