رویان‌زایی مگس سرکه
الگوی آسه پیشین - پسین در مگس سرکه > Maternal effect genes

The building-blocks of anterior-posterior axis patterning in Drosophila are laid out during egg formation (oogenesis), well before the egg is fertilized and deposited. The maternal effect genes are responsible for the polarity of the egg and of the embryo. The developing egg (oocyte) is polarized by differentially localized mRNA molecules.
The genes that code for these mRNAs, called maternal effect genes, encode for proteins that get translated upon fertilization to establish concentration gradients that span the egg. Bicoid and Hunchback are the maternal effect genes that are most important for patterning of anterior parts (head and thorax) of the Drosophila embryo. Nanos and Caudal are maternal effect genes that are important in the formation of more posterior abdominal segments of the Drosophila embryo.
In embryos from bicoid mutant mothers, the head and thoracic structures are converted to the abdomen making the embryo with posterior structures on both ends, a lethal phenotype.
Cytoskeletal elements such as microtubules are polarized within the oocyte and can be used to allow the localization of mRNA molecules to specific parts of the cell. Maternally synthesized bicoid mRNAs attach to microtubules and are concentrated at the anterior ends of forming Drosophila eggs. In unfertilized eggs, transcripts are still strictly localized at the tip, but immediately after fertilization, a small mRNA gradient is formed in the anterior 20% of the eggs. Another report documents a mRNA gradient up to 40%.
nanos mRNA also attaches to a Drosophila egg's cytoskeleton but is concentrated at the posterior end of the egg.
hunchback and caudal mRNAs lack special location control systems and are fairly evenly spread throughout the entire interior of the egg cells.
It has been shown that the dsRNA-binding protein STAUFEN (STAU1) is responsible for guiding bicoid, nanos and other proteins, which play a role in forming the anterior-posterior axis, to the correct regions of the embryo to build gradients.
When the mRNAs from the maternal effect genes are translated into proteins, a Bicoid protein gradient forms at the anterior end of the egg. Nanos protein forms a gradient at the posterior end. The Bicoid protein blocks translation of caudal mRNA so Caudal protein is of lower concentration at the anterior part of the embryo and at higher concentration at the posterior part of the embryo. This is of opposite direction of the Bicoid protein. The caudal protein then activates later to turn genes on to form the posterior structures during the segmentation phase. Nanos protein creates a posterior-to-anterior slope and is a morphogen that helps in abdomen formation. Nanos protein, in complex with Pumilio protein, binds to the hunchback mRNA and blocks its translation in the posterior end of Drosophila embryos.
The Bicoid, Hunchback, and Caudal proteins are transcription factors. The Bicoid protein is a morphogen as well. The Nanos protein is a translational repressor protein. Bicoid has a DNA-binding homeodomain that binds both DNA and the nanos mRNA. Bicoid binds a specific RNA sequence in the  3′ untranslated region , called the Bicoid 3′-UTR regulatory element, of caudal mRNA and blocks translation.
Hunchback protein levels in the early embryo are significantly augmented by new hunchback gene transcription and translation of the resulting zygotically produced mRNA. During early Drosophila embryogenesis, there are nuclear divisions without cell division. The many nuclei that are produced distribute themselves around the periphery of the cell cytoplasm. Gene expression in these nuclei is regulated by the Bicoid, Hunchback, and Caudal proteins. For example, Bicoid acts as a transcriptional activator of hunchback gene transcription. In order for development to continue, Hunchback is needed in an area that is declining in amount from anterior to posterior. This is created by the Nanos protein whose existence is at a declining slope from posterior to anterior ends.