Abstract
Circulating androgens in adult reproductively active male vertebrates influence a diversity of organ systems and thus are considered costly. Recently, we obtained evidence that androgen receptors (AR) are expressed in several skeletal muscles of three passeriform birds, the golden-collared manakin (Manacus vitellinus), zebra finch (Taenopygia guttata), and ochre-bellied flycatcher (Mionectes oleagieus). Because skeletal muscles that control wing movement make up the bulk of a bird’s body mass, evidence for widespread effects of androgen action on these muscles would greatly expand the functional impact of androgens beyond their well-characterized effects on relatively discrete targets throughout the avian body. To investigate this issue, we use quantitative PCR (qPCR) to determine if androgens alter gene mRNA expression patterns in wing musculature of wild golden-collared manakins and captive zebra finches. In manakins, the androgen testosterone (T) up-regulated expression of parvalbumin (PV) and insulin-like growth factor I (IGF-I), two genes whose products enhance cellular Ca2+ cycling and hypertrophy of skeletal muscle fibers. In T-treated zebra finches, the anti-androgen flutamide blunted PV and IGF-I expression. These results suggest that certain transcriptional effects of androgen action via AR are conserved in passerine skeletal muscle tissue. When we examined wing muscles of manakins, zebra finches and ochre-bellied flycatchers, we found that expression of PV and IGF-I varied across species and in a manner consistent with a function for AR-dependent gene regulation. Together, these findings imply that androgens have the potential to act on avian muscle in a way that may enhance the physicality required for successful reproduction.
Highlights
Androgenic hormones act on intracellular androgen receptors (AR) to influence local patterns of cellular gene expression
Males used in this study were captured from late July to early September, as we have previously shown that T levels during this time of the year in males are virtually nondetectable [38]
PCR Amplification of Gene Products We used PCR to confirm the presence of PV, insulin-like growth factor I (IGF-I), myogenic differentiation factor D (MyoD) and MSTN mRNA in skeletal muscle tissues of male golden-collared manakins, ochre-bellied flycatchers, and zebra finches
Summary
Androgenic hormones act on intracellular androgen receptors (AR) to influence local patterns of cellular gene expression. A recent study in goldencollared manakins (Manacus vitellinus), zebra finches (Taenopygia guttata), and ochre-bellied flycatchers (Mionectes oleagieus) showed that AR are expressed in the main muscles that lift [supracoracoideus (SC) and scapulohumeralis caudalis (SH)] and retract [pectoralis (PEC)] the wings during flight [20]. This group of muscles make up nearly 40% of a bird’s body weight [21,22], meaning that an enormous proportion of a bird’s mass is sensitive to androgen action. No studies have shown that androgens have functional (i.e. transcriptional) effects directly on avian skeletal muscle tissues that likely modulate muscle performance or morphology
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