Segmentation gene

Gene involve in early stages of segmentation

Segmentation genes of Drosophila embryo[1]

A segmentation gene is a gene involved in the early developmental stages of pattern formation. It regulates how cells are organized and defines repeated units in the embryo. Segmentation genes have been documented in three taxa: arthropods (i.e. insects and crabs),[2] chordates (i.e. mammals and fish), and annelids (i.e. leeches and earthworms).[3][4] In Drosophila melanogaster, a common fruit fly, segmentation genes divide the embryo into 14 parasegments[5] which are among the first compartments to form within the embryo.[6] Rare variants in segmentation genes can cause changes in appearance of differing severity depending on its type. The genes can be classified into 3 groups: Gap genes, Pair-rule genes and Segment polarity genes.[7]

## Gap genes

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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.[8] 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.[9] 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.[7] 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.[10] This creates a concentration gradient where bicoid expression is highest in the anterior and gradually decreases towards the posterior.[10] Bicoid along with other maternal effect genes like nanos create multiple concentration gradients that regulate the expression of gap genes.[7] Gap genes are expressed in large sections of the embryo multiple parasegments wide. Kruppel, for instance, is expressed in parasegments 4-6.[11] There are at least 6 types[12] of gap genes but the three[13] that are well-known are hunchback,[14] knirps, and kruppel.

Different concentration gradients of gap genes establish parasegment boundaries.[15] These parasegment boundaries help regulate or control the expression of pair-rule genes as well as segment polarity genes.[16] 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.[15]

## Pair-rule genes

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Pair-rule genes are genes that are expressed in alternating parasegments of the embryo for a total of 7-8 parasegments.[17] 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.[18] An expression of a pair-rule gene in one parasegment is followed by a region of no expression in the following parasegment.[19] 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).[20] 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.[21] Lastly, the pair-rule genes regulate the expression of segment polarity genes.[22]

## Segment polarity genes

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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.[23] One segment polarity gene, engrailed, is expressed in the anterior part of each parasegment while another, wingless, is expressed in the posterior region.[24] 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.[25][26]

