F-Actin is a marker of dorsal induction in earlyPatella embryos

Localization and quantification of cyclin A and B mRNA during the embryonic development of Patella vulgata

As the first five cleavages of the Patella vulgata embryo are synchronous, they are well suited to determine the mRNA level of cyclin A and B genes in an embryo. During the third and fourth cleavage cycle the quantity of A and B mRNA is regulated in a cell-cycle-dependent way, reaching a high level between cleavages and a lower level just after mitosis. This implies that transcription of the cyclin genes occurs before the overall transcription increases directly after the fifth cleavage. During the first cleavages cyclin A and B mRNA is localized in distinct parts of the cytoplasm. Between two successive cell devisions it is found as a crescent-shaped domain at the peripheral side of the nucleus. At cytokinesis it is present between two separating nuclei and at newly formed cell membranes. At the fifth cleavage this localization disappears. Changes in the expression pattern of cyclin A and B may be expected after the fifth cleavage, when the first cells become arrested in cell division and differentiate. The mechanism causing cell division arrest of these primary trochoblasts is still unknown. Cell division arrest caused by the absence of cyclin A and/or B mRNA could be conditional for further differentiation. However, a decrease in cyclin A and B mRNA level in the trochoblasts is not detectable until 4 h after their last division. Later in development no cyclin A and B mRNA can be detected in these cells, whereas cyclin A and B mRNA is present in other cells of the embryo. Thus, the absence of cyclin A and B mRNA in primary trochoblasts, and in the later differentiating secondary and accessory trochoblasts is not obligatory for cell division arrest or cell differentiation.

Actin genes expressed during early development ofPatella vulgata

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.