Temporal and Spatial Expression of a Muscle Actin Gene during Embryogenesis of the Ascidian Halocynthia roretzi

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Abstract
Screening a cDNA library from tailbud embryos of the ascidian Halocynthia roretzi with a Styela plicata mantle actin probe yielded several muscle-type actin clones. These clones differed from each other in the nucleotide sequences of their 3'non-coding regions, although the sequences of the coding region were almost identical. One of the clones, HrcMA4, was selected and characterized. HrcMA4 contains an open reading frame of 1137 bp and a 100 bp 3'non-coding region followed by a poly(A) tail. An antisense probe consisting of a small segment of 3'coding region and a large portion of 3'non-coding region of the clone was constructed. In situ hybridization analysis with this probe demonstrated that expression of HrMA4 mRNAs was restricted to differentiating muscle cells in tailbud embryos. Signal was not detectable in other regions of the embryo. Northern blot analysis showed that HrMA4 mRNA was undetectable in unfertilized eggs, zygotes and cleavage-stage embryos. A single band of the HrMA4 transcripts about 1.5 kb in length was first observed in gastrulae. The amount of HrMA4 mRNAs increased rapidly as development progressed. The mRNA was evident in tadpole larvae and newly metamorphosed juveniles. The amount of transcripts, however, decreased after metamorphosis and became undetectable by a week after metamorphosis. Thus, the HrMA4 gene showed strict zygotic expression restricted to the muscle lineage cells.