Biology Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2749
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Item Physiological dynamics of injury and regeneration in the clonal freshwater annelid Pristina leidyi(2022) Rennolds, Corey William; Bely, Alexandra E; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The threat that mechanical injury poses to homeostasis and survival has spurred the evolution of diverse processes to mitigate these effects. The most dramatic of these is regeneration, a process that restores the form and function of lost body parts. The apparent benefits of regeneration may come at considerable cost, however, and these may substantially diminish regeneration’s adaptive value in certain contexts, potentially contributing to evolutionary losses of regeneration. The costs and benefits of regeneration are poorly understood in most animals, precluding more than speculation of the evolutionary drivers of regeneration. Naids are a group of small, clonally reproducing freshwater annelids that feature great diversity of regenerative ability and are well suited to experimental studies. I used the species Pristina leidyi to determine how injury and regeneration affect organismal function and fitness, integrating physiological and molecular approaches. I first investigated how injury and regeneration differentially affect an individual’s ability to tolerate environmental stress, an ecologically relevant and energetically demanding task. I found that stress tolerance is reduced by regeneration in a stressor- and tissue-specific manner while, unexpectedly, tolerance is temporarily improved shortly after injury. These effects are unrelated to whole-organism metabolic rate, which surprisingly does not differ between early and late injury recovery. Using 3’ TagSeq, I found that, while injury and heat stress elicit largely distinct responses, both upregulate certain shared damage control pathways. I then tested whether the physiological cost of regeneration has potential to translate into fitness costs by examining the interaction between regeneration and reproduction, which occurs by asexual fission in this species. By modulating resource availability, I found evidence for an energetic trade-off between regeneration and reproduction that is masked when food is abundant. This tradeoff is manifested through a reduction in per-offspring allocation rather than reproductive rate. Overall, my results demonstrate that injury and regeneration costs are highly context dependent in P. leidyi. More broadly, these findings contrast in key ways from evolutionarily distant animals with very different life history traits, illustrating the importance of investigating the physiological mechanisms that may mediate selection on regeneration in diverse lineages.Item Anatomical and Physiological Characterization of the Turtle Brain Stem Auditory Circuit(2014) Willis, Katie Leann; Carr, Catherine E; Neuroscience and Cognitive Science; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The goal of this dissertation is to add to understanding of the evolution of hearing by studying the testudine taxon. This dissertation focuses on central auditory processing in the context of evolution. The experiments described are designed to give insight into how binaural hearing evolved. Follow the findings of Christensen-Dalsgaard and colleagues (2012) that an amphibious turtle had lower hearing thresholds under water than in air and that this difference is conferred by resonance of the middle ear cavity, I examined middle ear cavities across families of Testudines. I found that middle ear cavity structure and function is shared by all testudines (Willis, et al., 2013). Modern neuroanatomical tract tracing techniques were used to understand the connections among the auditory nuclei in the brain stem of the turtle. Turtles have brain stem nuclei that are connected in the same pattern as the other reptiles, including birds. These nuclei are nucleus angularis, nucleus magnocellularis, nucleus laminaris, superior olive, and torus semicircularis. Details of neuron structure were also examined and quantified. Finally, I developed an isolated head preparation that enables in vivo-like physiological recording. As proof of principle, neurons were characterized by best frequency response, threshold, phase locking. Additionally, binaurally responsive neurons were found, which have a range of interaural time difference sensitivity responses. Although the evolutionary position of testudines is not yet resolved, it is most likely that testudines share their most recent common ancestor with the archosaurs. I hypothesize that testudines likely reflect the ancestral condition of auditory processing for the archosaur clade. All experiments described in this dissertation were performed according to the guidelines approved by the Marine Biological Laboratory (Woods Hole, MA, USA), the University of Maryland Institutional Animal Care and Use Committees (IACUC) and the Danish National Animal Experimentation Board (Dyreforsøgstilsynet).Item POST-COPULATORY SEXUAL SELECTION AND GAMETIC ISOLATION IN STALK-EYED FLIES(2011) Rose, Emily G.; Wilkinson, Gerald S; Behavior, Ecology, Evolution and Systematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Understanding the forces that drive lineage splitting, i.e. speciation, has been a goal of evolutionary research since Darwin but remains poorly understood. Sexual selection is frequently invoked as a possible explanation, but focus is typically placed on precopulatory activities where males compete for access to females or females choose among males. The possibility that postcopulatory sexual selection, a powerful evolutionary force which involves interactions between sperm and the female reproductive tract, may contribute to reproductive isolation has only recently been considered. Using diopsid stalk-eyed flies as a model system, I examine divergence in fertilization systems among closely related populations of a single species (Teleopsis dalmanni), in order to assess whether gametic isolation