PUBLICATION

Analysis of maternal molecules in the early patterning of the zebrafish embryo

Authors
Howley, C.E.
ID
ZDB-PUB-021016-32
Date
2001
Source
Ph.D. Thesis : (Thesis)
Registered Authors
Howley, Cristin
Keywords
none
MeSH Terms
none
PubMed
none
Abstract
Studies in Drosophila and Xenopus have shown that localized maternal mRNAs commonly establish polarity within the early embryo, determining dorsal/ventral (D/V) and anterior/posterior (A/P) axis specification. While embryological experiments suggest that maternal factors are important in axis formation in the zebrafish, Danio rerio , it is unclear whether mRNA localization is used to achieve this. When this study was initiated, most of the identified maternal mRNAs had been found to be expressed throughout the early embryo. However, these initial studies overlooked the possibility that localization may have occurred earlier during oogenesis. To investigate this possibility, in situ hybridizations to known mRNAs were performed on zebrafish ovaries. The results establish that maternal mRNA localization during zebrafish oogenesis can be categorized into the following four patterns: ubiquitous, animal pole, vegetal pole, and cortical localization. While these patterns suggest that the animal/vegetal axis, which correlates with the A/P axis, may be established by localized mRNAs, they fail to define a role for mRNA localization in D/V establishment. Prior to this study, a maternal T-box gene like Xenopus VegT, known to localize and pattern the early frog embryo, had not yet been identified in the zebrafish. A screen to identify a similar maternal T-box gene in zebrafish yielded eomesodermin (eom). eom transcript localizes cortically during oogenesis. During the cleavage stages of development, maternal eom mRNA localizes in a vegetal to animal gradient, with most of the mRNA concentrating at the yolk/blastomere interface. This expression pattern is novel among maternally expressed genes in the zebrafish. Overexpression studies show that eom can induce ectopic expression of a number of genes, including organizer markers. Furthermore, eom overexpressing embryos form secondary axes, the hallmark of an ectopically induced organizer. These studies suggest a role for eom in D/V patterning. Overexpression of eom-VP16 (transcriptional activator) also induces secondary axes, suggesting eom behaves as an activator in organizer induction. Interestingly, eom-engrailed (transcriptional repressor) overexpression studies suggest a separate role for eom in early cell intercalation movements. In summary, this work demonstrates that maternal mRNA localization is a mechanism for establishing polarity in the early zebrafish embryo, as exemplified by eom.
Errata / Notes
Ph.D. Thesis, Princeton University
Genes / Markers
Figures
Expression
Phenotype
Mutations / Transgenics
Human Disease / Model
Sequence Targeting Reagents
Fish
Antibodies
Orthology
Engineered Foreign Genes
Mapping