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The actin gene family of the marine molluscPatella vulgata was chosen as a model system to study the regulation of genes expressed during early development in molluscs. Using a hamster actin cDNA clone as a probe, we isolated nine actin cDNA clones from trochophore larvae. The total nucleic acid sequence of three of these clones has been determined. Each clone contains the whole protein encoding region. The deduced amino acid sequences resemble actin proteins from other species to a high extent. The nucleotide sequence from the 3'UTR (UnTranslated Region) and 5'UTR from all nine clones has been resolved. In this way we could identify four different subtypes. Southern blots with genomic DNA were probed with different 3'UTR's corresponding to each subtype to determine the genomic organization. One 3'UTR detected one band probably corresponding with one gene. Another 3'UTR detected one or two genes and the third 3'UTR between two and four genes. Northern blots were used to detect the presence of actin mRNA during different stages of development. In the mature oocyte, actin mRNA is present in low amounts. The level of actin mRNA starts to rise steadily from 8 h after fertilization (88-cell stage) onwards. The level of the different subtype mRNAs, as specified by their 3'UTR rises at different developmental stages and to various extents. This indicates that the expression of each type is regulated independently and in relation to the developmental stage of the embryo.

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