What Advantage Does It Provide For Sessile Animals To Have Radial Symmetry Brainly

• Journal List
• Biol Direct
• v.vii; 2012
• PMC3438024

Biol Direct. 2012; 7: 22.

The manoeuvrability hypothesis to explain the maintenance of bilateral symmetry in animate being evolution

Gábor Holló

¹Establish of Psychology, Academy of Debrecen, P.O.B. 28, Debrecen, H, 4010, Hungary

Mihály Novák

²Faculty of Applied Sciences, Campus du Solbosch, bâtiment U, Université Libre de Bruxelles, artery F. D. Roosevelt fifty, 1050, Brussels, Kingdom of belgium

³Establish of Nuclear Research of the Hungarian Academy of Sciences (MTA ATOMKI), Debrecen, Hungary

Received 2011 December five; Accepted 2012 Jul 12.

Abstract

Background

The overwhelming majority of animal species exhibit bilateral symmetry. Nevertheless, the precise evolutionary importance of bilateral symmetry is unknown, although elements of the understanding of the phenomenon have been nowadays inside the scientific customs for decades.

Presentation of the hypothesis

Hither nosotros prove, with very simple concrete laws, that locomotion in three-dimensional macro-earth space is itself sufficient to explain the maintenance of bilateral symmetry in animal evolution. The power to change direction, a key element of locomotion, requires the generation of instantaneous "pushing" surfaces, from which the beast can obtain the necessary force to depart in the new direction. We show that bilateral is the just type of symmetry that can maximize this strength; thus, an actively locomoting bilateral body can have the maximal manoeuvrability as compared to other symmetry types. This confers an obvious selective advantage on the bilateral animal.

Implications of the hypothesis

These considerations imply the view that animal development is a highly channelled process, in which bilateral and radial trunk symmetries seem to be inevitable.

Reviewers

This article was reviewed by Gáspár Jékely, L. Aravind and Eugene Koonin.

Keywords: Bilateral symmetry, Radial symmetry, Manoeuvrability, Drag, Drag coefficient

Background

Animals show diverse types of symmetry including spherical, cylindrical (also known as perfect radial), radial, biradial, bilateral and asymmetric (for review, see ref. [1]). In this paper, the terms cylindrical and radial will be used as synonyms because in the context it makes no substantial difference. More than than 99 % of animal species are bilaterally symmetrical. The few exceptions are the amorphous parasitic placozoan Trichoplax, sponges (with asymmetry, spherical symmetry, and elements of radial symmetry in the skeleton), cnidarians (which include jellyfish, hydras, corals and bounding main anemones with radial and biradial symmetry), ctenophores or comb jellies (biradial symmetry), and echinoderms (bounding main lilies, sea urchins, and bounding main stars with radial symmetry).

Bilateral symmetry with two body axes arose early on in animal evolution, probably in slow, flat, worm-similar organisms locomoting on a substrate [ii]. Genetic analyses have ended that the genes responsible for bilateral symmetry most likely appeared prior to the cnidarian–bilaterian split [3-6], in the Precambrian [vii,viii]. However, even if the slow, cilium-based locomotion on a substrate may explain the generation of bilateral symmetry, it certainly cannot account for its survival over millions of years of animal development. Most recent animal species are bilaterally symmetrical, muscle-based locomoters, either living a pelagic life in water, or locomoting on the state and/or in the air. Why has bilaterality, which probably formed in benthic lifestyle, as well proved so succesful in gratuitous-moving animals?

Presentation of the hypothesis

Now we focus on the aquatic surround because bilateral symmetry (and animate being life, itself) formed there, and had to be maintained at that place for millions of years, before bilaterians conquered the country. Let united states offset with the elementary physical fact that to locomote in a fluid, a body has to overcome drag (the resistance of the medium in which the torso moves, interim in opposition to the direction of locomotion).

