Which Of The Following Animals Does Not Display Segmentation

Division of some brute and constitute body plans into a series of segments

Partition in biology is the division of some animal and plant body plans into a serial of repetitive segments. This commodity focuses on the partition of animal torso plans, specifically using the examples of the taxa Arthropoda, Chordata, and Annelida. These three groups course segments by using a "growth zone" to direct and define the segments. While all three accept a generally segmented body plan and apply a growth zone, they utilise unlike mechanisms for generating this patterning. Even inside these groups, different organisms have different mechanisms for segmenting the body. Segmentation of the torso programme is important for allowing free motility and development of certain body parts. Information technology also allows for regeneration in specific individuals.

Definition [edit]

Partition is a difficult process to satisfactorily define. Many taxa (for example the molluscs) take some form of serial repetition in their units but are not conventionally thought of equally segmented. Segmented animals are those considered to have organs that were repeated, or to have a body composed of cocky-similar units, but normally it is the parts of an organism that are referred to as existence segmented.^[1]

Animals [edit]

Segmentation in animals typically falls into iii 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 cistron gradients. Vertebrates like the zebrafish utilize oscillating gene expression to ascertain segments known as somites. Annelids such equally the leech use smaller blast cells budded off from big teloblast cells to define segments.^[2]

Arthropods [edit]

Expression of Hox genes in the body segments of dissimilar groups of arthropod, as traced past evolutionary developmental biological science. The Hox genes 7, 8, and 9 correspond in these groups just are shifted (by heterochrony) past upwards to 3 segments. Segments with maxillipeds accept Hox gene vii. Fossil trilobites probably had three body regions, each with a unique combination of Hox genes.

Although Drosophila sectionalisation is not representative of the arthropod phylum in general, information technology is the nearly highly studied. Early screens to identify genes involved in cuticle evolution led to the discovery of a form of genes that was necessary for proper segmentation of the Drosophila embryo.^[3]

To properly segment the Drosophila embryo, the inductive-posterior centrality is defined by maternally supplied transcripts giving rise to gradients of these proteins.^[2] ^[iii] ^[iv] This gradient so defines the expression pattern for gap genes, which fix the boundaries between the unlike segments. The gradients produced from gap cistron expression then ascertain the expression pattern for the pair-rule genes.^[ii] ^[4] The pair-rule genes are mostly transcription factors, expressed in regular stripes down the length of the embryo.^[4] These transcription factors and then regulate the expression of segment polarity genes, which ascertain the polarity of each segment. Boundaries and identities of each segment are later defined.^[4]

Within the arthropods, the body wall, nervous organization, kidneys, muscles and torso crenel are segmented, equally are the appendages (when they are nowadays). Some of these elements (eastward.g. musculature) are not segmented in their sis taxon, the onychophora.^[1]

Annelids: Leech [edit]

While not every bit well studied as in Drosophila and zebrafish, sectionalisation in the leech has been described as "budding" sectionalization. Early divisions within the leech embryo upshot in teloblast cells, which are stalk cells that dissever asymmetrically to create bandlets of smash cells.^[2] Furthermore, there are five different teloblast lineages (North, 1000, O, P, and Q), with one fix for each side of the midline. The N and Q lineages contribute two nail cells for each segment, while the M, O, and P lineages only contribute 1 cell per segment.^[5] Finally, the number of segments inside the embryo is defined by the number of divisions and blast cells.^[2] Segmentation appears to be regulated by the gene Hedgehog, suggesting its common evolutionary origin in the antecedent of arthropods and annelids.^[half-dozen]

Within the annelids, as with the arthropods, the body wall, nervous system, kidneys, muscles and trunk cavity are generally segmented. However, this is non truthful for all of the traits all of the time: many lack partition in the torso wall, coelom and musculature.^[1]

Chordates: zebrafish and mouse [edit]

Zebrafish grade segments known as somites through a process that is reliant upon gradients of retinoic acid and FGF, as well as periodic oscillation of cistron expression.

Although perhaps not too studied as Drosophila, partitioning in zebrafish, chicks, and mice is actively studied. Segmentation in chordates is characterized as the formation of a pair of somites on either side of the midline. This is frequently referred to equally somitogenesis.