## References

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1. ^ Schroeder, Mark D.; Pearce, Michael; Fak, John; Fan, HongQing; Unnerstall, Ulrich; Emberly, Eldon; Rajewsky, Nikolaus; Siggia, Eric D.; Gaul, Ulrike (2004-08-31). "Transcriptional Control in the Segmentation Gene Network of Drosophila". PLOS Biology. 2 (9): e271. doi:10.1371/journal.pbio.0020271. ISSN 1545-7885. PMC 514885. PMID 15340490.
2. ^ "Arthropod - Exoskeleton, Segmented, Jointed | Britannica". www.britannica.com. 2024-10-25. Retrieved 2024-12-05.
3. ^ Deutsch, Jean S. (2004). "Segments and parasegments in Arthropods: a functional perspective". BioEssays. 26 (10): 1117–1125\. doi:10.1002/bies.20111. ISSN 1521-1878. PMID 15382136.
4. ^ Balavoine, Guillaume (2014). "Segment formation in Annelids: patterns, processes and evolution". The International Journal of Developmental Biology. 58 (6–7–8): 469–483\. doi:10.1387/ijdb.140148gb. ISSN 0214-6282. PMID 25690963.
5. ^ Clark, Erik; Akam, Michael (2016-08-15). Wittkopp, Patricia J (ed.). "Odd-paired controls frequency doubling in Drosophila segmentation by altering the pair-rule gene regulatory network". eLife. 5: e18215. doi:10.7554/eLife.18215. ISSN 2050-084X. PMC 5035143. PMID 27525481.
6. ^ Hughes, Sarah C.; Krause, Henry M. (2001-04-01). "Establishment and maintenance of parasegmental compartments". Development. 128 (7): 1109–1118\. doi:10.1242/dev.128.7.1109. hdl:1807/16229. ISSN 0950-1991. PMID 11245576.
7. ^ a b c Gilbert, Scott F. (2000), "The Origins of Anterior-Posterior Polarity", Developmental Biology. 6th edition, Sinauer Associates, retrieved 2024-12-04
8. ^ Patel, Nipam H.; Liu, Paul Z. (2009-01-01), "Chapter 231 - Segmentation", in Resh, Vincent H.; Cardé, Ring T. (eds.), Encyclopedia of Insects (Second Edition), San Diego: Academic Press, pp. 909–912, doi:10.1016/b978-0-12-374144-8.00240-x, ISBN 978-0-12-374144-8, retrieved 2024-12-04
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10. ^ a b Stauber, Michael; Jäckle, Herbert; Schmidt-Ott, Urs (1999-03-30). "The anterior determinant bicoid of Drosophila is a derived Hox class 3 gene". Proceedings of the National Academy of Sciences. 96 (7): 3786–3789\. Bibcode:1999PNAS...96.3786S. doi:10.1073/pnas.96.7.3786. PMC 22372. PMID 10097115.
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13. ^ Hoy, Marjorie A. (2019-01-01), Hoy, Marjorie A. (ed.), "Chapter 4 - Genetic Systems, Genome Evolution, and Genetic Control of Embryonic Development in Insects", Insect Molecular Genetics (Fourth Edition), Academic Press, pp. 103–175, doi:10.1016/b978-0-12-815230-0.00004-2, ISBN 978-0-12-815230-0, retrieved 2024-12-05
14. ^ Treisman, Jessica; Desplan, Claude (September 1989). "The products of the Drosophila gap genes hunchback and Krüppel bind to the hunchback promoters". Nature. 341 (6240): 335–337\. doi:10.1038/341335a0. ISSN 1476-4687. PMID 2797150.
15. ^ a b Jaeger, Johannes (2011-01-01). "The gap gene network". Cellular and Molecular Life Sciences. 68 (2): 243–274\. doi:10.1007/s00018-010-0536-y. ISSN 1420-9071. PMC 3016493. PMID 20927566.
16. ^ Schroeder, Mark D.; Pearce, Michael; Fak, John; Fan, HongQing; Unnerstall, Ulrich; Emberly, Eldon; Rajewsky, Nikolaus; Siggia, Eric D.; Gaul, Ulrike (2004-08-31). "Transcriptional Control in the Segmentation Gene Network of Drosophila". PLOS Biology. 2 (9): e271. doi:10.1371/journal.pbio.0020271. ISSN 1545-7885. PMC 514885. PMID 15340490.
17. ^ Hiromi, Yasushi; Gehring, Walter J. (1987-09-11). "Regulation and function of the Drosophila segmentation gene fushi tarazu". Cell. 50 (6): 963–974\. doi:10.1016/0092-8674(87)90523-X. ISSN 0092-8674. PMID 2887293.
18. ^ Larsen, Camilla; Bardet, Pierre-Luc; Vincent, Jean-Paul; Alexandre, Cyrille (2008-09-15). "Specification and positioning of parasegment grooves in Drosophila". Developmental Biology. 321 (2): 310–318\. doi:10.1016/j.ydbio.2008.04.026. ISSN 0012-1606. PMID 18692780.
19. ^ Williams, Terri A.; Nagy, Lisa M. (2017-05-01). "Linking gene regulation to cell behaviors in the posterior growth zone of sequentially segmenting arthropods". Arthropod Structure & Development. Evolution of Segmentation. 46 (3): 380–394\. Bibcode:2017ArtSD..46..380W. doi:10.1016/j.asd.2016.10.003. ISSN 1467-8039. PMID 27720841.
20. ^ Frasch, M.; Levine, M. (1987-11-01). "Complementary patterns of even-skipped and fushi tarazu expression involve their differential regulation by a common set of segmentation genes in Drosophila". Genes & Development. 1 (9): 981–995\. doi:10.1101/gad.1.9.981. ISSN 0890-9369. PMID 2892761.
21. ^ Frasch, M.; Warrior, R.; Tugwood, J.; Levine, M. (1988-12-01). "Molecular analysis of even-skipped mutants in Drosophila development". Genes & Development. 2 (12b): 1824–1838\. doi:10.1101/gad.2.12b.1824. ISSN 0890-9369. PMID 2907321.
22. ^ Wilson, Megan J.; Dearden, Peter K. (2012-09-28). "Pair-Rule Gene Orthologues Have Unexpected Maternal Roles in the Honeybee (Apis mellifera)". PLOS ONE. 7 (9): e46490. Bibcode:2012PLoSO...746490W. doi:10.1371/journal.pone.0046490. ISSN 1932-6203. PMC 3460886. PMID 23029534.
23. ^ Mohler, Jym (1995-04-01). "Spatial regulation of segment polarity gene expression in the anterior terminal region of the Drosophila blastoderm embryo". Mechanisms of Development. 50 (2): 151–161\. doi:10.1016/0925-4773(94)00332-H. ISSN 0925-4773. PMID 7619727.
24. ^ Lim, Jinsung; Choe, Chong Pyo (2020-03-01). "Functional analysis of engrailed in Tribolium segmentation". Mechanisms of Development. 161: 103594. doi:10.1016/j.mod.2019.103594. ISSN 0925-4773. PMID 31778794.
25. ^ Swarup, Sharan; Verheyen, Esther M. (2012-06-01). "Wnt/Wingless Signaling in Drosophila". Cold Spring Harbor Perspectives in Biology. 4 (6): a007930. doi:10.1101/cshperspect.a007930. ISSN 1943-0264. PMC 3367557. PMID 22535229.
26. ^ Bejsovec, Amy; Wieschaus, Eric (1993-10-01). "Segment polarity gene interactions modulate epidermal patterning in Drosophila embryos". Development. 119 (2): 501–517\. doi:10.1242/dev.119.2.501. ISSN 0950-1991. PMID 8287799.