THE ZEBRAFISH EARLY ARREST MUTANTS

This report describes mutants of the zebrafish having phenotypes causi ng a general arrest in early morphogenesis. These mutants identify a g roup of loci making up about 20% of the loci identified by mutants wit h visible morphological phenotypes within the first day of development . There are 12 Class I mutants, which fall into 5 complementation grou ps and have cells that lyse before morphological defects are observed. Mutants at three loci, speed bump, ogre and zombie, display abnormal nuclei. The 8 Class II mutants, which fall into 6 complementation grou ps, arrest development before cell lysis is observed. These mutants se emingly stop development in the late segmentation stages, and maintain a body shape similar to a 20 hour embryo. Mutations in speed bump, og re, zombie, specter, poltergeist and troll were tested for cell lethal ity by transplanting mutant cells into wild-type hosts. With poltergei st, transplanted mutant cells all survive. The remainder of the mutant s tested were autonomously but conditionally lethal: mutant cells, mos t of which lyse, sometimes survive to become notochord, muscles, or, i n rare cases, large neurons, all cell types which become postmitotic i n the gastrula. Some of the genes of the early arrest group may be nec essary for progression though the cell cycle; if so, the survival of e arly differentiating cells may be based on having their terminal mitos is before the zygotic requirement for these genes.