Spicule matrix proteins of sea urchin embryo are the specific products of the micromere / primary mesenchyme cell (PMC) lineage, and are considered to be involved in spicule formation (Wilt, 1999). One of these proteins, SM50, has been described for three species: Strongylocentrotus purpuratus (SP), Lytechinus pictus (Lp) and Hemicentrotus pulcherrimus (Hp) (for references see Wilt, 1999). The nucleotide and amino acid sequences are well conserved in these species. SM50 proteins of these species have repetitive amino acid sequences in the carboxyl-terminal half of the proteins. Therefore, examination of SM50 sequences, especially the repetitive sequence region, in various species will help an understanding of the process of sea urchin ontogeny and evolution. In this study we tried to amplify, by PCR, the SM50 sequences of species for which no sequence data are reported.

Total DNA was extracted from the sperm of sea urchins by standard procedures. The purified DNA was subjected to PCR to amplify the repetitive amino acid region and its upstream region. The primers were designed based on the highly conserved sequences in the reported SM50 as Consensus-Degenerate Hybrid Oligonucleotide Primers (Rose et al., 1997). The amplified products were gel-purified, and sequenced using ABI PRISM 310 Genetic Analyzer using PCR primers. The determined nucleotide sequences were translated into amino acid sequences and compared among species with a phylogenetic tree constructed by the neighbour-joining method. For indirect immunofluorescent staining, embryos were fixed with 70% methanol and reacted with rabbit antiserum against recombinant SM50 protein.