Mothers, it would seem, were designed to meddle in our affairs right from the word go, as maternal gene products direct many embryonic processes before zygotic transcription gets going. To find out just what these genes might be, Mary Mullins' laboratory has carried out the first genetic screen of its kind in zebrafish, and has identified 68 mutants that should help to define the maternal and paternal contribution to vertebrate embryonic development.

Conventional zygotic screens are useless for picking up mutations that affect the very early stages of development, as zygotic gene expression has not yet begun. Mullins and colleagues therefore carried out a maternal-effect screen, involving a four-generation crossing scheme that was designed to maximize the recovery and propagation of mutant lines. Embryos of mothers that were homozygous for a given mutation were scored for abnormalities 24 h after fertilization. In this way, 68 maternal-effect mutations were found, which were divided into 2 groups — described in separate publications — according to whether they affect embryos before or after the mid-blastula transition, which marks the start of zygotic transcription.

The first paper describes the phenotype of 15 such mutants, which the authors classify into 5 groups, including those that affect egg activation and animal–vegetal polarity. The second paper focuses on 13 of the 48 mutations that affect the embryo after zygotic gene activation; these 'late' mutants also fall into 5 phenotypic classes, including those that affect cell viability and the body plan. Although the focus was on maternal genes, the screen also surprisingly picked up five paternal-effect mutants, which defied expectations by having substantial developmental effects.

Systematic, large screens such as this usually mark the beginning of many years of work into understanding which genes and pathways might be at work in the early vertebrate embryo, just as the output of maternal-effect Drosophila melanogaster screens has occupied hundreds of fly geneticists for years. In this case, the method is just as noteworthy as the results, as some inspired planning has allowed the authors to genetically map some of the mutations with relative ease.