Meeting Abstract

44.6  Monday, Jan. 5  Evolution of fiber type composition in a lizard locomotor muscle SCALES, J A*; KING, A A; BUTLER, M A; University of Hawaii, Manoa; University of Michigan; University of Hawaii, Manoa jscales@hawaii.edu

Locomotion has often served as a model system for adaptive evolution. Many studies have examined how morphology, especially limb proportions, is shaped by different locomotor demands. However, few studies have examined muscle properties such as fiber-type composition, although its importance has always been assumed. Here we explicitly test whether fiber-type composition of a locomotor muscle (the iliofibularis) is adaptive for the behavior of lizards by testing various evolutionary models based on the predator escape and foraging strategies of lizards. An adaptive model based on predator escape strategies provided the best explanation for the evolution of fast-twitch fiber types. Lizards that depend on sprints to avoid predators should have high relative proportions of fast glycolytic fibers, while cryptic lizards should have high relative proportions of fast oxidative glycolytic fibers. This pattern suggests a trend in evolution toward muscles composed largely of one fast-twitch fiber-type associated with behavioral specialization. The best-fitting models for slow-twitch fiber type composition were a single global optimum suggesting a general selective pressure across these lizard species, or a Brownian motion model, suggesting some support for neutral evolution. These data provide evidence that different fiber-types within the same muscle may evolve under different evolutionary pressures.