The magnitude of the drag force is:

F = – ½ ρ c A v ²

where F is the elevate force, ρ is the density of the medium, c is the dimensionless drag coefficient dependent on the body shape, A is the surface area of the maximal department of the body in the direction of motion, and 5 is the body'south velocity [nine,x]. The negative sign on the right side indicates that drag is reverse to the direction of motion. It is of import to note that this equation is valid for situations where the gluey forces are negligible compared to inertial forces, in what is loosely described as the macroscopic world (i.eastward at high Reynolds numbers). In the microscopic world, the forces are dominated by the viscosity of the fluid rather than by the inertia (i.e. at low Reynolds numbers) [10], however a discussion of the locomotion in the micro scale world is non the concern of this paper (for an in-depth analysis come across ref. [11]).

Given the fact that the medium imposes resistance on the body, if resistance forces are unequally distributed around the body, their resultant force volition non exist nada compared to the rectilinear management (i.e. movement straight ahead), so the body will not motility on a linear path. This is the case when a moving body is disproportionate. Thus, it follows that a directionally locomoting animal has to be symmetric in order to avert this effect. To exist able to motion forward, the animate being can have any type of symmetry, so the arroyo outlined here is not sufficient to explain the success of bilateral symmetry. Rectilinear motion is, however, not the only chemical element of locomotion. One other of import element is changing management, the importance of which, in this regard, has been by and large ignored in the literature and so far. A slight deviation from the straight trajectory tin hands be obtained by flawing one element of symmetry, thus generating asymmetry in the original direction of motility. This tin can be achieved past whatever symmetrical torso. However, when a quick changeover is required, the situation becomes very unlike.

In quick changes in management, the body has to exercise a force in the opposite management to the desired new orientation. This ways that information technology has to have a "pushing" surface in water from which to depart in the new direction. This surface is formed by the h2o layer against which the trunk is standing in order to push itself abroad, and it is produced by creating a neat instantaneous elevate force. Since ρ in the equation is unchanged, and v is diminishing or constant, the creature has to increase the maximal surface A and/or the drag coefficient c.

We volition now overview the primary symmetry types in terms of their capacity to create a pushing-off drag force. Given that a swimming body has to minimize the overall drag, its pare friction [nine], and thus its wetted expanse, has to be fairly reduced. Thus, only three chief body forms tin can be considered: spherical (with endless symmetry planes and symmetry axes), cylindrical (with endless symmetry planes and ane symmetry axis) and bilateral (with 1 plane of symmetry). An elongated radial body that shows a star-like section is suboptimal since it has a very large surface that is far from ideal for swimming forwards.

A spherically symmetrical body cannot generate the pushing surface, existence of equal shape and drag in every direction. Since the forces – which are unlike from the one operating in the direction of its motion – acting on this body are all equalized, it will non be able to depart in a new direction. It tin can only rotate effectually itself to deviate to a small extent (equally soccer players bend the ball), but this is hardly an effective changeover and manifestly cannot guarantee accurate manoeuvrability (understood simply every bit the capacity to perform quick and accurate changeovers). In this context nosotros can disregard how it was able to move directly in the first place.

A cylindrical (Effigy ​ i.A) or approximately cylindrical (or radial) body locomoting with lateral or vertical undulation is able to increase A, which will be generated by a section of its body opposed to the direction in which it wants to move. The area of this surface is given approximately past the production of the diameter and the length of the body portion in question (and of class by its angular orientation to the centrality of translation). If its lateral drag coefficient (c) is greater than the frontal, then when the beast turns its body tin can also increase c in the equation. Nonetheless, regardless of the relationship between the anterior and lateral c, if the product (c A) from the lateral view is greater than that from the frontal 1, this body will be able to move forward also equally to modify management.

Schematic representation of a cylindrical (A) and two bilateral bodies (B and C) generating pushing surfaces while changing direction. Denser grids indicate a greater drag strength.

A bilateral torso (Effigy ​ 1.B and C) can alter both coefficients A and c as well. Since it has merely i plane of symmetry (in the principal direction of the motion), vertically information technology tin can carry structures with an extended surface expanse. The lateral area (A) of the body will be further increased by these structures (but call back of the vertically posed fins of a shark). Furthermore, equipped with these, and with the more or less flattened sides of the body, the animal tin can also profoundly increase c. Since it is streamlined but from the frontal view, its lateral (or vertical if the animal is dorsoventrally flattened) drag coefficient is very loftier compared to the frontal one.

