MYKLES, D.L.*; MULDER, E.P.; MEDLER, S.A.; CHANG, E.S.: Developmentally-regulated fiber-type switching in claw muscles of American lobster, Homarus americanus
Fibers in the claw closer muscles undergo a developmentally-regulated switching as the isomorphic claws of larvae and juveniles differentiate into the heteromorphic cutter and crusher claws of the adult. Fast-to-slow switching occurs in the presumptive crusher claw and slow-to-fast switching occurs in the presumptive cutter claw. This fiber switching occurs at the boundary between the central fast-fiber and the peripheral slow-fiber regions, and thus the switching of a specific fiber is determined by its position within the muscle. We have cloned cDNAs encoding fast-type isoforms of myosin heavy chain (fMHC), P75 regulatory protein (P75), and tropomyosin (fTm) and slow-type isoforms (sMHC and sTm1) to use as molecular markers for switching. The expression of myofibrillar protein isoforms during the intermolt cycle was determined using RNA blot analysis and in situ hybridization. Switching fibers were identified as those fibers expressing both fast and slow transcripts. Switching occurred in the postmolt period, during which accelerated protein turnover facilitates incorporation of newly-synthesized protein into the contractile apparatus. Supported by NSF (IBN 00-77422).