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

Abstract

Terrestrial opossums use their semi-prehensile tail for grasping nesting materials as opposed to locomotor maneuvering. The objective of this study is to relate the development of this adaptive behavior with ontogenetic changes in myosin heavy chain (MHC) gene regulation and isoform expression in the tail from 3 weeks to adulthood. Monodelphis domestica is expected to demonstrate a progressive ability to flex the tail up to age 7 months, when it will have full use of its tail and should exhibit routine nest construction. We hypothesize that juvenile stages (3-7 months) will be characterized by retention of the fast neonatal isoform (MHC-Neo), along with large expression of fast MHC-2X and 2B, which will transition into slow MHC-1 and fast 2A isoform fibers as tail development progresses. This hypothesis was tested using qPCR to quantify and compare gene expression of each isoform to its protein content found by gel electrophoresis and densitometry. These data were paired with video observations of 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, largely resulting in no isoform translation after age 7 months, whereas mRNA for MHC-1 and 2A increase and this corresponds with increasing protein content for these isoforms into late adulthood. A critical growth period for isoform transition is observed between 7 and 13 months, and this correlates with increased use of the tail for nest construction as adults. Ontogenetic shifts from faster- to slower-contracting isoforms may be associated with muscle 'tuning' for the repetitive nest remodeling tasks requiring sustained low force contractions of the caudal flexors.