Molecular genetics of maternally-controlled cell divisions.

Elliott W Abrams,Nadine L Vastenhouw,Lee Kapp,Ricardo Fuentes,Sumei Lu,Tripti Gupta,Hong Zhang,Mary C Mullins,Virginia Lemon,Amanda A Amodeo,Manami Kobayashi,Shai R Joseph,Greg Runke,Amy Kugath,Florence L Marlow,Gregory S Barsh

doi:10.1371/journal.pgen.1008652

Abstract

Forward genetic screens remain at the forefront of biology as an unbiased approach for discovering and elucidating gene function at the organismal and molecular level. Past mutagenesis screens targeting maternal-effect genes identified a broad spectrum of phenotypes ranging from defects in oocyte development to embryonic patterning. However, earlier vertebrate screens did not reach saturation, anticipated classes of phenotypes were not uncovered, and technological limitations made it difficult to pinpoint the causal gene. In this study, we performed a chemically-induced maternal-effect mutagenesis screen in zebrafish and identified eight distinct mutants specifically affecting the cleavage stage of development and one cleavage stage mutant that is also male sterile. The cleavage-stage phenotypes fell into three separate classes: developmental arrest proximal to the mid blastula transition (MBT), irregular cleavage, and cytokinesis mutants. We mapped each mutation to narrow genetic intervals and determined the molecular basis for two of the developmental arrest mutants, and a mutation causing male sterility and a maternal-effect mutant phenotype. One developmental arrest mutant gene encodes a maternal specific Stem Loop Binding Protein, which is required to maintain maternal histone levels. The other developmental arrest mutant encodes a maternal-specific subunit of the Minichromosome Maintenance Protein Complex, which is essential for maintaining normal chromosome integrity in the early blastomeres. Finally, we identify a hypomorphic allele of Polo-like kinase-1 (Plk-1), which results in a male sterile and maternal-effect phenotype. Collectively, these mutants expand our molecular-genetic understanding of the maternal regulation of early embryonic development in vertebrates.

Highlights

After fertilization the newly formed zygote undergoes cellular cleavages, which are under the control of maternally-supplied gene products
The earliest stages of animal development are regulated by factors in the egg that are made during oogenesis and are required for the embryo to develop prior to genome activation of the embryo itself
We identified three distinct classes of mutations affecting cell division in the early embryo

Summary

Introduction

After fertilization the newly formed zygote undergoes cellular cleavages, which are under the control of maternally-supplied gene products. In other mammalian systems the MZT takes place at later stages of development, at the 4 to 8-cell transition in humans [4], and at the 8 to 16-cell transition in rabbit and sheep [5]. In some vertebrates, such as frogs and fish, the MZT coincides with the mid-blastula transition (MBT) [3, 6]. Because genetic access to maternal gene functions in vertebrates remains challenging, the molecular underpinnings of genome stability, chromosomal architecture and cellular integrity during these rapid cell divisions devoid of checkpoints are still poorly understood

Methods

Results

Conclusion