| dc.description.abstract | Locusts are grasshoppers that swarm and exhibit an extreme form of phenotypic plasticity called density-dependent phase polyphenism. They exist in two different phases, solitarious and gregarious, which are found at low and high population densities, respectively. They differ in their behavior, nymphal coloration, morphology, physiology, ecology, and reproductive and developmental biology, among others. Density-dependent phase polyphenism evolved independently multiple times in the family Acrididae, even within the genus Schistocerca. This genus contains three locust species and about 50 non-swarming species. The ancestral condition to the genus is thought to be a form of polyphenism similar to the one found in S. gregaria, which is a sister species to the rest of the genus. This polyphenism was subsequently lost in the other species, but was regained at least twice, in the locusts S. piceifrons and S. cancellata.
In this dissertation, I focus on the Central American locust S. piceifrons, and three closely related non-swarming species that exhibit different degrees of density-dependent phase polyphenism, S. americana, S. serialis cubense, and S. nitens. I establish the extent of polyphenism in each of these four species at a behavioral, morphological and molecular level, and show that there is a spectrum of polyphenism in these four species that is the lowest in S. nitens and increases through S. serialis cubense and S. americana to S. piceifrons, which exhibits the full extent of polyphenism similar to other locust species. Additionally, I show that the polyphenism in different species is not regulated by differential expression of the same genes, but rather by different genes that exhibit similar functions, and that hexamerin-like proteins might play an important role at the molecular level. Using both laboratory-based and field-based experiments, I show that gregarization is a slow process in S. piceifrons, while solitarization occurs very fast in this species, which is reversed from the situation in the congeneric S. gregaria. Finally, I report that [His7]-corazonin influences the nymphal coloration and the size, but not the behavior of S. piceifrons, suggesting its functions are highly conserved among locust species. | en |
