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ItemBrain diversity develops early: a study on the role of patterning on vertebrate brain evolution(Georgia Institute of Technology, 2011-08-24) Sylvester, Jonathan BlaylockThe brain has been one of the central foci in studies of vertebrate evolution. Work in East African cichlids and other emerging fish models like the Mexican cavefish (Astyanax mexicanus) offer new insight on the role of patterning on brain evolution. These fish can be grouped into two major categories according to habitat; for cichlids it is rock-dwelling (known locally as mbuna) and sand-dwelling (non-mbuna) lineage. The brain development of mbuna versus non-mbuna is defined by changes in gene deployment working along the dorsal/ventral (DV) and anterior/posterior (AP) neuraxes, respectively. Comparison of disparate fish ecotypes offer a new perspective of the role of patterning on brain evolution; through the slight and early modification of signal pathways working across 3-D axes, and a subsequent magnifying effect across ontogeny, evolution can generate widespread changes in the brain. To illustrate this patterning model of brain evolution, two comparative studies were done between mbuna and non-mbuna, examining the action of gene pathways that work to pattern the cichlid forebrain. The first study found that non-mbuna cichlids have a more rapid AP expansion of a gene pathway (Wingless) into the presumptive midbrain and diencephalon versus mbuna. These forebrain structures are involved in sight processing and could be of ecological benefit to vision-focused non-mbuna. The second study described a difference within the developing telencephalon. The embryonic telencephalon is split into the pallium, which processes visual signals, and the subpallium, which develops into the olfactory bulbs. Mbuna possess a larger subpallium relative to non-mbuna, which have a larger pallium. This was correlated to a more rapid expansion of another gene pathway (Hedgehog) along the DV axis. The difference in size of the pallial vs. subpallialial comparments between cichlids can be correlated to expanded olfaction in mbuna and vision in non-mbuna adult brains. Overall, East African cichlids are an excellent system to investigate the role of patterning on brain evolution because they allow for the comparison of the earliest patterning events in brain ontogeny between distinct ecotypes. These fish systems link study in brain development to the brain morphology comparisons employed in classic studies of brain evolution.
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ItemThe generation and maintenance of diversity in a rapid adaptive radiation(Georgia Institute of Technology, 2011-08-23) Parnell, Nicholas FrancisThe Lake Malawi cichlid fishes are a pre-eminent example of adaptive evolutionary radiation. The diversity of species (nearly 1000 extant) is mirrored by an array of variation in dozens of phenotypes (e.g. trophic morphology, tooth shape, color patterns, behavior, development). The unique characteristics of this system have produced unparalleled diversity with very little genetic differentiation between species. This dissertation is composed of four studies addressing different aspects of the variation in the LM cichlids and the mechanisms generating and maintaining this level of diversity at multiple biological levels. Community-level diversity is investigated using null model analysis of species co-occurrence data. We detect signals of non-random community assembly at only the broadest and finest spatial scales. Based on the unique ecological and evolutionary characteristics of this assemblage we suggest that different mechanisms are responsible for these patterns. A core‟ group of species is posited to act as a foundation on which these diverse communities are created as a result of fine-scale species interactions. We identify both positive and negative depth-based correlations between species and suggest these interactions play an important role in species diversity in these fish. The Lake Malawi cichlids exhibit an array of trophic morphologies which may play a role in the fine-scale species interactions described in chapter one. In the second chapter we build a genetic model to predict the evolution of jaw morphology and a complex functional jaw trait. We use a complex biomechanical system, the 4-bar jaw linkage, to simulate trait evolution during interspecific hybridizations. We find rampant transgression (trait values beyond parental distributions) in jaw function in a large proportion of potential crosses. This result is characterized by a lack of novel morphological components but rather is the result of recombinations of existing component traits thus producing functional novelty. In the third chapter we create a laboratory cross of one of the parental combinations suggested from the genetic model. The results of this study serve as a proof of principle to the simulations as we observe a large proportion of transgressive 4-bar function in the F2. As predicted this diversity is produced in the absence of transgressive morphology. We contrast these results between this complex system and data generated from several simple jaw lever traits and report differences in the patterns. Using quantitative trait locus (QTL) mapping approaches we examine the genetic basis for complex and simple jaw traits and discuss correlative patterns within and between systems. Finally we examine the genetic architecture of sex-determination and color morphs in this hybrid cross. We find both ZW and XY sex systems segregating as well as linkage to sex-specific color patterns. Several loci and epistatic interactions are associated with sex-determination and color morphs in this cross. The orange-blotch (OB) color is found associated with ZW as predicted from previous work but a previously undescribed (in these species) male nuptial color (blue) is found associated with both ZW and XY genetic systems as well as other loci segregating for sex-determination. These results are discussed in the context of models of sex chromosome evolution as a result of sexual conflict and the potential importance of sexual selection in the diversification of Lake Malawi cichlids. Overall we observe various mechanisms generating and maintaining diversity at different levels of biological organization. We use community co-occurrence analyses, genetic simulation, and QTL analysis of an F2 hybrid population to examine these mechanisms in this rapidly radiating assemblage. These results bolster our understanding of the origins of diversity and the interplay between variation and aspects of evolution in all biological systems.
