Maternal alk8 promoter fragment directs expression in early oocytes

Zebrafish Alk8 is a novel type I TGFbeta family member receptor that functions in Bmp signaling pathways to direct dorsoventral patterning of the early embryo. Both alk8 mRNA and protein is expressed in a variety of tissues, including teeth, eye, heart, blood, notochord, muscle, neural tissues, and neural crest cells (NCC's). Previous functional analyses, performed by microinjection of constitutively active (CA) and dominant negative (DN) alk8 mRNAs, revealed early developmental requirements for alk8. The alk8 gene is expressed ubiquitously as a maternal mRNA, and later exhibits more restricted zygotic gene expression. To identify regulatory elements directing the temporospatial expression of alk8, we characterized fluorescence in transgenic zebrafish lines containing alk8 promoter/green fluorescent protein (GFP) reporter constructs. This approach identified transgenic alk8(2.6):GFP lines that first expressed GFP in early stage 1A oocytes, and maintained GFP expression throughout the embryo only for the first 72 hours of development. Analysis of the nucleotide sequence of the 2.6 kb alk8 promoter element provides insight into the regulation of maternal alk8 gene expression. Significantly, the identified maternal alk8 promoter is one of only two maternal-specific promoter elements that have been described to date.