p58, a Cytoskeletal Protein, Is Associated With Muscle Cell Determinants in Ascidian Eggs

The theory that p58, a cytoskeletal protein, has an important role in ascidian muscle cell development was tested by altering normal distributions of orange-pigmented myoplasm in Boltenia villosa embryos and determining if muscle development is correlated with the presence of p58. Removal of the animal region of fertilized Boltenia eggs resulted in the redistribution of myoplasm into the anterior endoderm cells of the embryo. Despite alterations in the normal distribution of myoplasm, these embryos developed into larvae. However, when four-celled embryos that exhibited altered distributions of pigmented myoplasm were stained with NN18, an antibody that stains p58, a maximum of two blastomeres were stained, as in control embryos. Compression of Boltenia embryos at the four-celled stage caused the myoplasm to be partitioned into four blastomeres of an eight-celled embryo, instead of into two blastomeres. Compressed and cleavage-arrested eight-celled embryos developed myosin and muscle actin RNA in a maximum of four blastomeres, compared to a maximum of two blastomeres in control embryos. When compressed eight-celled embryos were stained with NN18, p58 was present in a maximum of four blastomeres. These results support the idea that the cytoskeletal protein p58 is associated with muscle cell determinants in ascidian eggs.