Living organisms are exceedingly complex. Simple, atomistic approaches to studying biological diversity have led to an inadequate understanding of the ways in which complex traits evolve. The general goal of the proposed research is to use a holistic approach to elucidate how a complex behavior (voluntary activity level) changes genetically in response to controlled and replicated cross-generational selective breeding. Two general hypotheses will be tested. First, increases in voluntary activity level will be associated with identifiable changes in brain structure and function that cause increased motivation for high activity. Second, increases in activity will be accompanied by changes in subordinate traits (e.g., muscle contractile properties) that support the behavior by enhancing organismal performance abilities (e.g., locomotor endurance or energetic efficiency). Thus, it is hypothesized that exceptional athletic performance requires both "brains" and "brawn." With previous NSF support, artificial selection for high voluntary wheel running was applied to an outbred population of laboratory mice. At generation 41, mice in the 4 replicate selected (S) lines ran almost three times as much as the 4 randombred control (C) lines (for females, 15.3 vs. 5.3 km/day; for males, 12.2 vs. 4.1). In the proposed research, selective breeding will continue and the S and C lines will be compared to elucidate the morphological, physiological, biochemical, and genetic bases of locomotor adaptations, as well as the neurobiological underpinnings high voluntary activity. General (consistent) responses to selection will be determined by statistical comparisons of all 4 S lines with all 4 C lines. Specific (unique) responses (multiple adaptive solutions) will be identified by comparing the 4 S lines. A key discovery of previous work is the "mighty mini-muscle" allele that has been favored by selection and gone to fixation in one of the S lines. This autosomal recessive causes a 50% reduction in hindlimb muscle mass, while doubling mass-specific aerobic capacity and altering contractile characteristics in ways that are hypothesized to enhance endurance running and the energetic efficiency of locomotion, but compromise sprinting abilities. This research will elucidate the physiological underpinnings of elevated maximal oxygen consumption, the power source for sustained locomotion, in the S lines and in mini-muscle vs. normal individuals. Mice will be exercise-tested on a treadmill and then submitted to biochemical and molecular analyses. The project includes tests of hypotheses that mini-muscles exhibit higher contractile efficiency and that this phenotype leads to reduced whole-animal costs of transport and reduced maximal sprint speeds. The investigators will map the putative mini-muscle locus to a chromosome, and identify possible candidate genes (in conjunction with in situ hybridizations and micro-array data). Finally, the researchers will test the hypotheses that S mice possess higher motivation to run on wheels, exhibit physiological withdrawal symptoms when wheel-deprived, and have altered dopamine physiology in key regions of the brain reward system. The proposed research will have general implications for how to study complex phenomena from an integrative and multidisciplinary perspective. It will identify biological determinants of a genetically defined, voluntary behavior that has major relevance for human health. The selectively bred lines of mice are a unique genetic resource and will be made available to other researchers for several additional projects. These studies will help to develop and promote the selected lines as an important model for studying effects of exercise on physical fitness, body-weight regulation, and aging. Research will emphasize training of undergraduates (including women and under-represented groups) and graduate students (four continuing Ph.D. students will participate), and will involve collaborations with eight other faculty and six other universities.
