THE MUSCLEBLIND GENE PARTICIPATES IN THE ORGANIZATION OF Z-BANDS AND EPIDERMAL ATTACHMENTS OF DROSOPHILA MUSCLES AND IS REGULATED BY DMEF2

We report the embryonic phenotype of muscleblind (mbl), a recently des cribed Drosophila gene involved in terminal differentiation of adult o mmatidia. mbl is a nuclear protein expressed late in the embryo in pha ryngeal, visceral, and somatic muscles, the ventral nerve cord, and th e larval photoreceptor system. All three mbl alleles studied exhibit a lethal phenotype and die as stage 17 embryos or first instar larvae. These larvae are partially paralyzed, show a characteristically contra cted abdomen, and lack striation of muscles. Our analysis of the somat ic musculature shows that the pattern of muscles is established correc tly, and they form morphologically normal synapses. Ultrastructural an alysis, however, reveals two defects in the terminal differentiation o f the muscles: inability to differentiate Z-bands in the sarcomeric ap paratus and reduction of extracellular tendon matrix at attachment sit es to the epidermis. Failure to differentiate both structures could ex plain the partial paralysis and contracted abdomen phenotype. Analysis of mbl expression in embryos that are either mutant for Dmef2 or ecto pically express Dmef2 places mbl downstream of Dmef2 function in the m yogenic differentiation program. mbl, therefore, may act as a critical element in the execution of two Dmef2-dependent processes in the term inal differentiation of muscles. (C) 1998 Academic Press.