has the potential to contribute to speciation. In chapter 2, I measure a suite of reproductive and non-reproductive morphological traits in eight closely related populations to determine their relative rates of evolution. I find that reproductive traits have diverged more rapidly than non-reproductive traits and that male and female postcopulatory traits, i.e. sperm length and sperm storage organ dimensions, have coevolved. Chapters 3 and 4 describe experiments aimed at elucidating the importance of gametic isolation among these populations. Chapter 3 is an examination of non-competitive gametic isolating barriers. I performed 275 crosses between four populations and measured mechanisms of non-competitive gametic isolation including sperm transfer, sperm survival, sperm motility and ability of sperm to reach the site of fertilization. I conclude that non-competitive gametic isolation exists among these population pairs and specifically identify the inability of sperm to reach the site of fertilization in between-population crosses as a mechanism of reproductive isolation. Chapter 4 is an investigation of competitive gametic isolation which occurs when sperm of males from different populations compete for fertilization. Using two pairs of populations, I carry out every possible combination of crosses and genotype over 1200 offspring to determine paternity. The results demonstrate that sperm competition further inhibits successful hybridization among these closely related populations. I conclude that postcopulatory sexual selection and gametic isolation have the potential to play an important role in the formation of new species in this system.Item EVOLUTION, DEVELOPMENT, AND GENETICS OF OPSIN GENE EXPRESSION IN AFRICAN CICHLID FISHES(2011) O'Quin, Kelly E; Carleton, Karen L; Behavior, Ecology, Evolution and Systematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The molecular genetic mechanisms that underlie phenotypic evolution include mutations within protein-coding, cis-regulatory, and trans-regulatory factors. Although many studies have examined how these mutations individually contribute to phenotypic divergence and the formation of new species, none have examined how they may do so collectively. In this study, I examine how these molecular genetic mutations collectively contribute to the evolution of color vision among African cichlid fishes. I show that phenotypic divergence in cichlid color vision is achieved by mutations affecting the coding sequence and expression of seven opsin genes. After contrasting the roles of these two mechanisms, I use bioinformatic-, association-, and experimental genetic analyses to determine what role mutations in cis- and trans-regulatory DNA play in the evolution of cichlid opsin expression. Specifically, I demonstrate that: (1) Protein-coding mutations primarily affect cichlid opsins sensitive to the ends of the visible light spectrum (SWS1 [ultraviolet-sensitive] and LWS [red-sensitive]). (2) Changes in opsin gene expression contribute to large differences in color vision among closely related species. These analyses also reveal that the expression of the SWS1 and SWS2B opsins have diverged among closely related cichlids in association with foraging preferences and ambient light intensity, suggesting that their expression has evolved due to natural selection. Ancestral state reconstructions reveal that changes in opsin expression have evolved repeatedly among cichlids in Lakes Tanganyika and Malawi; further, I find that this repeated evolution has likely been achieved by repeated changes to cichlid development. (3) Bioinformatic analyses suggest that cichlids have diverged in multiple cis-regulatory sequences surrounding the opsin genes, and association mapping identified three putative single nucleotide polymorphisms upstream of the SWS2A (blue), RH2B (blue-green), and LWS (red) opsins that may contribute to cichlid opsin expression differences in cis. (4) Genetic mapping in experimental crosses suggests that divergence in multiple trans-regulatory factors also contribute to the evolution of SWS2B (violet), RH2A (green), and LWS (red) opsin expression. The contribution of these trans-regulatory factors to the evolution of cichlid opsin expression may outweigh those in cis. These results reveal that multiple molecular genetic mechanisms can contribute to phenotypic evolution among closely related species.Item Sex, Dispersal, and Deep Divergence: the Population Genetics of the Protistan Parasite Perkinsus marinus(2010) Thompson, Peter Christian; Hare, Matthew P; Rosenthal, Benjamin M; Behavior, Ecology, Evolution and Systematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The protistan parasite Perkinsus marinus causes Dermo disease in the eastern oyster, Crassostrea virginica. This parasite causes reduced growth and fecundity in its ecologically and economically important host, and as such has become a focal point for shellfish research. Though much is known regarding the seasonal dynamics and interactions between host and parasite, little research has focused on the basic biology of this parasite. In the research presented here, I used population genetic approaches to investigate the reproduction, dispersal, and origins of extant populations of P. marinus. First, I determined the extent of clonality in P. marinus populations. Repeated sampling of the same multilocus genotypes and extensive multilocus linkage disequilibrium indicated that clonal reproduction is prevalent. However, genotypic diversity was great and recombination occurred between genetic loci, supporting sexual reproduction as an important source of new genetic variation in P. marinus. An interesting consequence of sexual reproduction is that genotypic correlations may be maintained through inbreeding when sex occurs. Next, I investigated the genetic connectivity among locations. Clustering analyses revealed that local geographic samples are collections of independent clonal lineages rather than freely interbreeding populations. Some lineages were widespread while others were found at high