The flatter its sides – including the appendages – are, the greater its lateral drag coefficient will be as compared to that of a cylindrical body. And knowing that a rectangular plate has an approximately fifty to 70 % higher drag coefficient (depending on the height to length ratio) than a cylinder (at Re = 10^v) [9], we tin can say that bilateral symmetry offers the evolutionary possibility of increasing F by as much every bit fifty to 70 % compared to cylindrical symmetry, thanks simply to the drag coefficient. In other words, when a hypothetical cylindrical and a bilateral body have the same A (and frontal c), the bilateral trunk will enjoy a greater reward in turning because information technology tin can produce a pushing forcefulness much greater than the cylindrical body because laterally information technology is less streamlined. Is this condition sufficient to clinch a marked evolutionary advantage for bilateral symmetry? Since this capacity offers a very constructive locomotion with potentially excellent manoeuvrability, we suggest that it is. Otherwise nosotros would take to argue that effective locomotion is not a great advantage for an organism for whom a bones characteristic is precisely locomotion. Compared to a bilateral body, the cylindrical form has lower resistance in sideways motility, so the cylindrical torso "slides" laterally in changeovers, equally we do when we attempt to change direction on ice.

One could argue that a bilateral body can manoeuvre well only in left-right directions while a cylindrical body can, in theory, turn in every direction away from that of the move. Bilateral animals, being non rigid objects (like ships or aircraft are), solve the problem simply past twisting the body and the appendages in the desired directions.

Based on the arguments explained and so far information technology could be stated that a symmetry that is streamlined in but ane direction, while non-streamlined in other directions, is favourable for manoeuvrable locomotion.

It is important to say that the changeover does not necessarily accept to be drag-assisted. Some radially symmetrical animals, such as jellyfish, use asymmetric contractions of the bong, thus generating asymmetric jet flows to steer. However, the accuracy and the speed of this medusan-blazon manoeuvring [12] are much more than modest than the drag-based manoeuvring of bilateral pelagic animals.

Bilateral symmetry has as well proved to be succesful both on land and in the air. On land, the force-generating office of the drag in h2o is replaced by gravitation and and then by the necessity of leaning on the state. In this regard, locomotion on land is coordinating to that on the fluid–solid interface. This locomotion essentially occurs in ii dimensions, thus, direction shift on country requires the body to be capable of turning left or right, and so of existence supported from the right as well every bit from the left. The effectiveness of creeping locomotion has been improved past the evolution of limbs, which, placed on the two sides of the bilateral torso, satisfy the above-mentioned condition. (For the sake of simplicity, we will not deal with the limbless evolution of snakes and limbless lizards here.)

Flying, similarly to swimming, requires the fauna to create pushing surfaces in the air. The evolution of big-surface wings immune the animals to locomote in a medium which, compared to water, has a lower density, and every bit a consequence, is almost completely defective in the hydrostatic pressure that to a certain extent counterbalances the force of gravity in water.

The combination of bilaterality with the centralisation of the nervous arrangement and cephalisation immune the evolution of really successful body plans ensuring precise locomotion and rapid information processing.

Implications of the hypothesis

Radial symmetry

From the principles adult so far it follows that asymmetry or radial symmetry could have evolved only in animals which do not locomote or locomote slowly. (We use the term 'tiresome' intuitively because an exact speed limit depends on the beast'due south mass, form, and size as well equally on the mode of locomotion and on the speed of h2o flow around information technology. Hence it would vary from species to species. We hope that the understanding of the essence of this concept will not be disturbed by the absence of a clearly divers value.) Past sacrificing quick locomotion, these animals necessarily go more vulnerable to predators. Thus they accept to be well protected (e.chiliad. echinoderms, cnidarians), and this protection tin can be farther strengthened by being of very low nutritive value (sponges, ctenophores, cnidarians).

The radial body symmetry will be ideal for these animals because it confers on the body the ability to react to environmental forces in every direction (sessile cnidarians and echinoderms), to be able to catch food around with the same probability (cnidarians, ctenophores, echinoderms) and to maintain a static position, adhering to the substratum against water currents (locomoting echinoderms) [1,x,13].

