Local adaptation in montane leaf beetles
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 when gene flow among subpopulations is reduced. Geographic barriers can restrict genetic exchange among populations even further. I have measured differentiation among populations using polymorphic enzymes.
I study populations of the montane leaf beetle Chrysomela aeneicollis in the Sierra Nevada mountains of California. These populations occur in three drainages at the southernmost tip of the insect's range. In the Sierra Nevada, the beetles only occur at high elevations (2500-3500 m). The abundance of the beetles has fluctuated dramatically over the last 15 years. These fluctuations suggest that non-equilibrium processes may strongly influence the genetic structure of these populations.
In 1988, I collected over 1600 beetles from the three drainages and measured differentiation at five polymorphic enzyme loci. I found significant differentiation among drainages, localities within a drainage, and individual host plants. Four enzymes varied among populations to about the same degree. However, divergent frequencies at the enzyme phosphoglucose isomerase (PGI) suggested that natural selection acts on it (abstract 7 ). In 1996, Dr. John Smiley and I collected new beetles at the same study localities that I had previously used. Most populations had gone extinct and been recolonized since 1988. I have nearly completed a second study of genetic differentiation. Results indicate that PGI frequency differences among drainages have persisted. In addition, the frequency of one allele has increased by 15% in Bishop Creek, providing further evidence that natural selection acts on this locus.
Current Research- In 1998, Professor Elizabeth Dahlhoff of Santa Clara University and I were awarded a one-year NSF grant to study thermal adaptation in this leaf beetle, focusing on mechanisms of biochemical adaptation. We documented that
We have therefore demonstrated that environmental differences occur accross the gradient where the differences in PGI frequencies occur, and we have shown that stress protein levels in the insects reflect those differences. We plan to complete further studies of thermal adaptation in the coming summers.
Natural enemies and herbivore host use
Insect herbivores are incredibly diverse, yet most have specialized onto only a few host plant species. Evolutionary biologists have long puzzled over the reasons for this specialization. The Predation Hypothesis proposes that herbivores specialize onto plants where predation and parasistism is reduced. I tested this hypothesis using two herbivores where the relationship between host plant chemistry and beetle defensive secretions were well known. I conducted two studies of host plant effects on larval survival of North American and European leaf beetles (Chrysomela aeneicollis and Phratora vitellinae). Both beetles require host plant salicylates to produce a larval defensive secretion. When they feed on salicylate-poor willows, larvae produce very little defensive secretion. Adult beetles prefer salicylate-rich hosts over salicylate-poor ones (abstract 2 & 3 ).
Contrary to the predation hypothesis, I found that these beetles did not survive best on the plants on which they obtain their defensive secretion (abstracts 3 & 4). In addition, I showed that the beetle secretion did not repel the most important natural enemies of P. vitellinae and C. aeneicollis. The major predators were specialists that are not repelled by the host-derived larval secretion. Syrphid flies in the genus Parasyrphus attack the beetle larvae most often on their preferred salicylate-rich host plants (abstract 5 ). The fly larvae use the secretion to locate beetle larvae (abstract 6 ). Another specialist predator, the eumenid wasp Symmorphus cristatus, also probably uses the beetle secretion to locate beetle larvae.
My studies have uncovered a new role of predators as selective factors on herbivore host use. Important predators orient to the chemicals in their prey in much the same way that beetles use plant compounds to locate their hosts. The broad geographical distribution of these predator-prey interactions suggests that they may be strongly influenced by shared evolutionary history. This has largely been neglected in empirical and theoretical studies of the relationships between insect herbivores and their predators.
Evolutionary history of host plant preference
Beetles in the genus Phratora differ in host plant use and larval defensive secretion chemistry. Most Phratora species feed on poplar or willow, but two species have shifted to birch. One European species, Phratora vitellinae, uses host plant compounds to produce its defensive secretion, while the rest of them make an autogeneous secretion. To reconstruct the evolutionary history of host plant preference and secretion chemistry, my collaborators and I conducted a phylogenetic study of 11 species, based on 1400 base pairs of the mt DNA cytochrome oxidase I gene (COI). This study showed that species of Phratora usually prefer chemically similar host plants. However, in one clade, species appear to be rapidly shifting between chemically heterogeneous hosts. It was in this clade that at least one host shift from willows (family Salicaceae) to the chemically distinct birch family (Betulaceae) occurred (abstract 1).
One of my research goals is to add the five North American species of Phratora to the phylogenetic tree of the genus. My results will determine whether the beetle species in Europe and North America diversified and specialized on willow, poplar, and birch convergently.
Research Home|Nathan Rank's Homepage |Department of Biology | Sonoma State University