frequencies only in specific locations indicating that P. marinus has a high capacity for dispersal, but local conditions may determine the success of certain lineages. Finally, I examined an interesting pattern of di-allelism observed in P. marinus DNA sequences. Two allelic classes were discovered at six out of seven nuclear loci where large divergences indicated the alleles had been independent for possibly millions of years. Balancing selection may be responsible for the retention of ancient diversity in this parasite, but it seems more likely that a recent hybridization event has occurred between two formerly allopatric lineages. These results underscore risks in the anthropogenic movement of protistan parasites as there may be no reproductive barriers between ancient lineages. Resulting hybrids could result in increased parasite virulence with increased disease in host populations.Item Patterns of song and preference variation in Laupala cerasina and their evolutionary implications(2010) Grace, Jaime Leigh; Shaw, Kerry L; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Closely related species often differ conspicuously in secondary sexual characters, many of which may be shaped by sexual selection. Changes in these characters may directly influence speciation, since females use them to assess males and to choose a mate. I investigate the hypothesis that in closely related populations, divergence in preference for sexually selected characters may promote sexual isolation through assortative mating. Ultimately, if reproductive isolation is complete, this process of divergent sexual selection through preference divergence may result in the formation of new species. I have investigated the relationship between divergence in the pulse rate of male song and female preference for pulse rates in a group of Hawaiian crickets, Laupala cerasina. Within a population, males sing at a characteristic pulse rate, but males from different populations sing with significantly different pulse rates. The degree of population divergence is almost an order of magnitude less than that which distinguishes closely related species of Laupala. Using phonotaxis trials, I found that females show acoustic preferences for males whose pulse rates fall nearest to the population mean. Across populations, male pulse rates and female preferences are significantly correlated, suggesting that they are coevolving. I have demonstrated that females discriminate among songs whose pulse rates vary within the range of pulse rates found among neighboring populations. Furthermore, in choice trials, females prefer the songs of males from the same population to those from neighboring populations. These females also exhibit significant assortative mating based on their acoustic preferences. Divergence in female acoustic preference among populations thus provides a mechanism for generating and maintaining biodiversity through divergence in sexually selected characters used in mate choice.Item COGNITIVE PERFORMANCE AND THE EVOLUTION OF MULTIPLE BEHAVIORAL DISPLAY TRAITS(2010) Keagy, Jason Christopher; Borgia, Gerald; Behavior, Ecology, Evolution and Systematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Males of many species have complex behavioral sexual displays and it is possible that these displays indicate aspects about male quality because of their relationship with cognitive ability. However, the relationship between behavioral display traits, cognitive ability, and reproductive success has received little attention. Satin bowerbirds, Ptilonorhynchus violaceus, are an excellent species for studying this relationship because their complex male courtship suggests a selective advantage to individuals with superior cognitive abilities. Here I show that cognitive performance can have important effects on mating success and the quality of behavioral displays. First, I present males with two novel problem solving tests and find that males who are better at solving these problems have higher mating success, establishing a link between male cognitive ability and sexual attractiveness. Second, I add four additional cognitive tests to expand the diversity of cognitive abilities tested and construct two measures of general cognitive ability using these data: the scores from an analysis that best explains covariation among performance on the different cognitive tests (g), and the average rank score on these cognitive tests ("IQ"). I show that these measures of general cognitive ability, as well as the majority of the independent cognitive tests, are positively associated with mating success. Third, I construct a different composite measure of male cognitive ability using an analysis that determines the combination of cognitive traits that make males most successful in attracting females (f). I show that this measure is significantly associated with my measures of general cognitive ability, suggesting that evolution of cognitive ability in bowerbirds is possible if there is sufficient heritability. I construct three different aggregate measures of male display quality (produced from four behavioral display traits) and find that they are all associated with f and with mating success. Finally, I show some support for the hypothesis that separate display traits may indicate different aspects of male cognitive ability. These results suggest that behavioral display traits may have a special role in sexual selection because of their connection to cognitive ability. Overall, this research highlights the importance of considering the interrelationship between cognitive ability and sexual selection.Item MATING COSTS, MALE CHOICE DISPLACEMENT, AND THE EFFECTS ON HYBRIDIZATION AND SPECIATION IN THE HAWAIIAN CRICKET LAUPALA (SUBFAMILY:TRIGONIDIINAE)(2009) Jadin, Jenna; Shaw, Kerry L; Behavior, Ecology, Evolution and Systematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Contact zones between two closely-related species provide unique laboratories for studying the processes of speciation. This is because, within these zones, species barriers will be