7/29/2019 Muscles dual origins

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E V O L U T I O N A R Y B I O L O G Y

Muscles dual originsJllyfish mov using a s of muscls ha look makably simila o siadmuscls in vbas. Howv, nw daa show ha h wo muscl ys conaindiffn molculs, imlying ha hy volvd indndnly. See Letter p.231

A N D R E A S H E J N O L

Jellyfish engage in a unique mode of loco-

motion. They pump their umbrella-shapedbodies through a cyclical series of expan-

sions and contractions, which requires mus-culature every bit as impressive as that used byan Olympic sprinter to break the latest worldrecord. In fact, jellyfish muscles look almostidentical to the striated muscle found in ver-tebrates, and for a long time many biologistshave thought that the two shared an ancientevolutionary origin1,2. On page 231 of thisissue3, Steinmetz et al. present evidence thatthis intricate musculature in fact evolved inde-pendently, at least twice.

In the seventeenth century, z oologist

Antonie van Leuvenhoek dissected a cod fish

under his newly developed microscope andnoticed that the animals muscles consist ofa complex set of interwoven fibres. His sub-

sequent report of these observations becamethe first description of striated muscle. Modernstudies have gone on to show that each striatedmuscle fibre is constructed of subunits calledsarcomeres which, under the microscope, aredemarcated by optically dense vertical discs(Z-discs). The fibres consist of an intricatearrangement of thick and thin filaments ofmyosin and actin proteins, respectively. Thegenes encoding myosin and actin are justtwo of more than 100 genes involved in thedevelopment of striated muscle in vertebrates.

Skeletal muscles, which make up around50% of our body weight, are striated, but

so too are cardiac muscles, which control

would reverse the voltage and the magneticfield. The fact that the voltage did not changesign upon magnetic-field reversal implies thatthe mirror symmetry must be broken, evenat zero magnetic-field strength. This sym-metry breaking is probably associated withthe particular crystallographic orientation ofthe semiconductor (the arrangement of thematerials atoms within the sample) chosenby the authors, although no detailed studyof this orientation dependence is reported intheir paper.

There is a price to pay for not using a magnet.Jaworski et al. obtained a sizeable effect only atlow temperatures: the spin Seebeck voltage at amagnetic field of 2.7 tesla becomes very smallat temperatures above 40 kelvin. The authorsrelate the maximum temperature at which theeffect is observed to the Zeeman energy. Thisdescribes the energy difference between spin-up and spin-down electron states in a magneticfield. It increases with the fields strength andis especially large in InSb. One might think,therefore, that increasing the magnetic fieldwould increase the temperature range overwhich the effect can be observed. However,

the magnetic-field dependence of the effect iscomplex, as seen in both the strength and eventhe sign of the measured spin Seebeck coeffi-cient. It is clear that more research is needed toobtain a plausible theoretical understanding ofthese measurements.

It is interesting to remark on a recurringparallel between thermoelectricity and topo-logical phenomena in condensed matter,which have been under intense scrutiny overthe past several years. Curiously, recently dis-covered7 three-dimensional topological insu-lators, which insulate electric current in theirbulk but conduct it on their surface, are based

on alloys that have been known for decades to

properties with thermoelectricity could workto mutual advantage for elucidating the formerand using the latter.

Jaworski and colleagues use of a magneticfield instead of a magnet shows that the spinSeebeck effect is more general than previouslyenvisaged. It remains to be seen whether, ulti-mately, a magnetic field is needed at all: for

example, linking the spin orientation to thedirection of electron motion in the surfacesof topological insulators10, combined with abroken crystallographic symmetry, might besufficient. Who will take up the challenge?

