MEDLER, S; MULDER, EP; CHANG, ES; MYKLES, DL: Muscle fiber-type switching in the developing lobster claw: timing during the intermolt cycle

During the larval and early juvenile stages of the American lobster, Homarus americanus, the claw closer muscles are identical, each being composed of a central core of fast muscle fibers surrounded by slow fibers. During subsequent stages, one claw undergoes a transformation into a cutter claw composed mostly of fast fibers and the other becomes a slow-fibered crusher claw. These transformations are essentially complete by the 13th stage. Unlike some transformations in crustacean muscles that involve degeneration of certain muscle fibers, the process in lobster claws involves an extensive remodeling of intact, functional fibers. In this study, our goal was to define the stage of the intermolt cycle when switching occurs. Claws were sampled starting at 1 day post-molt and at subsequent time points throughout the cycle. Fiber-specific gene expression was studied in 7th to 8th stage juveniles using RNA probes specific for fast and slow myosin heavy chain (MHC) transcripts in conjunction with in situ hybridization. In addition, expression of a fast fiber protein (P75) was determined using immunohistochemistry. We identified switching fibers in several claws, with some of the more compelling examples occuring 1-7 days postmolt. Some of these claws exhibited complex patterns of overlap in the expression of fast and slow MHC genes. However, we were unable to determine a precise time during which switching occurs. It appears that fiber transformation occurs largely, but not exclusively, during postmolt when protein synthesis is elevated. Supported by NSF (IBN-0077422) and NIH (AR 08597-01A1).