MUSCLE-SPECIFIC REGULATION OF TROPOMYOSIN GENE-EXPRESSION AND MYOFIBRILLOGENESIS DIFFERS AMONG MUSCLE SYSTEMS EXAMINED AT METAMORPHOSIS OF THE GASTROPOD HALIOTIS-RUFESCENS

The spatial and temporal association of muscle-specific tropomyosin ge ne expression, and myofibril assembly and degradation during metamorph osis is analyzed in the gastropod mollusc. Haliotis rufescens. Metamor phosis of tile planktonic larva to the benthic juvenile includes rearr angement and atrophy of specific larval muscles, and biogenesis of the new juvenile muscle system. The major muscle of the larva - the larva l retractor muscle - reorganizes at metamorphosis, with two suites of cells having different fates. The ventral cells degenerate, while the dorsal cells become part of the developing juvenile mantle musculature . Prior to these changes in myofibrillar structure, tropomyosin mRNA p revalence declines until undetectable in the ventral cells, while incr easing markedly in the dorsal cells. In the foot muscle and right shel l muscle, tropomyosin mRNA levels remain relatively stable, even troug h myofibril content increases. In a population of median mesoderm cell s destined to form de novo the major muscle of the juvenile and adult (the columellar muscle), tropomyosin expression is initiated at 45 h a fter induction of metamorphosis. Myofibrillar filamentous actin is not detected in these cells until about 7 days later. Given that patterns of tropomyosin mRNA accumulation in relation to myofibril assembly an d disassembly differ significantly among the four major muscle systems examined, we suggest that different regulatory mechanisms, probably o perating at both transcriptional and post-transcriptional levels, cont rol the biogenesis and atrophy of different larval and postlarval musc les at metamorphosis.