Editing Maternal effect (section)

===In ''Drosophila'' early embryogenesis===
[[File:Ribonucleoprotein particles in the Drosophila egg chamber.png|thumb|right|Protein and RNA are transported in particles (white dots) from the nurse cells (maternal) to the developing oocyte in ''Drosophila melanogaster''. Scale bar shows 10&nbsp;μm.]]
{{further|Drosophila embryogenesis}}

A ''[[Drosophila melanogaster]]'' [[oocyte]] develops in an egg chamber in close association with a set of cells called [[nurse cell]]s. Both the oocyte and the nurse cells are descended from a single germline [[stem cell]], however [[cytokinesis]] is incomplete in these [[cell division]]s, and the cytoplasm of the nurse cells and the oocyte is connected by structures known as [[ring canal]]s.<ref>{{cite journal |vauthors=Bastock R, St Johnston D |title=''Drosophila'' oogenesis |journal=Curr. Biol. |volume=18 |issue=23 |pages=R1082–7 |date=December 2008 |pmid=19081037 |doi=10.1016/j.cub.2008.09.011 |s2cid=14924601 |doi-access=free |bibcode=2008CBio...18R1082B }}</ref> Only the oocyte undergoes [[meiosis]] and contributes [[DNA]] to the next generation.

Many maternal effect ''Drosophila'' mutants have been found that affect the early steps in embryogenesis such as [[axis determination]], including ''[[bicoid]], [[dorsal (gene)|dorsal]], [[gurken]]'' and ''[[oskar (gene)|oskar]]''.<ref>{{cite journal |vauthors=Nüsslein-Volhard C, Lohs-Schardin M, Sander K, Cremer C |title=A dorso-ventral shift of embryonic primordia in a new maternal-effect mutant of ''Drosophila'' |journal=Nature |volume=283 |issue=5746 |pages=474–6 |date=January 1980 |pmid=6766208 |doi= 10.1038/283474a0|bibcode = 1980Natur.283..474N |s2cid=4320963 }}</ref><ref>{{cite journal |vauthors=Schüpbach T, Wieschaus E |title=Germline autonomy of maternal-effect mutations altering the embryonic body pattern of ''Drosophila'' |journal=Dev. Biol. |volume=113 |issue=2 |pages=443–8 |date=February 1986 |pmid=3081391 |doi= 10.1016/0012-1606(86)90179-X}}</ref><ref>{{cite journal |vauthors=Nüsslein-Volhard C, Frohnhöfer HG, Lehmann R |title=Determination of anteroposterior polarity in ''Drosophila'' |journal=Science |volume=238 |issue=4834 |pages=1675–81 |date=December 1987 |pmid=3686007 |doi= 10.1126/science.3686007|bibcode = 1987Sci...238.1675N }}</ref> For example, embryos from homozygous ''bicoid'' mothers fail to produce head and [[thorax]] structures.

Once the gene that is disrupted in the ''bicoid'' mutant was identified, it was shown that ''bicoid'' mRNA is [[Transcription (genetics)|transcribed]] in the nurse cells and then relocalized to the oocyte.<ref>{{cite journal  |vauthors=Berleth T, Burri M, Thoma G, etal |title=The role of localization of ''bicoid'' RNA in organizing the anterior pattern of the ''Drosophila'' embryo |journal=EMBO J. |volume=7 |issue=6 |pages=1749–56 |date=June 1988 |pmid=2901954 |pmc=457163 |doi= 10.1002/j.1460-2075.1988.tb03004.x}}</ref> Other maternal effect mutants either affect products that are similarly produced in the nurse cells and act in the oocyte, or parts of the transportation machinery that are required for this relocalization.<ref>{{cite journal |vauthors=Ephrussi A, St Johnston D |title=Seeing is believing: the Bicoid morphogen gradient matures |journal=Cell |volume=116 |issue=2 |pages=143–52 |date=January 2004 |pmid=14744427 |doi= 10.1016/S0092-8674(04)00037-6|s2cid=1977542 |doi-access=free }}</ref> Since these genes are expressed in the (maternal) nurse cells and not in the oocyte or fertilised embryo, the maternal genotype determines whether they can function.

'''Maternal effect genes'''<ref>{{Cite book|title=Life Sciences : Fundamentals and practice.| last1 = Pranav | first1 = Kumar | last2 = Mina | first2 = Usha | name-list-style = vanc |date=2013|publisher=Pathfinder Academy |isbn=9788190642774 |edition=3rd |location=New Delhi |oclc=857764171 }}</ref> are expressed during oogenesis by the mother (expressed prior to fertilization) and develop the anterior-posterior and dorsal ventral polarity of the egg. The anterior end of the egg becomes the head; posterior end becomes the tail. the dorsal side is on the top; the ventral side is in underneath. The products of maternal effect genes called maternal mRNAs are produced by nurse cell and follicle cells and deposited in the egg cells (oocytes). At the start of development process, mRNA gradients are formed in oocytes along anterior-posterior and dorsal ventral axes.

About thirty maternal genes are involved in pattern formation have been identified. In particular, products of four maternal effect genes are critical to the formation of anterior-posterior axis. The product of two maternal effect gene, bicoid and hunchback, regulates formation of anterior structure while another pair nanos and caudal, specifies protein that regulates formation of posterior part of embryo.

The transcript of all four genes-bicoid, hunchback, caudal, nanos are synthesized by nurse and follicle cells and transported into the oocytes.