Student Research Project
Hybrids and evolution in fragile ferns from the mountainous regions of the northeastern United States

Student
Michael Paler
Major: Botany
Future Plans: A career in the arts or plant biology

Professor
David S. Barrington, Professor, Department of Botany, University of Vermont, Burlington

Hybridization and subsequent establishment of a distinct species from the resulting hybrid is a common mode of evolution in the ferns. Mike Paler and I teamed up to study two hybrid species, Cystopteris fragilis and Cystopteris tenuis. These hybrid species test our limits for recognition as separate species; because the two share a progenitor species, they are similar in appearance. At the outset, Cystopteris fragilis was known to Vermont botanists only from the cool, wet high-mountain notches characteristic of the state, while C. tenuis was known to be widespread at middle and lower elevations.

We sought the answers to two questions about these candidate species. First, were the species difficult to tell apart because of their common heritage, or because of characters that were similar because of environmental factors operating on the plants? Second, was Cystopteris fragilis truly rare and limited to high altitudes, or had confusion about the species boundaries obscured a broader range in Vermont?

To answer these two questions, we searched for the two species in various habitats at various altitudes in the Green Mountains of Vermont. In each case, we tentatively identified the plant to species and recorded altitude and habitat. In the laboratory, we used isozyme electrophoresis to identify the species, since previous work had demonstrated that differences in the migration of isozymes in electric fields allowed species identification. It quickly became clear that the source of confusion was neither shared heritage nor environmental factors: we found a new sterile hybrid between the two hybrid species we had chosen to study. Since we had hybrids as well as hybrid species, we isolated cells undergoing meiosis to study chromosome number and meiotic pairing behavior. We also developed a set of about two dozen measurement characters with the goal of rigorously defining the morphological characters most likely to be useful in distinguishing the two species. We recorded these characters for each plant to decide whether structural differences do indeed allow the hybrid species to be distinguished.

Our work in the field and laboratory revealed that the two species were indeed distinguishable by inspection of the plants. The greatest barrier to discerning the species was the hybrid between them, which once identified could be removed from consideration. We were particularly fascinated by the habitat preferences: it turned out that Cystopteris fragilis was indeed widespread in the state-but at lower altitudes it sought the cooler and moister microsites that most resembled the higher-altitude habitats where it was originally encountered in Vermont.

We intend to pursue similar studies of hybrids and their progenitor species with the goal of understanding more about the role of sterile hybrids as well as polyploid hybrid species in the evolutionary process in the ferns.

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