In chordates, partitioning 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 a gradient of FGF with somites forming at the low cease of this slope. In higher vertebrates including mouse and chick, just not zebrafish, the wavefront also depends upon retinoic acrid generated just anterior to the caudal FGF8 domain which limits the anterior spreading of FGF8; retinoic acid repression of Fgf8 gene expression defines the wavefront every bit the signal at which the concentrations of both retinoic acid and diffusible FGF8 poly peptide are at their everyman. Cells at this point volition mature and form a pair of somites.^[7] ^[viii] The interaction of other signaling molecules, such equally myogenic regulatory factors, with this gradient promotes the development of other structures, such every bit muscles, across the basic segments.^[9] Lower vertebrates such every bit zebrafish do not require retinoic acid repression of caudal Fgf8 for somitogenesis due to differences in gastrulation and neuromesodermal progenitor function compared to higher vertebrates.^[10]

Other taxa [edit]

In other taxa, there is some evidence of sectionalization in some organs, but this segmentation is non pervasive to the full listing of organs mentioned above for arthropods and annelids. One might retrieve of the serially repeated units in many Cycloneuralia, or the segmented trunk armature of the chitons (which is not accompanied by a segmented coelom).^[1]

Origin [edit]

Segmentation can be seen as originating in 2 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 organisation of organ systems is 'formalized' from loosely defined packets into more rigid segments.^[1] Every bit such, organisms with a loosely defined metamerism, whether internal (as some molluscs) or external (as onychophora), can be seen equally 'precursors' to eusegmented organisms such as annelids or arthropods.^[1]

References [edit]

^ ^a ^b ^c ^d ^e ^f Budd, Grand. E. (2001). "Why are arthropods segmented?". Evolution and Evolution. 3 (5): 332–42. doi:10.1046/j.1525-142X.2001.01041.x. PMID 11710765.
^ ^a ^b ^c ^d ^east Tautz, D (2004). "Sectionalisation". Dev Jail cell. vii (3): 301–312. doi:10.1016/j.devcel.2004.08.008. PMID 15363406.
^ ^a ^b Option, L (1998). "Division: Painting Stripes From Flies to Vertebrates". Dev Genet. 23 (1): ane–ten. doi:10.1002/(SICI)1520-6408(1998)23:1<1::AID-DVG1>iii.0.CO;2-A. PMID 9706689.
^ ^a ^b ^c ^d Peel AD; Chipman AD; Akam Thou (2005). "Arthropod Segmentation: Across The Drosophila Paradigm". Nat Rev Genet. 6 (12): 905–916. doi:ten.1038/nrg1724. PMID 16341071.
^ Weisblat DA; Shankland Thou (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.
^ Dray, North.; Tessmar-Raible, M.; Le Gouar, M.; Vibert, L.; Christodoulou, F.; Schipany, G.; Guillou, A.; Zantke, J.; Snyman, H.; Béhague, J.; Vervoort, M.; Arendt, D.; Balavoine, Yard. (2010). "Hedgehog signaling regulates segment formation in the annelid Platynereis". Science. 329 (5989): 339–342. Bibcode:2010Sci...329..339D. doi:10.1126/science.1188913. PMC3182550. PMID 20647470.
^ Cinquin O (2007). "Understanding the somitogenesis clock: what's missing?". Mech Dev. 124 (seven–8): 501–517. doi:x.1016/j.mod.2007.06.004. PMID 17643270.
^ Cunningham, T.J.; Duester, K. (2015). "Mechanisms of retinoic acid signalling and its roles in organ and limb development". Nat. Rev. Mol. Jail cell Biol. 16: 110–123. doi:10.1038/nrm3932. PMC4636111. PMID 25560970.
^ Chang, CN; Kioussi, C (xviii May 2018). "Location, Location, Location: Signals in Muscle Specification". Journal of Developmental Biology. 6 (2). doi:ten.3390/jdb6020011. PMC6027348. PMID 29783715.
^ Berenguer, K.; et al. (2018). "Mouse but not zebrafish requires retinoic acid for control of neuromesodermal progenitors and torso axis extension". Dev. Biol. 441: 127–131. doi:10.1016/j.ydbio.2018.06.019. PMC6064660. PMID 29964026.