MECHANISM OF AN EVOLUTIONARY CHANGE IN MUSCLE-CELL DIFFERENTIATION INASCIDIANS WITH DIFFERENT MODES OF DEVELOPMENT

We have investigated the mechanism of an evolutionary change in ascidi an muscle cell differentiation. The ascidians Molgula oculata and Molg ula occulta are closely related species with different modes of develo pment. M. oculata embryos develop into conventional tadpole larvae wit h a tail containing striated muscle cells, whereas M. occulta embryos develop into tailless larvae with undifferentiated vestigial muscle ce lls. The muscle actin gene MocuMA1 was isolated from an M. oculata gen omic library. MocuMA1 is a single-copy, larval-type muscle actin gene which appears to lack introns. However, the 5' upstream region of Mocu MA1 is sufficient to drive expression of a lacZ fusion construct in th e larval muscle cells, implying that it is a functional gene. MocuMA1 mRNA first appears in the prospective muscle cells of M. oculata embry os during gastrulation, and transcripts continue to be present through out embryogenesis. Muscle actin mRNA was not detected during M. occult a embryogenesis, although the same probe was capable of detecting musc le actin mRNA in more distantly related ascidian species with tail mus cle. cells. Interspecific hybrids produced by fertilizing M. occulta e ggs with M. oculata sperm recover the ability to express muscle actin mRNA in the vestigial muscle cells, suggesting that trans-acting facto rs responsible for muscle actin gene expression are conserved in M. oc culta. The presence of these trans-acting factors was confirmed by sho wing that the MocuMA1/lacZ fusion construct is expressed in the vestig ial muscle cells of M. occulta larvae. The orthologous larval muscle a ctin genes MocuMA1a and MocuMA1b were isolated from a M. occulta genom ic Library. The coding regions of these genes contain deletions, inser tions, and codon substitutions that would make their products nonfunct ional. Although the 5' upstream regions of the M. occulta muscle actin genes also contain numerous changes, expression of MocuMA1a/lacZ and MocuMA1b/lacZ fusion constructs showed that they both retain specific promoter activity, although it is reduced in MocuMA1b. The results sug gest that the regression of muscle cell differentiation is mediated by changes in the structure of muscle actin genes rather than in the tra ns-acting regulatory factors required for their expression. (C) 1996 A cademic Press, Inc.