Development of sibling inbred sea urchins: Normal embryogenesis, but frequent postembryonic malformation, arrest and lethality
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.
© 1994 Elsevier Science Ireland Ltd. Received 19 November 1993; accepted 27 November 1993. This project grew out of a sabbatical visit by Prof. Antonio Garcia-Bellido in our laboratory, and though he bears no responsibility for its outcome, we owe to Antonio the encouragement required to launch us into this attempt to explore the consequences of homozygosity in sea urchin embryos and larvae. We are grateful to Professor Paul Sternberg of Caltech for careful review of drafts of this manuscript. Prof. Joel Rothman of University of Wisconsin-Madison was both generous and gracious in allowing us to quote his fascinating unpublished data on zygotic mutations in C. elegans. This research was supported by an NIH Division of Research Resources grant (RR-06591), as part of a program designed to support the further development of useful non-mammalian research model systems.