Refertilization in eggs of the sea urchin Strongylocentrotus purpuratus

To determine the role of the sea urchin egg plasma membrane in the species-specificity of fertilization, the ability of denuded activated eggs to be heterospecifically refertilized was determined. Our initial studies included evaluating the effectiveness of three commonly used methods of vitelline envelope (VE) removal using indirect immunofluorescence microscopy with antibodies directed against the VE. Unfertilized Strongylocentrotus purpuratus eggs were extracted with 0.01 M dithiothreitol (DTT) for 3 min or digested with 1.0 mg/ml pronase for 1 hr. Eggs were also fertilized, then diluted into a divalent-free medium to produce thin, elevated envelopes (VE*s) that were mechanically removed by sieving the eggs through nylon mesh. We found that both DTT extraction and pronase digestion were not completely effective in VE removal, and mechanical removal methods gave rise to a mixed population of eggs, those that had their VEs removed and those with a collapsed envelope that was not detectable at the light microscope level. Therefore, a new method of VE removal was developed. Eggs with VE*s were prepared followed by treatment with 0.01 M DTT to solubilize the envelopes. Nearly 100% of the denuded activated eggs incorporated one or more homologous and heterologous sperm, suggesting that the egg plasma membrane does not function in determining the species-specificity of fertilization.