Muscle fiber size, myonuclear domain, and fat mass phenotypes in pre-migratory snow buntings

In long-distance migrants, preparation for migration is typically
associated with increases in fat and body mass, and with an enlargement of
pectoralis muscle mass that likely improves flight performance. Although
changes in muscle mass or size have been well described in migratory
birds, potential changes in muscle ultrastructure during this transition
still deserves scrutiny. Using outdoor captive snow buntings
(Plectrophenax nivalis n = 15) measured during their transition into a
spring migratory phenotype as a model system, we studied changes in
pectoralis muscle ultrastructure and predicted that muscle fiber diameter
could increase in parallel with the gain in body mass. We also expected
that larger fibers could either recruit satellite cells to
support cellular maintenance and protein turnover, increase myonuclear
domain (cytoplasm per nuclei) with the potential increase for protein
turnover load per myonucleus, or existing myonuclei could undergo
endoreduplication. Buntings increased body mass by 46% within a month,
largely due to a > 6-fold increase in body fat. However, this
increase in body mass was also associated with a 36% increase in muscle
fiber diameter. Both pectoralis muscle mass (r2 = 0.57-0.77) and fiber
diameter (r2 = 0.32) correlated with total body mass, without any change
in the number of nuclei per fiber. Consequently, variation in myonuclear
domain (i.e. the amount of cytoplasm per nucleus), was also positively
associated with body mass (r2 = 0.51). Therefore, buntings preparing for
migration may experience an increase in muscle contraction force due to
larger muscle fibers, but this is also coupled with increases in
myonuclear domain, which may force these cells to increase protein
production to safeguard satellite cells.