Browsing by Author "Edward L. Vargo, Committee Member"

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Chemical Communication in the German Cockroach: Pheromones and Heterospecific Courtship Eliciting Compounds
(2007-08-07) Eliyahu, Dorit; Coby Schal, Committee Chair; Edward L. Vargo, Committee Member; Robert R. H. Anholt, Committee Member; Charles S. Apperson, Committee Member
Sexual communication is vital for the reproductive success of many species. Chemical communication is considered highly effective in being both species-specific and indicative of reproductive status. However, there are cases in which this intraspecific intersexual system breaks down, either by sexual mimicry or by convergence on similar compounds in different species. In the first chapter I review the literature for cases where courtship displays are chemically elicited by non-reproductive conspecifics and by heterospecifics. I group cases as being (1) adaptive to courting males; (2) maladaptive to courting males; or (3) incidental consequences of convergence on similar courtship releasing cues of no adaptive value to either participant in courtship. Volatile sex pheromones, responsible for bringing of the sexes together, have been studied extensively. On the other hand, contact sex pheromones which are vital for reproductive success in many species, have received much less attention and little is known about their chemistry and biochemical and hormonal regulation. An exception is the German cockroach, Blattella germanica, whose contact sex pheromone has been extensively studied in relation to endocrine regulation of pheromone biosynthesis. Here I report on investigations of chemical structure-activity relationship in the German cockroach using naturally occurring and synthetic courtship eliciting compounds. I show that contrary to expectation, the naturally occurring stereoisomer of the pheromone, (3S,11S)-dimethylnonacosan-2-one, is the least active of the four possible stereoisomers. Extensive behavioral assays with synthetic pheromone analogs and the natural pheromone were conducted to validate this result. Next I identify two additional contact sex pheromone components, predicted from the proposed biosynthetic pathway, and confirm their behavioral activity with synthetic compounds. I used a behavioral-assay guided approach whereby fractions from flash column chromatography, high performance liquid chromatography and gas chromatography were behaviorally assayed and the active components identified with mass spectrometry. In addition, I describe two intriguing phenomena where courtship is directed toward non-receptive individuals: the German cockroach male courts immatures of its own species, as well as members of five other cockroach species. I used a similar behavior-guided chemical fractionation approach to purify and identify from nymphs compounds responsible for eliciting courtship. The results show that last instar female nymphs share common pheromonal components with the adult female contact sex pheromone. I also isolated a different, yet to be identified compound, or set of compounds, from young nymphs and last instar female nymphs that elicit courtship in Blattella males. The nymphs may elicit courtship as a potentially adaptive strategy of sexual mimicry, whereby they avoid aggression from males or gain nutritional benefits from the courting male. Five other cockroach species can elicit courtship in German cockroach males, and the courtship eliciting compounds in Blatta orientalis were identified as related to, but different from any of the sex pheromone components of B. germanica. This suggests that the German cockroach male responds to a broader range of chemical structures than necessary for sexual and species recognition. Possible conditions favoring this broad sensory tuning are discussed.
Neotropical Ant-Gardens: Behavioral and Chemical Ecology of an Obligate Ant-Plant Mutualism
(2008-12-03) Youngsteadt, Elsa; Edward L. Vargo, Committee Member; Fred Gould, Committee Member; Jules Silverman, Committee Member; Coby Schal, Committee Chair
Seed dispersal mutualisms are essential for the survival of diverse plant species and communities worldwide. An outstanding but poorly understood ant-seed mutualism occurs in the Amazonian rainforest, where arboreal ants collect seeds of several taxonomically diverse plant species and cultivate them in nutrient-rich nests, forming abundant hanging gardens known as ant-gardens (AGs). AG ants and plants are dominant members of lowland Amazonian ecosystems, and their interaction is obligate and apparently species-specific. Though established AGs are limited to specific participants, it is unknown at what stage specificity arises. Seed fate pathways in AG epiphytes are undocumented, and the recognition cues that mediate the mutualism are unknown. Here the species specificity of the AG ant-seed interaction is assessed, and chemical cues are characterized that elicit seed-finding and seed-carrying in AG ants. To examine the specificity of the ant-seed interaction, general food baits and seeds of the AG plant Peperomia macrostachya were offered on alternate days at 108 bait stations. Seventy ant species were detected at food baits and could have interacted with AG seeds, but only three species collected P. macrostachya seeds, and 84% of observed seed removal by ants was attributed to the AG ant Camponotus femoratus. In a separate experiment, arthropod exclusion significantly reduced AG seed removal rates, but vertebrate exclusion did not. Thus species specific seed dispersal, rather than post-dispersal processes, appears to be the primary determinant of the distribution of AG plants. The results also begin to quantify factors that affect seed fate in this unusually specific plant-animal mutualism. To characterize behavioral cues on AG seeds, solvent extracts of three species (Anthurium gracile, Codonanthe uleana, and P. macrostachya) were subjected to chromatographic fractionation. To test behavioral activity, each fraction was applied to other seeds that ants ordinarily ignore. At least one fraction of each seed extract elicited retrieval behavior in C. femoratus, but the active fractions of the three species differed in chemical composition, indicating that each plant species elicits seed-carrying with a different class of chemical attractants. Volatile attractants from P. macrostachya were further investigated using Y-tube olfactometer assays and gas chromatography-electroantennographic detection. In the olfactometer, C. femoratus preferred the odor of P. macrostachya seeds over that of control seeds, and P. macrostachya extract over solvent control. Five electrophysiologically active phenolic and terpenoid compounds from P. macrostachya extract were identified and combined in a synthetic blend, which was also preferred over solvent control and over a single component alone. Although seed dispersal by ants is common worldwide, this is the first documentation of volatile attractants from any ant-dispersed seed. Furthermore, the most abundant component of the attractive blend, geranyl linalool, is toxic to many ant species, hinting at one mechanism by which AG seeds may avoid being collected by inappropriate dispersers. Future studies of AGs should address costs and benefits of the interaction to its various participants, evaluate the consequences of the interaction for gene flow and population structure in the plants, and assess geographic variation in ant and seed traits that facilitate the interaction. Such studies would bear on central themes in the theory of mutualism, including the origin of mutualism, the evolution and coevolution of traits that facilitate the interaction, evolution and maintenance of species-specificity, and prevention of cheating. Furthermore, the taxonomic diversity of AG participants provides a unique opportunity to examine these questions in distantly related species that have all converged upon a similar mutualistic survival strategy.