At that place is also another evolutionary situation in which the torso has to be externally cylindrical: a burrowing lifestyle. This lifestyle develops when an animal lives and locomotes in a very dense medium: in earth or equivalents and in the body of other organisms. In these media the density is then high that whatsoever lateral structures which increment surface area will be disadvantageous. And so the external body form will be the 1 that assures a minimum cantankerous section and hence a minimum friction per body mass; and at the same time also reduces the vulnerable body surface. This form is cylindrical. Naturally, this does not necessarily mean that externally bilateral burrowing animals cannot exist but, even in this case, their main body form tends to exist nearly cylindrical and the surface-augmenting effects of limbs must be counterbalanced past their burrowing or other functions (due east.k. subterranean rodents; ref. [14]).

Here it is crucial to note that animal body symmetry is often dissimilar externally and internally [1,iii]. It is enough to consider that the external side of the brute interacts directly with the environment while the internal side does not. Hence they face very unlike weather condition that may require unlike symmetries.

Apparent problems with the association of symmetry and locomotion

Since those animals which are non bilaterally symmetrical are typically sessile or planctonic drifters, while most bilaterals are free locomoting, the clan of bilateral symmetry with directed locomotion seems obvious. Beklemishev [2] pointed out that when the body is asymmetric, as it reaches a certain speed, rectilinear locomotion becomes incommunicable and the trunk begins to movement in a helical trajectory. Equally he explains, the advantage of bilateral symmetry is precisely that the ecology pressures on the two sides of the torso are equalized, guaranteeing a rectilinear locomotion. Following this view, the shut association betwixt free pond and bilaterality has also become widespread in textbooks (e.g. refs. [fifteen-18]). However, it could likewise be due to the lack of an acceptable caption for this – otherwise widely accepted – relationship that several authors have questioned it. It has been hypothesized that the origin of bilateral symmetry in animals could accept been favoured by internal transport, not by directed locomotion [19]. Based partly on this view, information technology seemed problematic to couple the tetraradial symmetry and the active locomotion of the endoparasite cnidarian Buddenbrockia, so a farther dissociation of symmetry from locomotion has been proposed [20]. It has also been reported that the bilateral body class [21] and the bilateral spine distribution [22] of body of water urchin species was connected to efficient body protection, non to efficient locomotion.

Based on the concept presented hither information technology can be understood that the cylindrical external form and the internal tetraradiality of Buddenbrockia is not inconsistent with its active locomotion [twenty], and that the irksome locomotion of a sea urchin does not take to be closely related to its bilateral trunk course [21] or its bilateral spine distribution [22].

Another potential question may sally if ane examines the earliest trace fossils from the Precambrian. These traces are retained horizontal burrowings in the upper layer of the sediment [23,24] and are also attributed to bilaterian animals [24]. Even so, this view has been challenged by the discovery of trace maker behemothic protists [25], put forwards as candidates for the producers of those ancient trails. Now, co-ordinate to our hypothesis, it seems easy to reconcile the putative burrowing behaviour and bilaterality in the precambrian animals mentioned above (if they really existed) considering that the upper layer of the sediment is likely to take a loose structure with low density, hence it does not necessarily require the body burrowing in it to be cylindrical.

Conclusions

It has been suggested that radial and bilateral body plans could have been generated with the same or like genetic toolkit but with different regulatory networks [8,26-28]. This means that most likely there was no genetic barrier to the emergence and evolutionary competition of the two body plans. Whatever the case, we debate that this competition was strongly adamant past the physical laws of locomotion.

Hither, we practice non consider the temporal priority of radial or bilateral symmetry in early animal development (just come across refs. [i,5,6,nineteen,28,29]), and similarly, we do not take a stand up on the lifestyle of the first bilaterians [8,19,30,31]. Nosotros but country that, from the moment bilateral symmetry arose in macro-fauna evolution information technology represented a potentially enormous selective advantage over other body plans assuring faster changeovers and a more precisely directed locomotion. This is a fundamental to survival both for prey and for predators.

