Photoperiodic induction of pre-migratory phenotype in a migratory songbird: identification of metabolic proteins in flight muscles
Migratory birds need to undergo physiological changes during their preparation for migration. The current study characterized those changes in photoperiodic migratory black-headed buntings (Emberiza melanocephala), which initiate their northward spring migration in response to increasing day lengths. We measured differences in body mass, testis size and triglycerides levels in buntings between groups exposed to short (8 h light:16 h darkness, 8L:16D; SD) and long (16L:8D; LD) days, and identified proteins that showed significant differences between SD and LD in the flight muscle. To confirm that photostimulated changes were linked with migration, similar measurements were done on photoperiodic non-migratory Indian weaverbirds (Ploceus philippinus), which share the habitat with buntings for almost half-a-year. Buntings were fattened and gained weight and had elevated serum triglyceride levels and recrudesced testes under LD, but not SD. The SDS-polyacrylamide gel electrophoresis revealed differences between SD and LD conditions in the flight muscle protein profiles of buntings, but not of weaverbirds. Two-dimensional gel electrophoresis of flight muscle of bunting separated three proteins, of which two were upregulated under LD condition. Mass spectroscopic analysis and a protein database search identified them as the fatty acid binding protein (FABP), myoglobin and creatine kinase (CK). Further semi-quantitative and quantitative PCR assays revealed that FABP and myoglobin transcript levels in buntings, but not in weaverbirds, were upregulated under LD condition. However, there was no difference in CK mRNA levels between SD and LD in both the species. High FABP is perhaps linked with increased energy demands and high myoglobin with intense physical activity during migration. A difference in the CK protein, but not in mRNA levels between SD and LD may possibly indicate its photoperiodic regulation at the translational level.