The role of local adaptation and fitness trade-offs in the sympatric divergence of long term experimental study populations of Enchenopa treehoppers (Hemiptera: Membracidae)
Date
2004
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Publisher
University of Delaware
Abstract
The role of sympatric speciation in the animal kingdom has long been controversial. Ecological models of speciation are becoming more popular, and, in particular, those that invoke host shifts by phytophagous insects. The treehopper Enchenopa binotata, a cryptic species complex comprised of nine species, is an excellent model system for this hypothesis. It has been hypothesized that this complex of species diverged through assortative mating initiated by shifts to novel hosts and driven by the host phenology. This has become known as the Host Water Phenology Hypothesis. Although many of the components of this system have been considered, the question of whether fitness trade-offs play a role in isolating host-associated populations within this complex is still unresolved. This study used a long-term experimental system to determine if host shifts cause predicted fitness trade-offs in egg mass production by females. Ovipositional output (in the form of number of egg masses produced) is also used as a measure of fecundity to determine the extent of local adaptation (if any) to novel hosts after nine generations under both allopatric and sympatric conditions with respect to ancestral host populations. After mating, females were collected from both the ancestral host Viburnum lentago as well as from the novel host V. lantana in experimental cages that were both sympatric as well as alloptric with regard to V. lentago. Females were then reciprocally transferred to both V. lentago and V. lantana. They were individually confined to branches using sleeve cages to establish iso-female lines. After oviposition and female death, the cages were removed and the number of egg masses per female was recorded. Little local adaptation to novel hosts was evident, although comparable ovipositional output by insects that developed on novel hosts with respect to insects that developed on the ancestral host within a novel host environment may indicate that the novel host-associated insects have begun to overcome developmental hurdles. In addition, fitness trade-offs were only found when novel host associated insects were returned to an ancestral host that was supporting an ancestral host-population. That is, fitness trade-offs only exist when the insects colonize new hosts in the presence of old hosts that are already populated by the treehoppers. Both these results support the Host Water Phenology Hypothesis and a sympatric model of speciation in the E. binotata species complex.
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Keywords
Enchenopa binotata (treehopper), Sympatric speciation, E. binotata (treehopper), Insect phenology