Segmentation ( biology ) - wikipedia Segmentation ( biology ) Not to be confused with Segmentation contractions. Vertebrates have a segmented vertebral column. Segmentation in biology is the division of some animal and plant body plans into a series of repetitive segments. This article focuses on the segmentation of animal body plans, specifically using the examples of the taxa Arthropoda, Chordata, and Annelida. These three groups form segments by using a `` growth zone '' to direct and define the segments. While all three have a generally segmented body plan and use a growth zone, they use different mechanisms for generating this patterning. Even within these groups, different organisms have different mechanisms for segmenting the body. Segmentation of the body plan is important for allowing free movement and development of certain body parts. It also allows for regeneration in specific individuals. Contents 1 Definition 2 Animals 2.1 Arthropods 2.2 Annelids : Leech 2.3 Chordates : Zebrafish 2.4 Other taxa 3 Origin 4 See also 5 References Definition Segmentation is a difficult process to satisfactorily define. Many taxa ( for instance the molluscs ) have some form of serial repetition in their units, but are not conventionally thought of as segmented. Segmented animals are those considered to have organs that were repeated, or to have a body composed of self - similar units, but usually it is the parts of an organism that are referred to as being segmented. Animals Illacme plenipes, a millipede with 170 segments and 662 legs Segmentation in animals typically falls into three types, characteristic of different arthropods, vertebrates, and annelids. Arthropods such as the fruit fly form segments from a field of equivalent cells based on transcription factor gradients. Vertebrates like the zebrafish use oscillating gene expression to define segments known as somites. Annelids such as the leech use smaller blast cells budded off from large teloblast cells to define segments. Arthropods Expression of Hox genes in the body segments of different groups of arthropod, as traced by evolutionary developmental biology. The Hox genes 7, 8, and 9 correspond in these groups but are shifted ( by heterochrony ) by up to three segments. Segments with maxillopeds have Hox gene 7. Fossil trilobites probably had three body regions, each with a unique combination of Hox genes. Although Drosophila segmentation is not representative of the arthropod phylum in general, it is the most highly studied. Early screens to identify genes involved in cuticle development led to the discovery of a class of genes that was necessary for proper segmentation of the Drosophila embryo. To properly segment the Drosophila embryo, the anterior - posterior axis is defined by maternally supplied transcripts giving rise to gradients of these proteins. This gradient then defines the expression pattern for gap genes, which set up the boundaries between the different segments. The gradients produced from gap gene expression then define the expression pattern for the pair - rule genes. The pair - rule genes are mostly transcription factors, expressed in regular stripes down the length of the embryo. These transcription factors then regulate the expression of segment polarity genes, which define the polarity of each segment. Boundaries and identities of each segment are later defined. Within the arthropods, the body wall, nervous system, kidneys, muscles and body cavity are segmented, as are the appendages ( when they are present ). Some of these elements ( e.g. musculature ) are not segmented in their sister taxon, the onychophora. Annelids : leech While not as well studied as in Drosophila and zebrafish, segmentation in the leech has been described as `` budding '' segmentation. Early divisions within the leech embryo result in teloblast cells, which are stem cells that divide asymmetrically to create bandlets of blast cells. Furthermore, there are five different teloblast lineages ( N, M, O, P, and Q ), with one set for each side of the midline. The N and Q lineages contribute two blast cells for each segment, while the M, O, and P lineages only contribute one cell per segment. Finally, the number of segments within the embryo is defined by the number of divisions and blast cells. Segmentation appears to be regulated by the gene Hedgehog, suggesting its common evolutionary origin in the ancestor of arthropods and annelids. Within the annelids, as with the arthropods, the body wall, nervous system, kidneys, muscles and body cavity are generally segmented. However, this is not true for all of the traits all of the time : many lack segmentation in the body wall, coelom and musculature. Chordates : zebrafish Zebrafish form segments known as somites through a process that is reliant upon gradients of retinoic acid and FGF, as well as periodic oscillation of gene expression. Although perhaps not as well studied as Drosophila, segmentation in zebrafish is actively studied. The process is similar in zebrafish and other chordates, such as the chicken and the mouse. Segmentation in chordates is characterized as the formation of a pair of somites on either side of the midline. This is often referred to as somitogenesis. In the zebrafish, segmentation is coordinated by the clock and wavefront model. The `` clock '' refers to the periodic oscillation of specific genes, such as Her1, a