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ItemGenetic variation in fast-evolving East African cichlid fishes: an evolutionary perspective(Georgia Institute of Technology, 2011-06-23) Loh, Yong-Hwee EddieCichlid fishes from the East African Rift lakes Victoria, Tanganyika and Malawi represent a preeminent example of replicated and rapid evolutionary radiation. In this single natural system, numerous morphological (eg. jaw and tooth shape, color patterns, visual sensitivity), behavioral (eg. bower-building) and physiological (eg. development, neural patterning) phenotypes have emerged, much akin to a mutagenic screen. This dissertation encompasses three studies that seek to decipher the underpinnings of such rapid evolutionary diversification, investigated via the genetic variation in East African cichlids. We generated a valuable cichlid genomic resource of five low-coverage Lake Malawi cichlid genomes, from which the general properties of the genome were characterized. Nucleotide diversity of Malawi cichlids was low at 0.26%, and a sample genotyping study found that biallelic polymorphisms segregate widely throughout the Malawi species flock, making each species a mosaic of ancestrally polymorphic genomes. A second genotyping study expanded our evolutionary analysis to cover the entire East African cichlid radiation, where we found that more than 40% of single nucleotide polymorphisms (SNPs) were ancestral polymorphisms shared across multiple lakes. Bayesian analysis of genetic structure in the data supported the hypothesis that riverine species had contributed significantly to the genomes of Malawi cichlids and that Lake Malawi cichlids are not monophyletic. Both genotyping studies also identified interesting loci involved in important sensory as well as developmental pathways that were well differentiated between species and lineages. We also investigated cichlid genetic variation in relation to the evolution of microRNA regulation, and found that divergent selection on miRNA target sites may have led to differential gene expression, which contributed to the diversification of cichlid species. Overall, the patterns of cichlid genetic variation seem to be dominated by the phenomena of extensive sharing of ancestral polymorphisms. We thus believe that standing genetic variation in the form of ancestrally inherited polymorphisms, as opposed to variations arising from new mutations, provides much of the genetic diversity on which selection acts, allowing for the rapid and repeated adaptive radiation of East African cichlids.
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ItemThe evolution of cleaning mutualism and predator cooperation in a radiation of Caribbean fishes(Georgia Institute of Technology, 2010-07-07) Lettieri, Liliana B.The steps by which neutral, random and/or negative biological interactions evolve into mutualistic ones remain poorly understood. Here, we study Elacatinus gobies and the fishes from which they clean parasites, termed 'clients'. Colorful stripes are common to mutualist cleaners and non-cleaning sister species. Blue stripes are unique to obligate cleaners. We quantified the contrast potential of ancestral and novel stripe colors, using fish color vision models, and determined that color stripes have become more visible to clients over evolutionary time. In turn, we focused on the role of color as a potentially specialized signal. We show that cleaners possess a putative chemical defense (one multimedia file in .mov format included) and demonstrate that stripes are sufficient to elicit client stereotypical posing behavior and to deter attack. Analysis of previously published records show that yellow cleaners tend to predators less than expected, compared to green and blue cleaners. Our results highlight evolution from predator avoidance to tolerance with conspicuous advertising reinforced by chemical defense in cleaners. Similar trajectories, via recognizable signals to risky partners, may be common in other diffuse mutualisms. We discuss the generality of defense and signal traits in other species rich lineages of mutualists.