Nestmate recognition and population genetic structure in the Argentine ant, Linepithema humile.
(2003-11-16) Buczkowski, Grzegorz Andrzej; Jules Silverman, Committee Chair; Coby Schal, Committee Co-Chair; Charles S. Apperson, Committee Member; Edward L. Vargo, Committee Member
The Argentine ant, Linepithema humile, is a widespread invasive species characterized by reduced intraspecific aggression within its introduced range. To gain an understanding of mechanisms underlying nestmate recognition in the Argentine ant, I studied its population genetic structure and the role of genetic vs. environmental cues on aggression. I combine behavioral, genetic, and analytical data to explore the role of exogenous cues and recent changes in the population genetic structure on nestmate recognition in this ant. In Chapter II, I examined population genetic structure and intercolony aggression in two portions of the Argentine ant introduced range: California and the southeastern U.S. I describe factors that may have contributed to the present-day differences in genetic diversity between these two regions and I also suggest ecological factors that may have contributed to regional differences in current distribution patterns of the Argentine ant. In Chapter III, I examined the context-dependency of nestmate discrimination in the Argentine ant and the effect of action thresholds on exogenous cue recognition. I also tested hypotheses that explain a change in action thresholds as well as factors that affect action thresholds. I examined changes in intraspecific aggression in various discrimination contexts. I tested the importance of isolated nest referents on aggression thresholds. In Chapter IV, I examined the impact of different diet-derived hydrocarbons on intraspecific aggression in the Argentine ant and the potential of shared, diet-derived hydrocarbons to produce colony uniformity where intercolony genetic and/or environmental differences exist. In Chapter V, I examined the role of environmental cues on nestmate recognition in two populations of the Argentine ant. I tested the hypothesis that there is geographical variation in the response of Argentine ants to nestmate recognition cues derived from prey. In Appendix I, I use the Argentine ant to compare four aggression bioassays for consistency between replicates, similarity between assays, and ability to predict whole colony interactions. I tested four different aggression assays with two or three scoring methods per assay. I also determined whether isolated aggressive encounters could predict whole colony interactions.
Phylogenetic Utility of Two New Nuclear Genes, Opsin and CAD, within the stiletto flies (Diptera: Therevidae)
(2003-07-07) Hill, Hilary Nan; Brian M. Wiegmann, Committee Chair; Lewis L. Deitz, Committee Member; Edward L. Vargo, Committee Member
A need for multiple phylogenetic markers to reconstruct evolutionary relationships is increasingly apparent within both the Therevidae and insect systematics as a whole, especially markers that can accommodate the immense species diversity that arose during the Mesozoic (~65-250mya). Phylogenetic trees are often best reconstructed using datasets from distinct sources and from character sets that have been under different selective pressures. Many recent insect systematics studies use various combinations of markers from multiple genomes, morphology, and biogeography. Nuclear genes, particularly protein-encoding genes that are highly conserved and low copy-number, are increasingly attractive as phylogenetic markers, and there is a recent concerted effort to find and increase access to new nuclear genes. Chapter one of this study investigates the phylogenetic utility of opsin, a new multiple copy nuclear protein-encoding gene in the dipteran family Therevidae. The present analyses of nucleotide sequence data of opsin reconstructed a therevid phylogeny that is partially concordant with previous classifications which demonstrates that opsin may have some limited utility as a phylogenetic marker within the Therevidae and closely related Diptera, but also shows that opsin may be better used in combination with other molecular markers or morphological data sets. Phylogenetic analyses of opsin at multiple levels within insects supports these results, but also reveals the significant obstacles in technical manipulation of the gene and in the interpretaion of ortholog/paralog relationships. In the second chapter two new nuclear, protein-encoding genes, opsin and CAD, in combination with EF-1a and 28S rDNA, are applied to reconstruct evolutionary relationships among the major lineages of Therevidae for which previous molecular evidence has been insufficient. The analyses of 28S rDNA, EF-1a, opsin, and CAD nucleotide sequences provide phylogenetic resolution that is highly concordant with the current therevid classification. The analyses also reveal that the phycine tribe Xestomyzini is monophyletic and could represent a major therevid clade. The new molecular dataset generated from this study can be used to build a larger dataset and can also be used in conjunction with a morphological dataset that is currently being generated within the therevid PEET project.