At the organization level, these considerations mean animal evolution should be viewed as a strongly channelled procedure (cf. ref. [32]) – a product of which is an enormous variation of bilateral organisms rather than "countless forms" [33] of living systems.

Very probably, other key selective forces also influenced and influence the evolution of basic beast body plans. All the same, these factors are nonetheless to be explored. Nosotros advise i of them – one that may in itself be enough to favour bilateral symmetry against other symmetries.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

M.H. conceived the idea and formulated the chief lines of the theory. M.N. pointed out the importance of changeover in locomotion and provided important elements of the simplest physical explanation. 1000.H. elaborated the final version of the theory and wrote the paper. All authors read and canonical the final manuscript.

Reviewers' comments

Reviewer one: Gáspár Jékely

This is an interesting analysis providing a physical explanation for the maintenance of bilateral symmetry in animal development. I find the paper well written, the arguments convincing, and only have a few comments to clarify the discussion.

The authors refrain from discussing locomotion in the microscopic earth. However, I think that they miss an opportunity hither. We know that in the micro earth many organisms tin navigate very efficiently. They accomplished this not by being bilaterally symmetrical, but by using helical swimming and the aligning of the helical trajectories. This happens very ofttimes in diverse phototactic protist (eastward.g. Chlamydomonas, dinoflagellates) and in the close-to spherical ciliated larvae of bilaterians (e.g. annelids, hemichordates). One reason why this is an effective strategy for small organisms but not large ones is, as discussed in the paper, the different Reynolds numbers. This could imply that bilaterality just evolved once the early on metazoans had attained a sufficiently large size. This interesting physical threshold could be discussed in more detail.

Authors' response: First of all we give thanks Dr. Jékely for his invaluable work. The idea that bilaterality evolved when the animals at mitt had reached a certain size (e.m. Valentine Proc Natl Acad Sci U.s. 1994, 91: 6751–6757) cannot be proved by convincing bear witness at nowadays (see for example Chen et al. Science, 305: 218–222 – although this is controversial [east.g. Chapter 1 by Budd GE in Beast Development: Genomes, Fossils, and Trees edited by Telford MJ, Littlewood DTJ, 2009]). The fact that several small animals, living in the realm of depression Reynolds numbers, have bilateral symmetry probably indicates that bilaterality could have evolved in the microscopic globe, but well-nigh likely information technology does not offer the kind of advantage over other symmetries in that location every bit it does in the macro world. Until information technology tin can be excluded that certain factors could favour bilateral symmetry in the micro world, we would rather avoid taking a stand on this – otherwise exciting – evolutionary problem. Please run across also response Nr. 1 to Dr. Aravind.

The model implies that the first bilaterians were not burrowing but either freely swimming or crawling on the sediment. It would be interesting to see a discussion of this in the context of the earliest putatively bilaterian trace fossils. The estimation that these are traces of burrowing animals may be slightly at odds with the hypothesis (e.g. Jensen Integr. Comp. Biol., 43:219–228).

Authors' response: Given that the precise origin of the get-go bilaterians is unproven we would not like to accept sides on their lifestyle; yet, the topic of the trace fossils, even so highly controversial, is very interesting. The following paragraph has been added to the "Apparent problems with the clan of symmetry and locomotion" section:

"Another potential question may emerge if one examines the earliest trace fossils from the Precambrian. These traces are retained horizontal burrowings in the upper layer of the sediment [23,24] and are also attributed to bilaterian animals [24]. Still, this view has been challenged by the discovery of trace maker giant protists [25], put forward as candidates for the producers of those ancient trails. Now, according to our hypothesis, it seems like shooting fish in a barrel to reconcile the putative burrowing behaviour and bilaterality in the precambrian animals mentioned above (if they really existed) because that the upper layer of the sediment is likely to have a loose structure with depression density, hence it does not necessarily require the body burrowing in it to be cylindrical."

Accordingly, the cited references as well have been added to the paper.

I suggest to also include in Figure 1. a bilaterian with a cylindrical body simply with lateral appendages. A laterally flattened, fish-like body is not full general for actively moving bilaterians. For case, errant annelid polychaetes accept a trunk that is roughly cylindrical, but they take lateral appendages that tin provide the necessary drag during active locomotion.

