Ontogeny of myosin isoform expression and prehensile function in the tail of the gray short-tailed opossum (Monodelphis domestica)

Terrestrial opossums use their semi-prehensile tail for grasping nesting materials as opposed to arboreal maneuvering. We relate the development of this adaptive behavior with ontogenetic changes in myosin heavy chain (MHC) isoform expression from 21 days to adulthood. Monodelphis domestica is expected to demonstrate a progressive ability to flex the distal tail up to age 7 months, when it should exhibit routine nest construction. We hypothesize that juvenile stages (3-7 months) will be characterized by retention of the neonatal isoform (MHC-Neo), along with predominant expression of fast MHC-2X and 2B, which will transition into greater MHC-1β and 2A isoform content as development progresses. This hypothesis was tested using Q-PCR to quantify and compare gene expression of each isoform to its protein content determined by gel electrophoresis and densitometry. These data were correlated with nesting activity in an age-matched sample of each age group studied. Shifts in regulation of MHC gene transcripts matched well with isoform expression. Notably, mRNA for MHC-Neo and 2B decrease, resulting in little-to-no isoform translation after age 7 months, whereas mRNA for MHC-1β and 2A increase, and this corresponds with subtle increases in content for these isoforms into late adulthood. Despite the tail remaining intrinsically fast-contracting, a critical growth period for isoform transition is observed between 7 and 13 months, correlating primarily with use of the tail during nesting activities. Functional transitions in MHC isoforms and fiber type properties may be associated with muscle 'tuning' repetitive nest remodeling tasks requiring sustained contractions of the caudal flexors.

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