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By Anne M. Rosenthal and Kathleen G. Human Views -- Winter 1997 A Colorful Problem in Microevolution
Each spring, vivid pink flowers of Linanthus parvilorus blanket the northwest end of the Jasper Ridge serpentine exposure. Yet on adjacent sandstone soil, the same species grows differently -- the plants flower almost exclusively white, bloom later, are taller, and grow farther apart. Only an area at the base of the serpentine exposure where both soils are present contains the two morphs well interspersed.
Research Dr. Douglas Schmenske makes observations on L. parviflorus (right). Enclosures for the CO2 experiment, a separate investigation, are visible in the background. Photo by Nona Chiariello. The Significance of Genetic Diversity Research
The ongoing Linanthus parviflorus research at Jasper Ridge contributes to our understanding of genetic diversity and the adaptability of organisms to their environment. This is an important research area, in part because of two simultaneous trends. The first is the accelerating pace of environmental change, a consequence of human activities that are affecting the Earth's climate and ecosystems. The second is the general reduction in size and genetic diversity of plant and animal populations, a result of habitat alteration, fragmentation, and destruction.
For a population to survive environmental modification, it must have sufficient genetic diversity to adapt to new conditions; less genetically diverse populations may be at greater risk. The combination of mounting environmental change and dwindling genetic diversity means that many species must survive greater stress with reduced genetic resources. A number of species face an increased risk of extinction, and many important conservation decisions must be made. Basic research on genetic diversity, such as that at the Jasper Ridge Biological Preserve, serves as a foundation for these decisions. A Mystery
Side-by-side patches of the same species with distinct morphologies prompt a number of questions. How are both morphs maintained in proximity? Why are they segregated into patches almost exclusively of one color, except where serpentine and sandstone soils overlap? Is there a genetic basis for the differences?
This remarkable pattern of variation is being studied at the Jasper Ridge Biological Preserve by Dr. Douglas Schemske, an ecologist at the University of Washington in Seattle. His goal is to discover the basis for the two L. parviflorus, morphs and the mechanism behind their marked differentiation on such a small spatial scale. Dr. Schemske hopes to better understand how differences in environment can maintain different forms of the basic units of heredity within a population or species.
These basic units, called genes, are generally short coding segments of deoxyribonucleic acid (DNA), present in each cell of living organisms. Through a complex process, DNA molecules replicate, and the genetic code is passed from one generation to the next. Genes specify both structural and regulatory proteins, and contribute to an organism's phenotype, the observed traits of an organism such as appearance and behavior. Some traits may be controlled or affected by only one gene, others by two or more. Depending on the trait, the environment in which the organism develops and lives may have a lesser or greater degree of influence on how the genes are expressed. The genetic composition of an organism is referred to as the genotype of the organism. Genetic Diversity and Long Term Survival
A close-up of L. parviflorus (right) reveals the delicate beauty its blossom. In the Preserve, this species blooms pink on serpentine soil and white on sandstone soil. Photo by Woody Woodward.
Variation of individual genotypes within or among species is known as genetic diversity. An important attribute for long-term survival of a species, genetic diversity may enable a population to adapt to new conditions, such as those brought by environmental change (see the sidebar The Significance of Genetic Diversity Research). Consider, for example, a patch of annual plants similar to L. parviflorus. If all of the plants in that patch were genetically identical, then all might respond to changes in soil moisture in roughly the same way. If the plants were adapted to moderate rainfall, the patch might disappear after an unusually dry year. Conversely, if a patch was genetically diverse, this might be reflected as physiological differences that made it possible for some plants to reproduce even during dry years. The process described above, where plants with a particular set of genes reproduce more successfully than others during drought, is an example of natural selection, the differential reproduction of genotypes. For natural selection to occur, there must be variation in a trait, heritability for the various forms of the trait, and a selective force. Natural selection can lead to evolution-change in the genetic make-up of a population over time. A Clue from the Mojave Desert
Although sorting out the evolutionary forces acting upon L. parviflorus is a challenging task, Dr. Schemske came to the Jasper Ridge conundrum with almost a decade of experience on a similar problem in the Mojave desert, where he worked on the species L. parryae. This species had sparked the interest of two evolutionary biologists in the 1940s, Theodosius Dobzhansky and Sewall Wright. They had found that in the Mojave desert, most populations of L. parryae contained only white-flowering plants, but a few populations had blue-flowering plants as well. Some of the mixed populations contained only a few blue flowers, while other of the mixed populations were almost completely blue.
Plant breeding experiments had shown that color variation was clearly genetically controlled, with blue dominant to white. But the color patterns did not seem to be correlated with soil type, neighboring vegetation, topography, or any other environmental parameter. "The variation seems entirely haphazard," Dr. Dobzhansky wrote in Genetics and the Origin of Species in 1951. He attributed it to genetic drift-random fluctuations in the frequencies of genes within a population. Dr. Schemske felt that the populations of these plants-which numbered in the tens of thousands-were too large for genetic drift to play a role. By marking individual plants in the Mojave and following their reproductive success over time, he found a clear association between flower color and soil moisture. In years when soils dried out quickly after brief rains, plants with blue flowers reproduced more successfully. In years when soils remained wetter for a longer period, plants with white flowers flourished. A Hint from the Greenhouse
With this clue in hand, Dr. Schemske began to look at the Jasper Ridge population of L. parviflorus. In greenhouse experiments using standard potting soil, plants were grown using field-collected seed from both sandstone and serpentine flowers. Seed from sandstone populations resulted in white flowers, and seed from serpentine populations in pink flowers, even though the plants were grown under identical conditions. Other differences found in the field between the two morphs were also maintained. These greenhouse experiments showed that there is genetic variation in flower color, plant height, and flowering time between the serpentine and sandstone populations.