Authors' response: The figure has been added as Figure i.C.

The authors use the term 'aerodynamic' when writing about locomotion primarily in water. Using 'drag' or 'resistance' may be more fortunate.

Authors' response: The word "aerodynamic" has been replaced by more appropriate ones.

The sentences in question at present read: "… where F is the drag forcefulness, ρ is the density of the medium, c is the dimensionless drag coefficient dependent on the body shape, A is the surface area of the maximal department of the body in the direction of motion, and five is the body'south velocity [9,10]."

"A spherically symmetrical body cannot generate the pushing surface, beingness of equal shape and drag in every management."

"Since it is streamlined only from the frontal view, its lateral (or vertical if the animal is dorsoventrally flattened) drag coefficient is very high compared to the frontal 1."

"Compared to a bilateral trunk, the cylindrical course has lower resistance in sideways motility, so the cylindrical body "slides" laterally in changeovers, as we practice when we try to change direction on water ice."

The statement that "Bilateral symmetry with 2 body axes arose … in ho-hum, flatworm-like organisms" together with Ref. [7] seems to imply that the first bilaterians were phylogenetically related to acoel flatworms. The latest careful phylogenetic analyses evidence that acoels are deuterostomes (Nature 470, 255–258), so they practice not correspond the earliest extant bilaterian metazoans. I suggest to write 'worm-like organisms'.

Authors' response: Information technology has been inverse to "flat, worm-like organisms"; remaining, at the same time, preferably true-blue to the cited reference.

A reference to jellyfish navigation (eastward.g. Garm et al. The Journal of Experimental Biology 210, 3616–3623) would make the give-and-take about the medusa-type manoeuvering more convincing.

Authors' response: Thank you lot, the reference has been included.

Reviewer 2: L. Aravind

Since Beklemishev it has been more often than not accepted amongst students of zoology that the advantages of directed movements are the driving strength for the origin and maintenance of bilateral symmetry, the ascendant class of symmetry amidst metazoans. It is unremarkably imagined that such this symmetry emerged in benthic contexts – creeping on a substratum enable favored dorso-ventral differentiation, which coupled with choice for constructive directed motion resulted in a bilateral form. However, this has been questioned on the basis of the observations of the asymmetric expression of TGF-beta family members and Short gastrulation orthologs along the directive axis in cnidarians. This implies that even if the ancestral metazoan was outwardly radial symmetric, in that location might have been a pre-accommodation or pre-disposition for bilaterality as suggested by the situation in cnidarians. This has been used by Finnerty to argue for a role for internal circulation within the gut lumen as a major gene in the origin of bilateral symmetry. The molecular prove on the whole favors a unmarried major origin for bilaterality in animals, only is subsequent strong maintenance remains less explained.

Hither the authors nowadays a uncomplicated concrete explanation as to why bilaterality is a more stable strategy than whatever other symmetry once directed locomotion emerges. Cardinal to their explanation is its role in maneuverability that has plain not been used as washed by the authors in this article. The physical arguments by the authors betoken to bilaterality beingness an apparently stable strategy at large Reynolds number where the equation used by them for drag forces is appropriate.

However, the main question that arises how large should be the Reynolds number be for this argument to hold. Looking up these values it appears that an unicellular free-pond eukaryotes might accept Re = 10^-1. Here the gummy forces are probably dominant, allowing for the asymmetric morphology of such forms, e.g. ciliates and dinoflagellates. The smallest vertebrate is said to accept Re ~ 1 and bilateral metazoans similar chaetognatha and rotifer have Re in between those figures. It should exist noted that they take strongly bilateral forms. Further, the estimate sizes for the basal bilateralians do not place them much college than this range in terms of Re. And then the central question that arises is whether at these sizes the argument based on negligible viscous forces is entirely valid. It would be good for the authors to consider this issue and present potential tests for their hypothesis.

