segmentation gene

Gap genes are among the first genes expressed in the embryo. Here, expression refers to the translation of the gene. Gap genes were named as such because loss-of-function variants in gap genes resulted in large deletions (or gaps) in the neighbouring segments of the embryo.{{Citation |last1=Patel |first1=Nipam H. |title=Chapter 231 - Segmentation |date=2009-01-01 |encyclopedia=Encyclopedia of Insects (Second Edition) |pages=909–912 |editor-last=Resh |editor-first=Vincent H. |url=https://linkinghub.elsevier.com/retrieve/pii/B978012374144800240X |access-date=2024-12-04 |place=San Diego |publisher=Academic Press |doi=10.1016/b978-0-12-374144-8.00240-x |isbn=978-0-12-374144-8 |last2=Liu |first2=Paul Z. |editor2-last=Cardé |editor2-first=Ring T.|url-access=subscription }} The expression of gap genes is regulated by maternally deposited factors called maternal effect genes. Maternal effect genes encode factors like messenger RNA needed for early development such as cell division.{{Cite journal |last=Mitchell |first=Laura E. |date=2022-01-13 |title=Maternal effect genes: Update and review of evidence for a link with birth defects |journal=Human Genetics and Genomics Advances |language=English |volume=3 |issue=1 |doi=10.1016/j.xhgg.2021.100067 |issn=2666-2477 |pmc=8756509 |pmid=35047854}} One of their main roles is to provide polarity and sense of direction to the embryo: which region will become the anterior or the head region, and which region will become the posterior or the tail region.  For instance, the mRNA of bicoid, a maternal affect gene, is transported to the anterior region of the embryo and then spreads toward the posterior region.{{Cite journal |last1=Stauber |first1=Michael |last2=Jäckle |first2=Herbert |last3=Schmidt-Ott |first3=Urs |date=1999-03-30 |title=The anterior determinant bicoid of Drosophila is a derived Hox class 3 gene |journal=Proceedings of the National Academy of Sciences |volume=96 |issue=7 |pages=3786–3789 |doi=10.1073/pnas.96.7.3786 |doi-access=free |pmc=22372 |pmid=10097115|bibcode=1999PNAS...96.3786S }} This creates a concentration gradient where bicoid expression is highest in the anterior and gradually decreases towards the posterior. Bicoid along with other maternal effect genes like nanos create multiple concentration gradients that regulate the expression of gap genes. Gap genes are expressed in large sections of the embryo multiple parasegments wide. Kruppel, for instance, is expressed in parasegments 4-6.{{Citation |last1=Umulis |first1=David |title=Robustness of Embryonic Spatial Patterning in Drosophila melanogaster |date=2008-01-01 |work=Current Topics in Developmental Biology |volume=81 |pages=65–111 |editor-last=Schnell |editor-first=Santiago |series=Multiscale Modeling of Developmental Systems |publisher=Academic Press |doi=10.1016/s0070-2153(07)81002-7 |pmc=6388640 |pmid=18023724 |last2=O'Connor |first2=Michael B. |last3=Othmer |first3=Hans G. |isbn=978-0-12-374253-7 |editor2-last=Maini |editor2-first=Philip K. |editor3-last=Newman |editor3-first=Stuart A. |editor4-last=Newman |editor4-first=Timothy J.}} There are at least 6 types{{Cite web |title=Drosophila gene families: gap and pair rule genes |url=https://www.sdbonline.org/sites/fly/aignfam/gapnprl.htm#:~:text=Out%20of%20the%20twelve%20A-P,,%20and%20Odd-skipped). |access-date=2024-12-05 |website=www.sdbonline.org}} of gap genes but the three{{Citation |last=Hoy |first=Marjorie A. |title=Chapter 4 - Genetic Systems, Genome Evolution, and Genetic Control of Embryonic Development in Insects |date=2019-01-01 |work=Insect Molecular Genetics (Fourth Edition) |pages=103–175 |editor-last=Hoy |editor-first=Marjorie A. |url=https://linkinghub.elsevier.com/retrieve/pii/B9780128152300000042 |access-date=2024-12-05 |publisher=Academic Press |doi=10.1016/b978-0-12-815230-0.00004-2 |isbn=978-0-12-815230-0|url-access=subscription }} that are well-known are hunchback,{{Cite journal |last1=Treisman |first1=Jessica |last2=Desplan |first2=Claude |date=September 1989 |title=The products of the Drosophila gap genes hunchback and Krüppel bind to the hunchback promoters |url=https://www.nature.com/articles/341335a0 |journal=Nature |language=en |volume=341 |issue=6240 |pages=335–337 |doi=10.1038/341335a0 |pmid=2797150 |issn=1476-4687|url-access=subscription }} knirps, and kruppel.

