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First published online July 2, 2004
Journal of Experimental Biology 207, 2755-2767 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01094

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Fiber polymorphism in skeletal muscles of the American lobster, Homarus americanus: continuum between slow-twitch (S₁) and slow-tonic (S₂) fibers

Scott Medler^*, Travis Lilley and Donald L. Mykles

Department of Biology, Colorado State University, Fort Collins, CO 80523, USA

^* Author for correspondence (e-mail: smedler{at}lamar.colostate.edu)

Accepted 18 May 2004

In recent years, an increasing number of studies has reported the existence of single fibers expressing more than one myosin heavy chain (MHC) isoform at the level of fiber proteins and/or mRNA. These mixed phenotype fibers, often termed hybrid fibers, are currently being recognized as the predominant fiber type in many muscles, and the implications of these findings are currently a topic of great interest. In a recent study, we reported single fibers from the cutter claw closer muscle of lobsters that demonstrated a gradation between the slow-twitch (S₁) and slow-tonic (S₂) muscle phenotype. In the present study, we focused on S₁ and S₂ fibers from the superficial abdominal muscles of the lobster as a model to study the continuum among muscle fiber types. Complementary DNAs (cDNA) encoding an S₂ isoform of myosin heavy chain (MHC) and an S₂ isoform of tropomyosin (Tm) were isolated from the superficial abdominal flexor muscles of adult lobsters. These identified sequences were used to design PCR primers used in conjunction with RT-PCR and real-time PCR to measure expression levels of these genes in small muscle samples and single fibers. The relative expression of the corresponding S₁ MHC and S₁ Tm isoforms was measured in the same samples with PCR primers designed according to previously identified sequences. In addition, we measured the relative proportions of MHC, troponin (Tn) T and I protein isoforms present in the same samples to examine the correlation of these proteins with one another and with the MHC and Tm mRNAs. These analyses revealed significant correlations among the different myofibrillar proteins, with the S₁ and S₂ fibers being characterized by a whole assemblage of myofibrillar isoforms. However, they also showed that small muscle samples, and more importantly single fibers, existed as a continuum from one phenotype to another. Most fibers possessed mixtures of mRNA for MHC isoforms that were unexpected based on protein analysis. These findings illustrate that muscle fibers in general may possess a phenotype that is intermediate between the extremes of `pure' fiber types, not only at the MHC level but also in terms of whole myofibrillar assemblages. This study supports and extends our recent observations of mixed phenotype fibers in lobster claw and leg muscles. The existence of single fiber polymorphism in an invertebrate species underscores the generality of the phenomenon in skeletal muscles and emphasizes the need for an understanding of the proximal causes and physiological consequences of these intermediate fiber types.

Key words: skeletal muscle, myosin heavy chain, actin, isoform, lobster, Crustacea, Arthropoda, Homarus americanus

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