Nathan Egan Rank
Undergraduate and graduate students have been full and active participants in my research for the past several years. Students work with me in laboratory, behavioral and field studies. We are conducting these studies in collaboration with scientists in other laboratories. Below I describe some ongoing and recently completed projects. Please feel free to contact me or any of the students whose email addresses are linked to this page if you wish to know more about the project (email me at email@example.com ).
Evolutionary history of host plant preference
Students: Kim Hansard, Susanne Kuhlmann, Katherine Sauceda
Species in the leaf beetle genus Phratora differ in host plant preference. Most beetles feed only on willows or poplars (family Salicaceae). Some of these plants contain salicylates in their leaves, while others contain little or no salicylates. Phratora species are usually either attracted to salicylates or repelled by them. However, two Phratora species feed on birches (family (Betulaceae), which contain no salicylates and are very different chemically from plants in the Salicaceae. One of the birch leaf beetles, Phratora polaris, lives in Finnish Lapland and the other one, Phratora hudsonia, lives in Canada. We will establish whether these two species are closely or distantly related, which will allow us to determine whether the host shift to birch occurred once or more than once. We are also analyzing comparative data on differences in the host plant preference of Phratora hudsonia and Phratora polaris.
This project follows from earlier studies with European collaborators of the evolution of host plant use in Phratora (description of research). The results of those studies will be published shortly (abstract).
Collaborators: Drs. Cristian Orrego and Det Vogler of San Francisco State University, Dr. Seppo Neuvonen of the Kevo Subarctic Research Station in Kevo, Lapland.
Natural enemies and herbivore host use
Some leaf beetle species in the genera Chrysomela and Phratora use host plant salicylates to make a larval defensive secretion, which consists primarily of salicylaldehyde. These beetles prefer salicylate-rich plants over salicylate-poor ones. It has been suggested that the beetles prefer these plants because their larvae are protected from predators and parasites on them. Yet until recently, very little was known about the natural enemies of leaf beetles.
My research has shown that two kinds of specialist predators are commonly associated with leaf beetles in different parts of the world. One of these specialist predators belongs to the hover fly genus Parasyrphus (family Syrphidae). Female flies lay their eggs onto the beetle eggs. The fly larvae consume only beetle eggs and larvae. My earlier results have shown that the fly larvae are not repelled by the beetle secretion. Rather they are attracted to it. Students and I are studying more about the orientation of the fly larvae to compounds present in the beetle secretion.
The other specialist predator belongs to the trap nesting wasp genus Symmorphus (family Eumenidae). The female wasps capture and sting the beetle larvae, wipe their defensive secretion off, and carry them to a nest where they place them to feed their offspring. Symmorphus wasps were known to occur throughout the Northern Hemisphere, but they are considered to be rare and difficult to find. We are working with Symmorphus cristatus populations feeding on the leaf beetle Chrysomela aeneicollis in the Sierra Nevada mountains. Last summer, we found that Symmorphus cristatus also feeds on the leaf beetles of coastal California. We plan to study whether the wasps are attracted to the secretions that the beetle larvae produce.
Collaborators: Prof. Monika Hilker and Juergen Gross of the Free University of Berlin, Dr. Riitta Julkunen-Tiitto of the University of Joensuu in Finland
Population structure of montane leaf beetles
Students: Megan Arnold, Amy Fungi, Eva Lizer, Jennifer Michaud, Katherine Sauceda, Mark Smith, Jerry Zatorski
Natural populations of organisms often occur in isolated patches with limited exchange of migrants among them. The potential for evolution to local environmental conditions is enhanced by their isolation. On the other hand, populations that go through cycles of extinction and recolonization are less likely to be adapted to local conditions.
We are studying natural populations of the leaf beetle Chrysomela aeneicollis in the Sierra Nevada mountains of California. These populations occur in three drainages at the southernmost tip of the range of this insect (Big Pine Creek, Bishop Creek, Rock Creek). In these drainages, the beetles occur from 7500 to 10,500 feet (2500-3500 m). During the drought years of the 1980s, I observed that populations of C. aeneicollis declined rapidly. By 1992, no beetles could be found except at the highest elevations. Since the drought ended, populations have recovered and beetles have returned to the lower elevations.
In 1988 before the beetle populations crashed, I collected 1600 beetles from the three drainages and studied the amount of genetic differentiation among them, using enzyme electrophoresis. I found that populations differed genetically within and among drainages. I also found abnormally high differentiation at the enzyme phosphoglucose isomerase, which suggested that this enzyme might be under natural selection.
In 1996 after the beetle populations recovered, Dr. John Smiley and I collected 1300 beetles at the same localities where I had collected them in 1988. Students and I are using electrophoresis to compare the genetic structure of the populations before and after the dramatic changes in population density. Preliminary results show that the unusual pattern for phosphoglucose isomerase. has persisted, despite the large fluctuations in population density.
Collaborators: Dr. John Smiley of the University of California's Big Creek Reserve, Prof. Elizabeth Dahlhoff of the University of Santa Clara.
Host preference versus larval performance in a California leaf beetle
Student: Coby LaFayette
The genus Plagiodera is widely distributed in the tropics, but a few species occur in the Northern Hemisphere. Plagiodera versicolora was native to northern Europe, but it was introduced to North America and has rapidly spread across the eastern half of the U.S.A. Plagiodera californica occurs along the central and northern coast of California. Both species feed on willows. The larvae of both beetles produce large droplets of an autogeneously synthesized secretion that contains methylcyclopentanoid monoterpenes. Plagiodera versicolora is known to prefer salicylate-poor willows, but the host plant use of Plagiodera californica remains largely unknown.
We observed that Plagiodera californica adults were abundant on two willows that differ substantially in leaf chemistry. Salix lasiolepis leaves contain substantial amounts of salicylates, but those of Salix lucida are salicylate-poor. We are determining the effects of previous experience on the host plant preference and performance of P. californica. In addition, we have compared the host preference of P. californica and P. versicolora.
Collaborators: Prof. Robert Fritz of Vassar College and Dr. Riitta Julkunen-Tiitto of the University of Joensuu
Ecology and genetics of a biological invasion by the salt marsh grass Spartina alterniflora.
Spartina alterniflora is native to the eastern half of the USA, where it is a dominant species inhabiting salt marshes. It grows in large stands and grows to two meters in height. This grass was introduced to several locations along the West Coast, where it is flourishing. It poses a substantial threat to the native flora and fauna of the San Francisco Bay. The salt marshes and mud flats of California have been traditionally much more open and devoid of salt marsh grasses. They support a diversity of plants and animals, but they are are now menaced by S. alterniflora.
Spartina foliosa, which is native, grows in smaller, dispersed clumps throughout coastal California. This species is threatened by its sister species, S. alterniflora, for two reasons. First, S. alternflora tends to exclude S. foliosa from its own preferred habitat. Second, the two species hybridize with each other. Students are investigating the genetic and ecological threat by S. alterniflora to S. foliosa. They are also comparing the susceptability of populations of S. alterniflora and S. foliosa to the plant hopper Prokelisia marginata.
This research is being carried out and supported by the research group of Professor Donald Strong of the Bodega Marine Laboratory of the University of California-Davis.
Collaborators: Dr. Deborah Ayres of the Bodega Marine Laboratory, Prof. Curt Dahler of the University of Hawaii, Prof. Donald Strong of the Bodega Marine Laboratory.
November 1, 1997 NER