Journal of Heredity 2004:95(1)
© 2004 The American Genetic Association 95:90-91
Book Review |
Fungal Populations and Species
John Burnet.Oxford University Press, New York. 2003.
The fungi that most scientists are familiar with are tame and domesticated creatures indeed. My postdoctoral advisor, Bob Mortimer, was primarily responsible for the laboratory domestication of the baker's yeast Saccharomyces cerevisiae, which thanks to his efforts is now a favorite organism of geneticists around the world. This simple one-celled organism is far closer to us in evolutionary time than bacteria, but it has bacteria-like advantages in a laboratory setting. Like bacteria, it can be grown in separate colonies on a petri dish of defined medium, but it can also undergo meiosis and regular genetic recombination just like you or me. The first attempts by geneticists to utilize S. cerevisiae, in the 1930s, were disastrous—its genetics seemed inexplicable and its life cycle seemed quite deranged. Patiently Bob selected a laboratory strain that behaved itself. Its meiotic products all survived because they all had the proper number of chromosomes, and the haploid strains that resulted from meiosis could be maintained as haploids without suddenly reverting to diploids again as they did in most yeast strains.
Thanks to the work of Bob and other pioneers, we now have a much clearer idea of why most strains of Saccharomyces that have been isolated from the natural environment behave in the odd ways they do. And we are gaining a clearer idea of the even more complex life histories of the millions of other fungus species with which we share the world. John Burnet's useful and well-organized book takes us through many of these complexities, and it will prove a valuable tool for students and researchers who are interested in fungi, the least explored of the eukaryotic kingdoms.
Burnet begins with the basics, starting with a description of the different fungal life cycles and of the genetic and molecular techniques that have been used to understand them. He then links what we know of population genetics and evolutionary processes with the world of fungi, where things are often not as simple as they are in the world of humans or fruit flies. Fungi can come in a variety of sexes, and sometimes the sex of different members of a single species can be determined by different mating systems. We now know that the genes that determine mating type appear to have originated from mobile elements, a theme that Burnet does not explore, and their origin gives us a glimpse of the evolution of the sexual process itself, long before the evolution of specialized sex chromosomes.
But Burnet does explore the many fascinating ways in which fungi use sex and mechanisms that are akin to sex to adapt to their subterranean world. As fungal mycelia grow through their substrate they encounter constantly changing conditions. Many heterokaryotic fungi can alter the mix of nuclei in different cells, changing parts of their mycelium to exploit the different subenvironments of their world more effectively. This ability has been given the evocative name parasexuality, and it shows that there are more ways than sex to harness genetic variation.
Burnet's book draws together many different studies that have explored how fungi have adapted to varying ecological conditions. Some, like the Basidiomycete Armillaria, can grow to sizes that in the aggregate dwarf the great blue whale in volume, and can live for 1500 years or more—though one would never suspect that such ancient leviathans lurk beneath the ground of the northern Michigan countryside. Others cluster around the roots of trees, supplying them with essential nutrients and protecting them from invasion from more inimical fungi. Still other fungi can live under some of the most extreme conditions that are found on the surface of the Earth, growing on rocks that bake all day in the intense sun of the desert.
One of the book's great strengths is that it emphasizes the process of adaptation in fungi and details the many studies that have been carried out at the genetic level showing how adaptation has taken place. Because fungal populations evolve quickly, they provide wonderful opportunities to study adaptation of both molecules and morphologies, and molecular tools are now available to carry out effective studies.
The book also explores some of the challenges facing fungal taxonomists, who must sort through a bewildering variety of morphological species to try to find out where one species ends and another begins. Fungus speciation happens in ways very like speciation in the rest of the eukaryotic world, through the accumulation of prezygotic and postzygotic isolation mechanisms, but understanding and measuring these mechanisms is a challenge. Indeed, the challenges may be even greater than the book suggests—ecologists have recently found hundreds of fungal morphological species living in the interstices between leaf cells of almost every tropical tree and bush. What do they do there, and how do they find and exploit so many ecological niches in a single plant?
Although the book covers many aspects of the world of fungi, some things were inevitably left out. I missed a discussion of the place of fungi in the larger world, including the growing evidence that, even though fungi were once considered to be a colorless branch of the plant kingdom, they are actually closer to animals than to plants. We are actually more closely related to mushrooms than we are to petunias. And recent advances in genomics have begun to explain the evolution of fungi themselves. The entire genomes of S. cerevisiae, Candida albicans, and Neurospora crassa are now available for comparison, and it is possible to glimpse how multicellularity evolved in the fungi. But all these fungi are Ascomycetes, and the other major fungal groups remain largely unexplored genetically. What surprises await us among the Basidiomycetes or the poorly understood and animal-like Chytrids?
Today, all writers of compendia such as Burnet's book are faced with the grim fate that their painfully collected information will be eclipsed by dazzling new information. In the meantime, however, there are the seeds (or rather the spores) of many an exciting thesis in this book.
Division of Biological Sciences University of California–San Diego La Jolla, CA 92093
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