The timing of expression of four actin genes and an RNA polymerase III-transcribed repeated sequence is correct in hybrid embryos of the sea urchin species Strongylocentrotus purpuratus and Lytechinus pictus.

The accumulation of the mRNAs from four Strongylocentrotus purpuratus actin genes (the single muscle gene M, and three cytoskeletal genes CyI, CyIIIa, and CyIIIb) and of transcripts from an RNA polymerase III-transcribed repeated sequence family (SURF1) was followed throughout the early development of hybrid embryos of S. purpuratus and Lytechinus pictus. Each of the actin mRNAs appeared in hybrid embryos, constructed in either direction (Sp female X Lp male and Lp female X Sp male), at approximately the same time that they appear in normal S. purpuratus embryos. Transcripts of the repeated sequence family SURF1 also appeared at the correct time in the hybrid embryos, but were present at substantially reduced levels when contributed by the paternal genome (Lp female X Sp male). The accurate temporal expression of these genes indicates that both sets of hybrid embryos contain factors which regulate the timing of their transcription.