Authors' response: Nosotros thank Dr. Aravind for his valuable piece of work. The fact that bilateral symmetry is also present in the environment of low Reynolds numbers does not necessarily contradict our hypothesis. The very dissimilar pattern of primary trunk symmetries between low and high Re environments probably emerges from their different relations to viscous and inertial forces. Just this departure does not necessarily exclude bilateral symmetry from the modest-scale globe – although apparently it does not relish the advantage over other symmetries which it does in the large-calibration globe. Our hypothesis implies that bilateral symmetry is advantageous in the high Re-world but this does not mean bilaterality could not have been favoured by certain factors in the low Re-world. However, these factors – to our knowledge – have not been antiseptic since Beklemishev. Please run into likewise response Nr. i to Dr. Jékely.

Minor points

<<Bilateral symmetry with two torso axes arose early in animal development, probably in slow flatworm-like organisms locomoting on a substrate [two], probable prior to the Cnidarian–Bilaterian split up [3-vi] in the Precambrian [7,viii].>>

This sentence is potentially confusing, because it might nowadays a contradiction within information technology. The authors demand to analyze as to what they hateful by origin of bilateral symmetry prior to the origin of Bilateria (i.due east. the flatworm like organisms). Are they meaning the situation in cnidarians or reconstructing some bequeathed grade?

Authors' response: Give thanks you lot, this part has hopefully been clarified, and it reads now:

"Bilateral symmetry with two trunk axes arose early on in animal evolution, probably in slow, flat, worm-similar organisms locomoting on a substrate [2]. Genetic analyses have ended that the genes responsible for bilateral symmetry most likely appeared prior to the cnidarian–bilaterian split [3-6], in the Precambrian [7,eight]." <<"a symmetry that is streamlined in only 1 direction, while non-aerodynamic in other directions, is favourable for locomotion." >>

May be the final word should be replaced with maneuverable locomotion.

Authors' response: The word "manoeuvrable" has been inserted in the judgement, thank you:

"a symmetry that is streamlined in just ane direction, while non-aerodynamic in other directions, is favourable for manoeuvrable locomotion."

Reviewer three: Eugene Koonin

The authors of this manuscript strive to provide an caption for the domination of bilateral symmetry in animals. The come with the idea that bilateral symmetry provides for by far greater ability to swiftly alter the direction of move than any other torso program, hence a substantial reward for free moving animals. The authors submit that this is a major factor backside the near ubiquity of bilateral symmetry but they are careful in indicating that other factors could exist important equally well. In my view, this is an interesting and sensible hypothesis although I remember that it would gain in forcefulness should the authors coach their hypothesis in specific equations of mechanics.

Authors' response: We give thanks Dr. Koonin for taking charge of the review of the manuscript. While formulating the hypothesis we constantly endeavoured to provide the simplest explanation for the problem in the clearest way. We retrieve the bones statements (e.g. the torso has to overcome drag; to push button itself in a new direction it has to do a force in the opposite direction) and the equation of drag with some other minor considerations are necessary and sufficient arguments for the caption of the theory, but should it require a more than detailed rationale we would exist grateful for more specific instructions. Delight too consider that this is the main hypothesis that may serve equally a basis for more specific future analyses for particular scenarios.

Acknowledgements

The authors thank Gáspár Jékely, Aravind Lakshminarayanan Iyer and Eugene Koonin for their effective reviews. The authors are grateful to Balázs Papp (Evolutionary Systems Biology Grouping, BRC, Szeged), to Andrew H. Knoll (Harvard Academy) and to Bert Hobmayer (University of Innsbruck) for their insightful comments and the wide-ranging communication they gave. The authors as well thank Borbála Holló and George Seel for checking the English; and Ottó Jász for the preparation of Figure ​ 1. The technical aid provided by Zsófia Haase and Róbert Juhász was likewise much appreciated. G.H. thanks Zoltán Varga and Paolo Saiello for inspiration; Péter Csermely, György Székely, László Végh, Zsolt Karányi, Lóránt Székvölgyi, Imre Holb and Márton Miskei for encouragement. G.H. dedicates this commodity to the memory of István Holló and Zsófia Gárdos-Tóth.

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Articles from Biological science Directly are provided here courtesy of BioMed Central

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Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3438024/

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