Relationship Between Gene Expression Networks and Muscle Contractile Physiology Differences in Anolis Lizards.

Introduction: Muscles facilitate most animal behavior, from eating to fleeing. However, to generate the variation in behavior necessary for survival, different muscles must perform differently. For instance, sprinting requires multiple rapid muscle contractions, whereas biting may require fewer contractions but greater force.

Methods: Here, a transcriptomic approach was utilized to identify genes associated with variation in muscle contractile physiology among different muscles from the same individual. Differential gene expression was measured between a leg and jaw muscle of Anolis lizards (A. chlorocyanus, A. cybotes, A. osa, A. sagrei) known to differ in muscle contractile physiology and performance. For each individual, one muscle was used to measure muscle contractile physiology, including Vmax (the muscle contraction velocity at zero force), V40 (the muscle contraction velocity at 40% force), power ratio (a measure of the trade-off between force and velocity), and twitch time (the amount of time it takes to create and dissipate tension at 50% force). The contralateral muscle was used to extract RNA for transcriptomic sequencing. Weighted Gene Co-Expression Analysis (WGCNA) was performed to cluster differentially expressed genes into groups, or modules with color designations, based on an eigengene. Modules were correlated to physiologic metrics and analyzed for function with gene ontology (GO term) analysis.

Results: Using the transcriptomic data, clear clustering of muscle type was found indicating there were no specific differences among the four species. Several contractile metrics were significantly different between the jaw and leg: twitch time, V40, and power ratio. Expression of genes clustered in GO terms related to muscle contraction and extracellular matrix was negatively correlated with slower twitch times but positively correlated to power ratio and V40. Conversely, genes related to the GO terms related to aerobic respiration were downregulated in muscles with higher power ratio and V40 and over-expressed with slower twitch times.

Conclusions: Determining the molecular mechanisms that underlie variation in muscle contractile physiology can begin to explain how organisms are able to optimize behavior under variable conditions. Key areas of difference between gene expression of the jaw and leg included muscle contraction, energy synthesis, and extracellular structures. Modules relating to aerobic respiration are strongly correlated with slower twitch time likely due to slower utilization of ATP. Modules relating to muscle contraction and extracellular structure are negatively correlated with slower twitch time and positively correlated with V40 and power ratio indicating the increased need for structural components to increase and transmit force for greater power.