Different concentration gradients of gap genes establish parasegment boundaries.{{Cite journal |last=Jaeger |first=Johannes |date=2011-01-01 |title=The gap gene network |journal=Cellular and Molecular Life Sciences |language=en |volume=68 |issue=2 |pages=243–274 |doi=10.1007/s00018-010-0536-y |issn=1420-9071 |pmc=3016493 |pmid=20927566}} These parasegment boundaries help regulate or control the expression of pair-rule genes as well as segment polarity genes.{{Cite journal |last1=Schroeder |first1=Mark D. |last2=Pearce |first2=Michael |last3=Fak |first3=John |last4=Fan |first4=HongQing |last5=Unnerstall |first5=Ulrich |last6=Emberly |first6=Eldon |last7=Rajewsky |first7=Nikolaus |last8=Siggia |first8=Eric D. |last9=Gaul |first9=Ulrike |date=2004-08-31 |title=Transcriptional Control in the Segmentation Gene Network of Drosophila |journal=PLOS Biology |language=en |volume=2 |issue=9 |pages=e271 |doi=10.1371/journal.pbio.0020271 |issn=1545-7885 |pmc=514885 |pmid=15340490 |doi-access=free}} Lastly, the gap genes also play a role in later development such as giving rise to neurons along with formation of muscles and the gut.

Pair-rule genes are genes that are expressed in alternating parasegments of the embryo for a total of 7-8 parasegments.{{Cite journal |last1=Hiromi |first1=Yasushi |last2=Gehring |first2=Walter J. |date=1987-09-11 |title=Regulation and function of the Drosophila segmentation gene fushi tarazu |url=https://linkinghub.elsevier.com/retrieve/pii/009286748790523X |journal=Cell |language=English |volume=50 |issue=6 |pages=963–974 |doi=10.1016/0092-8674(87)90523-X |issn=0092-8674 |pmid=2887293|url-access=subscription }} The boundaries of parasegments are not determined by grooves that can be seen on the embryo but are compartments that show gene expression. One parasegment is made from the back half of a visible segment (not parasegment) and the front half of the visible segment behind it.{{Cite journal |last1=Larsen |first1=Camilla |last2=Bardet |first2=Pierre-Luc |last3=Vincent |first3=Jean-Paul |last4=Alexandre |first4=Cyrille |date=2008-09-15 |title=Specification and positioning of parasegment grooves in Drosophila |url=https://linkinghub.elsevier.com/retrieve/pii/S0012160608003072 |journal=Developmental Biology |volume=321 |issue=2 |pages=310–318 |doi=10.1016/j.ydbio.2008.04.026 |pmid=18692780 |issn=0012-1606|url-access=subscription }} An expression of a pair-rule gene in one parasegment is followed by a region of no expression in the following parasegment.{{Cite journal |last1=Williams |first1=Terri A. |last2=Nagy |first2=Lisa M. |date=2017-05-01 |title=Linking gene regulation to cell behaviors in the posterior growth zone of sequentially segmenting arthropods |url=https://linkinghub.elsevier.com/retrieve/pii/S1467803916301463 |journal=Arthropod Structure & Development |series=Evolution of Segmentation |volume=46 |issue=3 |pages=380–394 |doi=10.1016/j.asd.2016.10.003 |pmid=27720841 |bibcode=2017ArtSD..46..380W |issn=1467-8039|doi-access=free }} For example, odd-skipped genes are expressed in alternating even-numbered parasegments (stripe 2, 4, and so on) while even-skipped genes are expressed in odd-numbered parasegments (stripe 1, 3, and so on).{{Cite journal |last=Frasch |first=M. |last2=Levine |first2=M. |date=1987-11-01 |title=Complementary patterns of even-skipped and fushi tarazu expression involve their differential regulation by a common set of segmentation genes in Drosophila. |url=https://genesdev.cshlp.org/content/1/9/981 |journal=Genes & Development |language=en |volume=1 |issue=9 |pages=981–995 |doi=10.1101/gad.1.9.981 |issn=0890-9369 |pmid=2892761|doi-access=free }} They were termed as such because loss-of-function variants in even-skipped genes can cause the disappearance of odd-numbered parasegments only leaving behind the even-numbered parasegments, hence, the name.{{Cite journal |last=Frasch |first=M. |last2=Warrior |first2=R. |last3=Tugwood |first3=J. |last4=Levine |first4=M. |date=1988-12-01 |title=Molecular analysis of even-skipped mutants in Drosophila development. |url=https://genesdev.cshlp.org/content/2/12b/1824 |journal=Genes & Development |language=en |volume=2 |issue=12b |pages=1824–1838 |doi=10.1101/gad.2.12b.1824 |issn=0890-9369 |pmid=2907321|doi-access=free }} Lastly, the pair-rule genes regulate the expression of segment polarity genes.{{Cite journal |last1=Wilson |first1=Megan J. |last2=Dearden |first2=Peter K. |date=2012-09-28 |title=Pair-Rule Gene Orthologues Have Unexpected Maternal Roles in the Honeybee (Apis mellifera) |journal=PLOS ONE |language=en |volume=7 |issue=9 |pages=e46490 |doi=10.1371/journal.pone.0046490 |doi-access=free |issn=1932-6203 |pmc=3460886 |pmid=23029534|bibcode=2012PLoSO...746490W }}

