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NIH Post-Doctoral Research Fellowcontact me: arnegard@zoology.ubc.ca |
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Research Interests
The evolution of biological diversity depends on three interdependent processes: (i) adaptation within lineages; (ii) the splitting of lineages, or speciation; and (iii) the origin of novelty. While evolutionary biologists have made much progress in understanding the ecological causes of speciation in many different systems, the field has barely begun to uncover the genetic basis of ecological speciation. I am particularly interested in studying genes underlying sexual signals, mating preferences, and other behaviors related to pre-mating reproductive isolation. These aspects of animal speciation merge naturally with my broader interests in the evolution of animal communication. Understanding the origins of new modes of communication requires that we also understand, more generally, how innovative structures and physiological systems evolve de novo. Investigating genetic changes associated with novelty remains one of evolutionary biology's greatest challenges, with little insight into this process being offered by well-investigated cases of regressive trait loss.
Currently, my main project with Katie Peichel, Dolph Schluter, and Gina Conte investigates the genetic basis of parallel ecological speciation between benthic and limnetic sticklebacks. I started this project with Dolph at the University of British Columbia (UBC) in 2007; more recently, I received postdoctoral NRSA funds from NIH to continue this project with Katie at the Fred Hutchinson Cancer Research Center in Seattle, in collaboration with Dolph and Gina at UBC. In the Schluter Lab, Gina and I created a number of mapping populations of F2 intercross hybrids between sympatric benthic and limnetic sticklebacks endemic to each of two lakes in British Columbia (Paxton and Priest: see above). The hybrids were reared under fully natural conditions in experimental ponds at UBC. We have measured a number of F2 phenotypes related to important ecological differences between the parental species (e.g., functional morphology, stable isotopes of carbon and nitrogen, habitat choice). In the summer of 2011, we completed fieldwork needed to measure behaviors contributing to pre-mating reproductive isolation: e.g., nesting habitat choice of F2 males; and F2 female mating preferences for benthic versus limnetic males assessed by genetically fingerprinting offspring. Recently, our project received additional NIH funds needed to genotype all F2s using an array of 1,536 SNPs. We are in the process of testing the genetic architectures underlying ecological speciation for species pairs in both Paxton and Priest Lakes. We are particularly interested in asking whether the same QTL – either pleiotropic or closely linked genes – simultaneously govern ecological traits under divergent natural selection and behaviors contributing to reproductive isolation. Such a genetic architecture would favor rapid speciation under gene flow. We are also testing whether there is parallelism in the genetic architectures of species differences between the two lakes. We presented our initial results at the 2012 Evolution Conference in Ottawa, Canada, and at the 2012 International Conference on Stickleback Behavior and Evolution, which took place on Bainbridge Island near Seattle. The first manuscripts for this project are currently in the works.
In addition to research on speciation genetics, I also investigate the role of communication in speciation, the origins of novel communication systems, and the effects of neural innovations on the tempo and breadth of species radiation. I do so using weakly-electric fishes. In one of the most remarkable cases of convergence in the Animal Kingdom, speciose groups of electric fish (mormyroids and gymnotiforms) arose independently in Africa and South America, respectively. I recently led an effort to test whether a duplicated voltage-gated sodium channel gene (Scn4aa) directly contributed to independently-derived electric organs in mormyroids and gymnotiforms. Myogenic electric organs (EOs) produce electrical communication signals in both groups. EO is composed of cells called electrocytes, which are developmentally derived from skeletal muscle myoblasts and show striking similarities in structure and physiology between groups. To function, electrocytes require voltage-gated sodium channels (see the figure, below), two paralogs of which (Scn4aa and Scn4ab) I and co-authors Derrick Zwickl, Ying Lu, and Harold Zakon investigated in a large number of electrogenic and non-electrogenic fishes. We tested three predictions of our hypothesis that Scn4aa directly contributed to the parallel origins of this novel organ of communication. Our findings provide evidence in support of all three predictions, demonstrating that gene duplication can contribute to strikingly similar innovations even after extremely long waiting periods between gene duplication and the origins of novelty. We also found that amino acid replacements caused by diversifying natural selection on Scn4aa in electric fish occur at the same residues where deleterious substitutions in paralogs cause cardiac or neurological disease in humans (e.g., long QT syndrome). Read more about these findings in Arnegard et al., 2010 (PNAS 107: 22172-22177).
The origins of electrical communication are examples of 'key innovations' that have directly contributed to the independent evolutionary radiations of gymnotiforms and mormyroids. I also recently led a project showing that electric organ discharges (EODs) have evolved much faster than ecological traits in the Paramormyrops species flock of mormyroid electric fishes radiating in west-Central Africa (see the figure, below). Based on the prior work of myself and others, we know that EODs function as courtship signals underlying mate recognition, and that EODs exhibit dramatic sexual dimorphism in many mormyroids, including Paramormyrops. Together, these findings implicate sexual selection as a potentially important driver of mormyroid speciation (within specific lineages). Given certain features of the electrical communication modality, our findings also suggest that 'opportunity' in the communication landscape can augment rates of phenotypic divergence and species radiation by sexual selection, analogous to the well established role of 'ecological opportunity' in adaptive radiation. Read more about this research in Arnegard et al., 2010 (Am. Nat. 176: 335-356). Co-authors: Pete McIntyre, Luke Harmon, Miriam Zelditch, Will Crampton, Justin Davis, John Sullivan, Sébastien Lavoué, and Carl Hopkins.
More recently, Bruce Carlson, several of his lab members, Luke Harmon, and I investigated the evolutionary consequences of neural novelties on the sensory and motor sides of electrical communication in mormyroid fishes (Carlson et al., 2011. Science 332: 583-586). We found that the origin of developmental flexibility in one aspect of electrocyte morphology (the stalk system) resulted in an enhanced capacity for evolutionary divergence in EOD waveforms on the motor (or sender) side of electrical communication. On the receiver side, change in a sensory region of the brain established a newfound ability to detect subtle variation in electric signals for the purpose of species and sex recognition. In combination, these novel enhancements of electrical communication triggered dramatic increases in both the rate of EOD divergence and the rate of species diversification in the largest lineage of mormyroid fishes, which we now call 'clade A'. As Bruce and I go on to discuss (Carlson & Arnegard, 2011. Communicative & Integrative Biology 4: 720-725), neural innovations and resulting opportunities in the communication landscape may have played a general role in augmenting species radiation in a variety of animal groups. Examples might be found among orthopteran insects, anuran amphibians, songbirds, and bats, in addition to other animal groups.
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Peer-Reviewed Publications*The first two authors contributed equally to these studies.
Video of Electric Fish Behavior — filmed under natural conditions in Africa.
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Page Last Updated: 21 May 2013
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