reinforced and speciation will reach completion, or the barriers will break down, causing the two species to become one. Which of these two alternatives will occur depends on the degree of genetic differentiation and behavioral isolation between the species. If there is significant and non-combinable genetic variation between species, but behavioral isolation between the two incipient taxa is incomplete and allows hybrid offspring to be produced, these hybrid offspring will have lower fitness relative to parental types and selection should act directly to eliminate those offspring and indirectly against parents with broad mating preferences or traits. If however the genetic architecture is similar and behavioral isolation is incomplete, the populations would be expected to turn into a hybrid swarm and eventually become one species. Patterns of behavioral isolation and genetic variation in several Laupala species pairs suggest that contact zones between closely related species are marked by conflicting patterns of behavioral isolation and genetic differentiation. Evidence also suggests that the complex courtship system of Laupala may allow male choice to play an important role in sexual selection and speciation. Therefore I tested several hypotheses about the genetic differentiation, sexual selection, and behavioral isolation in a contact zone between the closely-related and morphologically indistinguishable L. tantalus and L. pacifica species pair. First, by using the mitochondrial COI gene and AFLPs as genetic markers, I demonstrated that there appears to be mitochondrial DNA introgression between sympatric, but not allopatric congeners, which suggests contemporary hybridization in the contact zone. Next, I found that males experience post-mating resource-limitation and show a significant tendency to invest less into a second mating, however, their investment is dependent upon female size. Finally, I found that there is apparent displacement of male choice, decreased variation in spermatophore production, and asymmetrical mating isolation within the contact zone. This evidence all suggests that there is increased behavioral isolation in this contact zone, which may be consistent with a hypothesis of speciation by reinforcement. However, this evidence also suggests that male costs may result in male choice conflicting with other isolating mechanisms. If so, this study may be another putative case of reinforcement, or it may be an entirely novel report of conflicting selection pressures within a hybrid zone. I suggest that further studies are needed to measure hybrid fitness as well as to evaluate relative male and female mating costs within the complex mating system of this rapidly-diversifying genus.Item Mechanism and evolutionary significance of the loss of melanin pigmentation in the cave fish Astyanax mexicanus.(2008-05-27) Hixon, Ernest R; Jeffery, William R; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The regressive evolution of traits such as eyes and pigmentation is common among cave organisms. As a model to study regressive evolution, I have used the teleost Astyanax mexicanus, which consists of eyed and pigmented epigean forms and many populations of cave-dwelling forms that have lost those traits. This study investigates the mechanism for the loss of melanin production, from the origin of chromatophores from the neural crest to the synthesis of melanin within the melanocyte. I show that cavefish retain a migratory population of neural crest derived cells that are tyrosinase positive and respond to exogenous signals as expected of a melanocyte. I then propose that the regressive evolution of melanin pigmentation is a selectively evolved trait that provides for an excess of dopamine, supported by the near two-fold increase in dopamine in cavefish brains, quantified via HPLC analysis. This study suggests that regressive evolution sometimes occurs via selection.Item Peripheral neural coding strategies for spectral analysis and sound source location in the non-teleost bony fish, Acipenser fulvescens(2008-04-26) Meyer, Michaela; Popper, Arthur N.; Fay, Richard R.; Biology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This work is an investigation of coding strategies for spectral analysis and sound source location in Acipenser fulvescens, the lake sturgeon. A. fulvescens belongs to one of the few extant non-teleost ray-finned fishes. The sturgeon taxonomic family, the (Acipenseridae), has a phylogenetic history that dates back about 200 million years. Studies on sensory coding in any species of this family or any other non-teleost species have not been conducted before. Thus, this is the first study on peripheral coding strategies by the auditory system in a non-teleost bony fish. For this study, a shaker system, similar to that used in previous experiments on teleosts, was used to simulate particle motion of sound at the ears and auditory periphery of A. fulvescens. Simultaneously, electrophysiological recordings of isolated single units from the eighth nerve were obtained. Peripheral coding strategies for spectral analysis and sound source location in A. fulvescens resembled those found in teleosts. Frequency data resembled the characteristics found for auditory afferents (with preferences for lower frequencies) in land vertebrates. In addition, the auditory periphery in A. fulvescens appears to be well suited to encode the intensity of sound. In terms of mechanisms for sound source location, eighth nerve afferents responded to directional stimuli in a cosine-like manner (as in teleosts), which can generally serve as the basis for coding the location of a sound source. Certain differences to teleosts were also found in A. fulvescens and these may have implications for the mechanisms for sound source location in azimuth. The common physiological characteristics between A. fulvescens, teleosts, and land vertebrates may reflect important functions (that are part of the process of auditory scene analysis) of the auditory system that have been conserved throughout the evolution of vertebrates.