Development of sibling inbred sea urchins: normal embryogenesis, but frequent postembryonic malformation, arrest and...

Inbred lines of Strongylocentrotus purpuratus descended from a single pair of wild animals were constructed by sibling mating. We describe results from a systematic series of crosses in which eggs from F2 and from F3 females were fertilized respectively with sperm from their sibling males. Observations were also made on self-fertilized cultures derived from several naturally occurring hermaphrodites. Morphological development, survival efficiency, and expression of three territorial embryonic markers were assayed in the embryos developing from these crosses. Unexpectedly, out of > 90 controlled crosses, we observed no developmental failures whatsoever, up to the end of embryogenesis (i.e., onset of feeding) that could be attributed to homozygous, zygotically acting recessive genes. However, during postembryonic larval development, lethality, morphological malformation, and arrest are observed in inbred cultures at a high frequency. The incidence of these zygotic developmental failures is such that it appears that there is at least one recessive genetic defect affecting larval development per haploid parental genome. The relative imperviousness of the basic embryonic process to defects arising from homozygosity is consistent with other evidence implying that territorial specification in sea urchin embryogenesis is controlled by maternally rather than zygotically expressed gene products.