Temperature Sensitivity of a Hybrid between Two Species of Sea Urchin Differing in Thermotolerance

Eggs of Dendraster excentricus can be fertilized with sperm of Strongylocentrotus purpuratus at a high success rate according to Whiteley and Baltzer's method. The resulting hybrids are well suited for investigations of the factors determining embryonic thermosensitivity, because the embryos of both the above species differ significantly with regard to egg size, blastula shape, and particularly thermotolerance and the temperature dependence of their developmental rate. Morphologically, the hybrids show some paternally inherited traits, but their thermosensitivity is strictly determined by the egg cytoplasm.