Microevolution of the mammalian limb skeleton: from genes and development to morphology and performance

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Background: Mammals show a remarkable diversity of limb proportions, which evolved as adaptations for specific types of locomotion (e.g., sprinting, hopping). The long-term vision of my research program is to uncover the developmental, functional and evolutionary mechanisms that make this skeletal diversity possible. To do so, I developed the Longshanks mouse, which has 15-20% longer shin bones (tibiae), but the same body mass as random-bred control mice.*** Research Objectives: Over the next five years, I will address three complementary objectives: (1) to define the contributions of specific developmental genetic pathways to the longer limbs and altered limb proportions of Longshanks; (2) to probe the relationship between Longshanks' modified limb morphology and its locomotor performance; and (3) to determine the impact of tibia selection on coevolution of other skeletal and soft tissue traits. *** Scientific Approach: For Objective 1, we will use histology and gene expression tools to: (1) test the hypothesis that the signaling factor Dlk1 regulates the number of proliferating cells in the growth plates during long bone development, and thus plays a major role in increasing bone length in Longshanks, and (2) determine whether different gene pathways are involved in increasing Longshanks' overall skeletal size, vs. the size of each limb bone. For Objective 2, we will use high-speed video and force platforms to test the hypothesis that Longshanks' increased limb bone length improves its running and sprinting performance, e.g., by increasing its maximum sprint speeds and/or endurance. For Objective 3, we will combine micro-CT imaging, and quantitative genetics tools, to determine if and how other parts of Longshanks' body (e.g., cranium, internal organs) have changed in response to tibia selection, and whether this has impacted Longshanks' overall capacity to evolve.*** Significance: My research program integrates data that spans genes to morphology to whole organism performance, to address long-standing questions on the mechanistic basis of evolutionary change. Anticipated outcomes will have broad impacts across evolutionary biology and related fields (e.g., evolutionary developmental biology, ecology, paleontology), through the following breakthroughs: (1) uncovering key cell and molecular drivers of morphological diversity, (2) understanding how species, both living and extinct, adapt to their environments through improved organismal performance, and (3) identifying evolutionary constraints that have shaped morphological diversity in living and fossil species. The proposed studies will also improve public understanding of evolution, by providing clear and relatable examples of evolution in action, and will contribute to the training of Canada's next generation of life scientists, with transferable skills in bioinformatics, movement science and digital imaging. **