Isolation of gonadal mutations in adult zebrafish from a chemical mutagenesis screen

A mutagenesis screen was conducted on zebrafish using N-ethyl N-nitrosourea as a mutagen and an F2 crossing scheme to obtain homozygous mutants in the F3 generation. Whole abdomens of 3-mo-old F3 zebrafish progeny were fixed and mass-embedded in paraffin blocks. Blocks were cut with a microtome to o btain cross-sections of the entire body cavity that included the ovaries an d testes. Slides of the cross-sections were analyzed for alterations in gon adal structure and gametogenesis and were compared with gonads of wild-type fish. A total of 125 mutagenized genomes in 81 families were screened and 11 mutations were observed that produced visible phenotypes in only one sex per family. Male mutations included testes without mature sperm that conta ined either predominantly spermatocytes or spermatogonia. Female mutations included ovaries containing 1) degenerating oocytes surrounded by hypertrop hied follicle walls or stroma, 2) extrafollicular tissue proliferation, 3) proliferating postovulatory follicle walls, and 4) large numbers of degener ating preovulatory and postovulatory oocytes. While past screens on zebrafi sh have concentrated on early developmental mutations, the results of this study demonstrate for the first time that mutagenesis can be used with zebr afish to study reproduction in adult animals.