In flowering plants, there are, generally speaking, two of every chromosome, with each member of a chromosome pair carrying the same set of genes. However, the two chromosomes may or may not carry the same information about a given trait. For example, with respect to the gene for flower color, a plant could have one allele, or form of a gene, for pink and one for white. Since, for the characteristics in question, all the L. parviflorus on the serpentine soil are similar to each other, and likewise, the plants on the sandstone soil are uniform, Schemske believes that at the loci (locations on the chromosomes) related to flower color, flowering time, plant height, and other characteristics differing between the serpentine and sandstone populations, most of the plants in the field are homozygous-that is, both alleles are the same. More Hints from the Greenhouse
Schemske's group wondered whether pink-flowering and white-flowering plants could interbreed. To test this, they grew parental(P) homozygous plants from field-collected seed, and then crossed pink-flowering individuals with white ones. For each locus, all of the progeny-the first filial (F1) generation-received one allele from a serpentine plant and one from a sandstone plant. Thus the F1 plants were heterozygous for traits differing between serpentine and sandstone plants, meaning that they had two different alleles for each trait (or sets of alleles if more than one gene controlled a trait).
For flower color, which was dominant-pink or white? This was revealed when the F1 plants flowered-they were pink like the homozygous plants growing on serpentine soil, showing that pink is dominant to white. Other experiments imply that flower color is controlled by a single gene (although shades of color may be controlled by multiple genes). Taken together, data from all of these genetic experiments "strongly suggest local adaptation to particular microhabitats," says Dr. Schemske. A Soil Moisture Connection?
L. parviflorus growing in serpentine soil on Jasper Ridge (right). Photo by Woody Woodward.
With a possible connection between soil moisture and flower color suggested by the studies of L. parryae in the Mojave, Dr. Schemske took a closer look at soil moisture at Jasper Ridge. Indeed, soil moisture data collected throughout the 1995 flowering season showed an interesting difference between serpentine and sandstone soils: although early in the season serpentine soils are wetter, they dry out sooner than the sandstone soils. Dr. Schemske then hypothesized that these differences in soil moisture were the major selective force that led to genetic differences between the pink and white populations, probably favoring the evolution of earlier flowering on serpentine soils. Other Possibilities
However, he also examined a number of other possibilities. For example, he showed experimentally that the plants cannot self-pollinate, which ruled out a self-perpetuated chance distribution of pink and white morphs. He also determined that the same bee fly pollinators visit both morphs and travel far enough to fly between them. Therefore, the distribution patterns could not be explained by pollinators that only visited flowers of one color.
Of course, the differential reproduction of the two morphs on their respective soil types may not be directly related to flower color, flowering time, or plant height. For example, the two morphs may differ in some physiological process that happens to affect these traits while also confering advantage to each morph on its respective soil type. Or, alternatively, the association between the different morphs and soil types might involve genetic linkage, where genes are located so closely together on a chromosome that they are almost always inherited together. For example, if color does not influence a plant's reproductive success, but is linked with a trait that does, color could come along for the ride. Continuing the Investigation: Field Studies
With many questions about L. parviflorus still unanswered, Dr. Schemske continued field work during spring of 1996 with the assistance of research assistant Lara Hawthorne. They looked at the same three sites on Jasper Ridge as in 1995, one each on serpentine soil, sandstone soil, and the intermediate area where both soils are present. Over the course of the season, flowers were counted on 12 plots at each site, and soil core samples were taken to determine moisture. The soil moisture data for 1996 mimicked that of 1995-the serpentine soils, while initially wetter than the sandstone soils, dried out more quickly. At the end of the 1996 season, Hawthorne collected the seeds from each study plot.
Hawthorne also spent time watching for the bee flies that pollinate the flowers. However the number of bee flies observed was very small, possibly because of the wet year. Schemske now thinks that Jasper Ridge populations of L. parviflorus at all sites are, to some extent, wind pollinated. In fact, wind pollination is probably important on the serpentine site because the plants there typically flower so early in the season that few bee flies are present, says Schemske. During the upcoming 1997 field season, Schemske plans to continue monitoring flowering times and pollinator abundance. Greenhouse and Laboratory Studies
Work is also proceeding in the greenhouse. Currently, Schemske's group is crossing F1 plants with each other, resulting in second filial or F2 plants, as well as back crossing F1 plants against parentals. The plants are being grown on both soil types, and will be scored for flower color, flowering time, plant height, and other characteristics. The frequencies of the different plant classes, such as pink-early flowering-tall, will be calculated, revealing further information about the genes involved. Interestingly, in preliminary experiments, pink-flowering plants did not grow well on sandstone soils, suggesting that ability to grow on each of the different soil types is a characteristic unto itself.
In additional experiments using multiple generations of crosses, Schemske is attempting to produce pink-flowering plants with all other traits characteristic of the white-flowering plants found on sandstone soils, and vice versa. He terms this "moving the pink gene into an otherwise white background, and the white gene into an otherwise pink background." In other words, he will reverse a single genetic locus without changing anything else. Says Schemske, "This is a way of measuring its effect." One of Schemske's long-term goals is to use molecular genetic markers to identify all of the genes associated with local adaptation to sandstone and serpentine soils. These markers could include, for example, small segments of DNA that are relatively easy to detect. Molecular markers are helpful in discerning how many genes contribute to a trait, the magnitude of the effect of an individual trait, and the mode of a gene's action-for example, whether it is dominant or recessive, notes Schemske. These markers can be used to screen plants for genotype, so that they can be crossed on the basis of the genes they carry rather than on their appearance. Such crosses would be useful in producing L. parviflorus plants with specific genotypes for testing in field experiments, all part of an effort to unveil the genetics behind the two morphs. Views Home | Views Archive | Views Printed Edition Archive |
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