hairy / Enhancer of split - gene. Expression starts at the posterior end of the embryo and moves towards the anterior. The wavefront is where the somites mature, defined by gradients of retinoic acid and FGF. Retinoic acid is high in the anterior and FGF is high in the posterior ; the wavefront is the point at which both concentrations are at their lowest. Cells at this point will mature and form a pair of somites. Elaboration of this process with other signalling chemicals allows for structures such as muscles to span the basic segments. Other taxa In other taxa, there is some evidence of segmentation in some organs, but this segmentation is not pervasive to the full list of organs mentioned above for arthropods and annelids. One might think of the serially repeated units in many Cycloneuralia, or the segmented body armature of the chitons ( which is not accompanied by a segmented coelom ). Origin Segmentation can be seen as originating in two ways. To caricature, the ' amplification ' pathway would involve a single - segment ancestral organism becoming segmented by repeating itself. This seems implausible, and the ' parcellization ' framework is generally preferred -- where existing organization of organ systems is ' formalized ' from loosely defined packets into more rigid segments. As such, organisms with a loosely defined metamerism, whether internal ( as some molluscs ) or external ( as onychophora ), can be seen as ' precursors ' to eusegmented organisms such as annelids or arthropods. See also Metamerism Pharyngeal arch Rhombomere References ^ Jump up to : Budd, G.E. ( 2001 ). `` Why are arthropods segmented ? ''. Evolution and Development. 3 ( 5 ) : 332 -- 42. doi : 10.1046 / j. 1525 - 142X. 2001.01041. x. PMID 11710765. ^ Jump up to : Tautz, D ( 2004 ). `` Segmentation ''. Dev Cell. 7 ( 3 ) : 301 -- 312. doi : 10.1016 / j. devcel. 2004.08. 008. PMID 15363406. ^ Jump up to : Pick, L ( 1998 ). `` Segmentation : Painting Stripes From Flies to Vertebrates ''. Dev Genet. 23 ( 1 ) : 1 -- 10. doi : 10.1002 / ( SICI ) 1520 - 6408 ( 1998 ) 23 : 1 < 1 : : AID - DVG1 > 3.0.CO ; 2 - A. PMID 9706689. ^ Jump up to : Peel AD ; Chipman AD ; Akam M ( 2005 ). `` Arthropod Segmentation : Beyond The Drosophila Paradigm ''. Nat Rev Genet. 6 ( 12 ) : 905 -- 916. doi : 10.1038 / nrg1724. PMID 16341071. Jump up ^ Weisblat DA ; Shankland M ( 1985 ). `` Cell lineage and segmentation in the leech ''. Philos Trans R Soc Lond B Biol Sci. 312 ( 1153 ) : 39 -- 56. Bibcode : 1985RSPTB. 312... 39W. doi : 10.1098 / rstb. 1985.0176. PMID 2869529. Jump up ^ Dray, N. ; Tessmar - Raible, K. ; Le Gouar, M. ; Vibert, L. ; Christodoulou, F. ; Schipany, K. ; Guillou, A. ; Zantke, J. ; Snyman, H. ; Béhague, J. ; Vervoort, M. ; Arendt, D. ; Balavoine, G. ( 2010 ). `` Hedgehog signaling regulates segment formation in the annelid Platynereis ''. Science. 329 ( 5989 ) : 339 -- 342. Bibcode : 2010Sci... 329... 339D. doi : 10.1126 / science. 1188913. PMC 3182550. PMID 20647470. Jump up ^ Cinquin O ( 2007 ). `` Understanding the somitogenesis clock : what 's missing ? ''. Mech Dev. 124 ( 7 - 8 ) : 501 -- 517. doi : 10.1016 / j. mod. 2007.06. 004. PMID 17643270. The development of phenotype Key concepts Genotype -- phenotype distinction Norms of reaction Gene -- environment interaction Gene -- environment correlation Operon Heritability Quantitative genetics Heterochrony Neoteny Heterotopy Genetic architecture Canalisation Genetic assimilation Dominance Epistasis Fitness landscape / evolutionary landscape Pleiotropy Plasticity Polygenic inheritance Transgressive segregation Sequence space Non-genetic influences Epigenetics Maternal effect Genomic imprinting Dual inheritance theory Polyphenism Developmental architecture Developmental biology Morphogenesis Eyespot Pattern formation Segmentation Modularity Evolution of genetic systems Evolvability Mutational robustness Neutral networks Evolution of sexual reproduction Control of development Systems Regulation of gene expression Gene regulatory network Developmental - genetic toolkit Evolutionary developmental biology Homeobox Hedgehog signaling pathway Notch signaling pathway Elements Homeotic gene Hox gene Pax genes eyeless gene Distal-less Engrailed cis - regulatory element Ligand Morphogen Cell surface receptor Transcription factor Influential figures C.H. Waddington Richard Lewontin François Jacob + Jacques Monod Lac operon Eric F. Wieschaus Christiane Nüsslein - Volhard William McGinnis Mike Levine Sean B. Carroll Endless Forms Most Beautiful Debates Nature versus nurture Morphogenetic field Index of evolutionary biology articles Retrieved from `` https://en.wikipedia.org/w/index.php?title=Segmentation_(biology)&oldid=819794201 '' Categories : Developmental biology Hidden categories : All articles with unsourced statements Articles with unsourced statements from January 2018 Talk Contents About Wikipedia Català Čeština Deutsch Eesti Español Galego हिन्दी Lietuvių 日本 語 Norsk Português Simple English Srpskohrvatski / српскохрватски Svenska 6 more Edit links This page was last edited on 11 January 2018, at 09 : 14 ( UTC ). Text is available under the Creative Commons Attribution - ShareAlike License ; additional terms may apply. By using this site, you agree to the Terms of Use and Privacy Policy. 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Question: what is an animal that has a segmented body
Answer: fruit fly