DNA synthesis by hybrid echinoid embryos produced from parent species of low and high temperature tolerance.

Synthesis of DNA, as measured by quantitative Feulgen microspectrophotometry, was studied in reciprocal echinoid hybrids and their homospermic controls. Both parent species develop at the same rate until hatching, at 20 degrees C. One parent (Lytechinus) will develop normally at 25 degrees C, a temperature lethal for the other parent employed in the studies (Strongylocentrotus). Strongylocentrotus will develop normally at 10 degrees C, a temperature at which Lytechinus fertilised eggs live, but fail to cleave. It was found that sperm of Lytechinus in eggs of Strongylocentrotus (SL hybrid) show a slowing of the S phase of the cell cycle at 10 degrees C, and that sperm of Strongylocentrotus in eggs of Lytechinus (LS hybrid) show a slowing of the S phase at 25 degrees C. These slowing effects are not noted during cleavage stages. The results are discussed in terms of proteins produced during the time of active transcription of the hybrid genome which may be rate-limiting in the synthesis of DNA in the hybrid embryos under these temperature extremes.