Graduate student's work may solve puzzle in plant genetic
In a study published in the April 15 issue of the journal Science, a University graduate student provides clues that may help solve a long-standing mystery in plant genetics.
The work by Jane Masterson, a graduate student in the Committee on Evolutionary Biology, provides insight into "polyploidy," the ability of plants to develop and thrive with tens or even hundreds of complete sets of chromosomes in each cell. Although even one extra chromosome in humans is enough to cause debilitating disorders such as Down syndrome, scientists speculate that the extra sets of chromosomes in many plants may make them hardier by enabling them to cope with a wider variety of environmental stresses.
A problem for evolutionary biologists studying the phenomenon of polyploidy has been how to trace its history in plants. Because scientists don't know how many chromosomes were present in the earliest plants, their estimates of how many plants are actually polyploid today range from as low as 30 percent to as high as 80 percent.
Masterson said it's difficult to study the genetics of fossil plants because "you can't just line up the chromosomes and count them." Instead, she applied an innovative approach, inferring the number of chromosomes in a plant cell based on the size of the cell.
Masterson used fossil leaves to estimate how many chromosomes were present in extinct ancestors of modern magnolia, laurel and sycamore trees. She compared the sizes of fossil-leaf guard cells, which open and close the plant's gas-exchange pores, with those of living species. She found that modern-day sycamores, which have 42 chromosomes, have ancestors with 28 and 14 chromosomes. Laurels, which today include species with 24 and 48 chromosomes, have a 12-chromosome ancestor.
The results of Masterson's study suggest that the earliest plants probably contained between 12 and 18 chromosomes (two sets of between six and nine), and that more than 70 percent of angiosperms (flowering plants) living today may be polyploid, based on the number of chromosomes they have.
"Jane has terrific insight that is enabling her to study plant evolution and genetics from an entirely new direction," said David Jablonski, Professor in Geophysical Sciences and the Committee on Evolutionary Biology and Masterson's adviser. "It's wonderful when a graduate student really makes great strides on her own. Her study will change current thinking on the evolution and impact of chromosomal numbers in flowering plants."
As in animals, the genes of plants are arrayed in chromosomes. But unlike most animals, plants often have more than two copies of their chromosomes. Humans have a total of 46 chromosomes in two sets of 23, but lilacs have 44 chromosomes in four sets of 11. The world-record holder is a fern that has more than 500 sets of its chromosomes in each cell. These multiples can arise either from duplication, yielding additional identical sets of chromosomes, or from combining with another species, yielding additional nonidentical sets of chromosomes.
Why plants not only survive but also thrive with multiple sets of chromosomes has intrigued evolutionary biologists and geneticists alike. Some see polyploidy as an evolutionary "dead end" -- with so many copies of one chromosome, it's difficult for a recessive gene to be selected, and this slows down the evolutionary process, some scientists say. And although seeds of polyploid species are larger and grow faster initially, the growth rate slows as the plant matures, and reproduction is slowed because duplication of all of the genetic material in the cell takes more time.
Others speculate that the additional chromosomes provide extra enzymes that enable a plant to survive harsh conditions, increasing its range and allowing it to adapt to different environments. In fact, induced polyploidy is one of the mechanisms employed today to produce disease-resistant cultivated crops -- disease-resistant properties are introduced by hybridizing the genome of a hardy wild-type plant with that of a cultivated cousin.
"It's easy to say a lot of things about polyploidy, but it's much harder to prove that any of them are true," Masterson said. The results of her study should help scientists begin to evaluate the evolutionary role of polyploidy.
For her study, Masterson collected living leaves from botanical gardens around the world and visited museums and field sites to gather fossil leaves. She used actual leaves, rather than the fossil imprints left behind in rock, in order to measure the sizes of the cells accurately.
Masterson's work was supported by a Searle graduate fellowship, a dissertation-improvement grant from the National Science Foundation and a fellowship from the American Association of University Women. She also received support in the form of grants from the Geological Society of America, the National Academy of Sciences (through Sigma Xi) and the University's Hinds Fund.
-- Diana Steele