White-throated sparrow (Zonotrichia albicollis) liver and pectoralis flight muscle transcriptomic changes in preparation for migration.

Migratory songbirds undertake challenging journeys to reach their breeding grounds each spring. They accomplish these nonstop flapping feats of endurance through a suite of physiological changes, including the development of substantial fat stores and flight muscle hypertrophy and an increased capacity for fat catabolism. In addition, migratory birds may show large reductions in organ masses during flight, including the flight muscle and liver, which they must rapidly rebuild during their migratory stopover before replenishing their fat stores. However, the molecular basis of this capacity for rapid tissue remodeling and energetic output has not been thoroughly investigated. We performed RNA-sequencing analysis of the liver and pectoralis flight muscle of captive white-throated sparrows in experimentally photostimulated migratory and nonmigratory condition to explore the mechanisms of seasonal change to metabolism and tissue mass regulation that may facilitate these migratory journeys. Based on transcriptional changes, we propose that tissue-specific adjustments in preparation for migration may alleviate the damaging effects of long-duration activity, including a potential increase to the inflammatory response in the muscle. Furthermore, we hypothesize that seasonal hypertrophy balances satellite cell recruitment and apoptosis, while little evidence appeared in the transcriptome to support myostatin-, insulin-like growth factor 1-, and mammalian target of rapamycin-mediated pathways for muscle growth. These findings can encourage more targeted molecular studies on the unique integration of pathways that we find in the development of the migratory endurance phenotype in songbirds.NEW & NOTEWORTHY Migratory songbirds undergo significant physiological changes to accomplish their impressive migratory journeys. However, we have a limited understanding of the regulatory mechanisms underlying these changes. Here, we explore the transcriptomic changes to the flight muscle and liver of white-throated sparrows as they develop the migratory condition. We use these patterns to develop hypotheses about metabolic flexibility and tissue restructuring in preparation for migration.