Comparative study of cytoplasmic actin DNA sequences from six species of Planorbidae (Gastropoda: Basommatophora).

Full-length actin-encoding sequences were PCR-amplified from genomic DNA of six planorbid species; Biomphalaria glabrata (Say; M-line strain), B. alexandrina (Ehrenberg), B. pfeifferi (Krauss), B. tenagophila (Orbigny), B. obstructa (Morelet) and Helisoma trivolvis (Say), using primers designed from a previously reported B.glabrata cytoplasmic (beta) actin cDNA. The amplified sequences contained two conserved exons (126 nt and 1005 nt, respectively), separated by an intron that varied in size between snail species (ranging from 671 to 794 nt). Sequence similarities occurred between the introns of the actin genes from B.glabrata, B. alexandrina and B. pfeifferi and between those from B. tenagophila and B. obstructa, vet considerable differences were evident between these two groups and the intron derived from H. trivolvis. Analysis of exons for sequence similarities, the presence of conserved residues (deduced amino acids), and construction of gene trees indicated that these planorbid genes encode cytoplasmic (beta) actins rather than muscular (alpha) actins. Southern blotting and hybridisation experiments suggested that B. glabrata and H. trivolvis may have multiple (up to 5) actin genes, and it can not be ruled out that actin sequences obtained from different planorbid species were derived from paralogous genes. Interestingly however, the gene trees resolved actins derived from gastropod, cephalopod and bivalve molluscs. The sequences presented add to the growing body of information on the molecular biology of planorbid snails.