xi, 103 p. : ill. A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / Although outcrossing is the most widespread mating system among animals and plants, the reason for this prevalence is not fully understood. Evolutionary theory has classified the potential selective pressures driving the evolution and maintenance of outcrossing into two broad categories: deleterious mutations and changing ecological conditions. Despite the inherent advantages of self-fertilization, exposure to either or both of these selective pressures is predicted to favor outcrossing over self-fertilization. I tested these predictions using experimental evolution in populations of Caenorhabditis elegans with genetically modified rates of outcrossing and selfing. I found that outcrossing reduces the fixation of deleterious mutations under mutation influx and that outcrossing expedites adaptation to a bacterial pathogen. Further, I identified facultative outcrossing, a novel life history characteristic, in specific C. elegans strains that predominantly reproduce by selfing but engage in outcrossing when stressed. The shift from a primarily selfing mating system to a predominantly outcrossing system is similar to the environmentally induced facultative sex observed in asexual species, which is thought to enable more rapid adaptation. Facultative outcrossing, although not previously documented, may play a major role in the life histories of many highly selfing species. Finally, most mutations are deleterious and therefore elevated mutation rates are generally thought to produce progressively larger reductions in fitness. Using the chemical mutagen ethylmethanesulfonate, I found the surprising result that populations exposed to a mutation rate at least fifty times greater than natural rates exhibited significantly greater fitness than populations exposed to substantially lower mutation rates. This unexpected fitness optimum may be the result of a volatile balance between the influx of deleterious mutations and compensatory mutations. This work confirms the predictions of several long-standing evolutionary theories by identifying both deleterious mutations and changing ecological conditions as selective pressures capable of driving the evolution and maintenance of outcrossing. These selective pressures, which are ubiquitous in nature, may explain the prevalence of outcrossing relative to selling. This dissertation includes previously published and co-authored materials. / Committee in charge: Barbara Roy, Chairperson, Biology; Patrick Phillips, Advisor, Biology; Karen Guillemin, Member, Biology; William Bradshaw, Member, Biology; Douglas Kennett, Outside Member, Anthropology

xv, 83 p. : ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / This dissertation describes research to identify genes involved in specification, patterning and development of zebrafish primary motoneurons. We first examined the spatiotemporal expression patterns of retinoic acid and retinoid X receptor mRNAs to determine whether particular ones might be involved in motoneuron specification or patterning. Retinoic acid and retinoid X receptor mRNAs are expressed at the right time to pattern motoneurons, but the expression patterns did not suggest roles for particular receptors. In contrast, netrin mRNAs are expressed in specific motoneuron intermediate targets and knockdown experiments revealed an important role in development of VaP motoneurons. Two identified motoneurons, CaP and VaP, initially form an equivalence pair. CaPs extend long axons that innervate ventral muscle. VaPs extend short axons that stop at muscle fibers called muscle pioneers; VaPs later typically die. Previous work showed that during extension, CaP axons pause at several intermediate targets, including muscle pioneers, and that both CaP and muscle pioneers are required for VaP formation. We found that mRNAs for different Netrins are expressed in intermediate targets before CaP axon contact: netrin 1a in muscle pioneers, netrin 1b in hypochord, and netrin 2 in ventral somite. We show that Netrins are unnecessary to guide CaP axons but are necessary to prevent VaP axons from extending into ventral muscle. Netrin 1a is necessary to stop VaP axons at muscle pioneers, Netrin 1a and Netrin 2 together are necessary to stop VaP axons near the hypochord, and Netrin 1b appears dispensable for CaP and VaP development. We also identify Deleted in colorectal carcinoma as a Netrin receptor that mediates the ability of Netrin 1a to cause VaP axons to stop at muscle pioneers. Our results suggest Netrins refine axon morphology to ensure final cell-appropriate axon arborization. To learn whether Netrin proteins diffuse away from their sources of synthesis to function at a distance, we are developing Netrin antibodies. If successful, the antibodies will provide the research community at large with a new tool for understanding in vivo Netrin function. This dissertation includes both my previously published and unpublished coauthored material. / Committee in charge: Monte Westerfield, Chairperson, Biology Judith Eisen, Advisor, Biology; Victoria Herman, Member, Biology; John Postlethwait, Member, Biology; Clifford Kentros, Outside Member, Psychology

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Studies have shown that facial sexual dimorphism, ranging on a continuum from very feminine to very masculine, affects observers’ ratings of dominance and threat. These studies, however, have used forced-choice paradigms, in which a pair of faces that have been manipulated to appear more masculine (masculinized) and feminine (feminized) are presented side by side. They are susceptible to demand characteristics, situational aspects of the experiment which produce the desired outcome, because participants may be able to draw the conclusion that faces which appear more masculine should be rated as more dominant. In this dissertation, I evaluated whether facial sexual dimorphism affects observers' threat perceptions in a way that minimizes the confounds caused by demand characteristics. In Experiment 1 and Experiment 2, I presented observers with masculinized and feminized faces individually, rather than in pairs, and for an extremely brief duration (Experiment 1). I predicted that observers would assign higher dominance (Experiment 1) and threat ratings (Experiment 2) to masculinized faces. Observers assigned higher dominance (Experiment 1) and threat (Experiment 2) ratings to masculinized faces, even when they were presented individually, and for as little as 100 milliseconds (ms) (Experiment 1). This would suggest that they can appraise differences in facial sexual dimorphism following very brief exposure. I proceeded to evaluate the effects of facial sexual dimorphism on men’s selective attention, while reducing the effects of demand characteristics, by testing for an attentional bias towards task-irrelevant masculinized men’s faces. In Experiment 3, forty-five men completed a Posner Cueing Paradigm in which they classified shapes, presented either right or left of center screen after a masculinized or feminized man’s face was presented in either the same or opposite location. Participants were faster to classify the shape following the presentation of a masculine face; however, they were not faster when a masculine face cued target position. In Experiment 4, forty-four men completed a Flanker Task in which they judged letter orientation (i.e., upright or upside-down), while ignoring flanking faces. Participants’ RT was not affected by Morph Type (i.e., whether the face was masculinized, feminized or unmodified). In Experiment 5, forty-one men completed a Dot Probe Task. They were presented with two facial photographs of different Morph Types to the left and right of center screen. This was followed by the presentation of a target shape, in the location of one of the faces. Participants’ objective was to classify shape orientation. Facial sexual dimorphism did not affect participants' classification speed. In Experiment 6, I primed participants with images meant to induce fear or arousal before each trial of a Dot Probe Task. Following the presentation of a fear inducing picture, participants RT to classify shapes when a masculinized face cued target position did not differ from when a feminized face cued target position. The two different presentation times did not create different patterns of results, indicating that masculinized faces did not induce either a cueing or inhibitory affect. Overall, my results do not support my hypothesis that men selectively attend to masculinized faces when they are presented as irrelevant information. I conclude by discussing future directions for evaluating the effects that facial sexual dimorphism has on observers’ interpersonal perceptions of threat and dominance, while controlling for the effects of demand characteristics.

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