SMALL-DIAMETER WHITE MYOTOMAL MUSCLE-FIBERS ASSOCIATED WITH GROWTH HYPERPLASIA IN THE CARP (CYPRINUS-CARPIO) EXPRESS A DISTINCT MYOSIN HEAVY-CHAIN GENE

A carp myosin heavy chain gene isoform was isolated from a genomic don e, restriction mapped and partially sequenced to reveal the location o f various exons. The clone contains a complete gene of approximately 1 2.0 kb which is half the size of the corresponding mammalian and avian myosin heavy chain genes, The mRNA transcript of this gene, however, is the same size as mammalian and avian striated muscle myosin heavy c hain genes (about 6000 nucleotides), illustrating that the difference in size at the genomic level is due to shorter introns. A 169bp NsiI r estriction fragment containing only the 3' untranslated region of this gene was subcloned and used as an isoform-specific probe to study the expression of this particular isoform, Hybridisation analysis could o nly detect expression of this myosin heavy chain gene in the white mus cle of adult carp that had been subjected to an increased environmenta l temperature. No expression of this gene was detected in carp under 1 year of age. In situ hybridisation demonstrated that expression of th is gene is limited to small-diameter white muscle fibres of adult carp , which are thought to be responsible for muscle growth by fibre hyper plasia.