Segment polarity genes are expressed in distinct regions within a parasegment. A parsegment is divided into anterior - the head -region, and the posterior - the tail - region.{{Cite journal |last=Mohler |first=Jym |date=1995-04-01 |title=Spatial regulation of segment polarity gene expression in the anterior terminal region of the Drosophila blastoderm embryo |url=https://linkinghub.elsevier.com/retrieve/pii/092547739400332H |journal=Mechanisms of Development |volume=50 |issue=2 |pages=151–161 |doi=10.1016/0925-4773(94)00332-H |pmid=7619727 |issn=0925-4773|doi-access=free }} One segment polarity gene, engrailed, is expressed in the anterior part of each parasegment while another, wingless, is expressed in the posterior region.{{Cite journal |last1=Lim |first1=Jinsung |last2=Choe |first2=Chong Pyo |date=2020-03-01 |title=Functional analysis of engrailed in Tribolium segmentation |url=https://linkinghub.elsevier.com/retrieve/pii/S0925477319301509 |journal=Mechanisms of Development |volume=161 |pages=103594 |doi=10.1016/j.mod.2019.103594 |pmid=31778794 |issn=0925-4773|url-access=subscription }} Loss-of-function variants in engrailed, for instance, can result in defects within the anterior portions of each parasegment. Lastly, certain segment polarity genes like wingless are involved in the planning and development of body parts such as the wings.{{Cite journal |last1=Swarup |first1=Sharan |last2=Verheyen |first2=Esther M. |date=2012-06-01 |title=Wnt/Wingless Signaling in Drosophila |url=https://cshperspectives.cshlp.org/content/4/6/a007930 |journal=Cold Spring Harbor Perspectives in Biology |language=en |volume=4 |issue=6 |pages=a007930 |doi=10.1101/cshperspect.a007930 |issn=1943-0264 |pmc=3367557 |pmid=22535229}}{{Cite journal |last1=Bejsovec |first1=Amy |last2=Wieschaus |first2=Eric |date=1993-10-01 |title=Segment polarity gene interactions modulate epidermal patterning in Drosophila embryos |url=https://journals.biologists.com/dev/article/119/2/501/37810/Segment-polarity-gene-interactions-modulate |journal=Development |volume=119 |issue=2 |pages=501–517 |doi=10.1242/dev.119.2.501 |pmid=8287799 |issn=0950-1991|url-access=subscription }}

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Category:Embryology