Tero T. Heikkil is at the Low TemperatureLaboratory (OVLL), Aalto University, FI-00076Aalto, Finland. Yaroslav Tserkovnyakis inthe Department of Physics and Astronomy,University of California, Los Angeles,Los Angeles, California 90095, USA.e-mails: tero.heikkila@aalto.fi;yaroslav@physics.ucla.edu

1. Jaworski, C. M., Myers, R. C., Johnston-Halperin, E.& Heremans, J. P.Nature487, 210213 (2012).

2. Seebeck, T. J. Magnetische Polarisation der Metalleund Erze durch Temperatur-DifferenzAbh. Preuss.

Akad. Wiss.265373 (182223).3. Uchida, K. et al. Nature455, 778781 (2008).4. Kimura, T., Otani, Y., Sato, T., Takahashi, S. &

Maekawa, S. Phys. Rev. Lett.98,156601 (2007).5. Uchida, K. et al. Nature Mater.9, 894897

(2010).6. Jaworski, C. M. et al. Nature Mater.9, 898903

(2010).7. Chen, Y. L. et al. Science325, 178181 (2009).8. Wilczek, F.Nature Phys.5, 614618 (2009).9. Mourik, V.et al.Science336, 10031007 (2012).10. Hsieh, D. et al. Nature460, 11011105 (2009).

Spincurrent

Detector

Solenoid

Temperaturegradien

t

Magneticeld

Voltage

Non-magneticsemiconductor

Electron

be among the best thermoelectric materials.In a reverse twist, not long after InSb was usedin the search for signatures of exotic Majoranafermions (particles that are their own anti-particles) in topological superconductors8,9,we are now informed of its remarkable spinSeebeck characteristics. The culprit in both thetopological and spin Seebeck properties is thestrong spinorbit coupling. It is thus difficultto resist speculating that marrying topological

Figure 1 | Spin Seebeck effect driven by a magnetic field. Jaworskiet al.1 observed the spin Seebeck effectin a non-magnetic semiconductor made of indium antimonide, InSb. The authors placed the system ina large magnetic field that has a direction parallel to an applied temperature gradient; in this illustration,the field is produced using a solenoid. Owing to the large field, more electrons have spins (black arrows)that align parallel with the field than antiparallel to it. This strong spin polarization, combined with thetemperature gradient and other properties of InSb, generates a spin current that can be detected as a voltage.

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7/29/2019 Muscles dual origins

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heartbeats. Another type of muscle, smoothmuscle, also consists of thin actin and thickmyosin filaments, but these are not organ-ized into sarcomeres as in striated muscle.Smooth muscle is the predominant muscletype in the gut, bladder and blood vessels. It iswidely believed that both smooth and striatedmusculature were present in the last commonancestor of bilaterally symmetrical animals(bilaterians), which includes most animals,and jellyfish (cnidarians), which have radialsymmetry. This is because jellyfish possessmuscles that look nearly identical to verte-brate striated muscle when imaged by fluores-cence microscopy (Fig. 1a) or high-resolutionelectron microscopy2.

Steinmetz and co-workers investigated theevolutionary history of musculature in moredetail. They searched the fully sequencedgenomes of 22 animal species for 47 human

muscle genes. The species included not onlyanimals that have muscle, but also animalsthat lack muscle, such as sponges, and unicel-lular non-animals both of which are evenless closely related tovertebrates than are jel-lyfish. By exploring the genomes of a widearray of species in an evolutionary framework,the authors were able to infer when individualmuscle genes arose. They found that some ver-tebrate muscle genes were present in unicellularnon-animal species, implying that these genespredate the origin of multicellular animals. Sur-prisingly, however, they did not find the genesthat code for some of the most fundamental

components of vertebrate striated musculature

such as the proteins troponin and titin ininvertebrate species that possess striated mus-cles, including jellyfish.

These results led the authors to investigatethe expression of human muscle genes in someof the most distant animal relatives of verte-brates, including two sponges (animals withno musculature), a sea anemone (an animalrelated to a jellyfish, but which lacks striatedmusculature) and a jellyfish. Their most strik-ing finding was that only one of the vertebratestriated muscle genes was expressed in jellyfishmuscle. These results suggest that, despite theirremarkable physical resemblance, the striatedmuscles of jellyfish and humans are con-structed using a vastly different set of genes.Steinmetz and colleagues have revealed anextraordinary instance of convergent evolu-tion the evolution of highly similar traits indistantly related organisms (Fig. 1b).

So how often did striated musculatureevolve in animals? To address this question,similar studies will need to be conductedusing animals with striated musculature thathave close relatives lacking such muscula-ture. One obvious candidate is a member ofthe comb jellies (ctenophores), which areamong humans most distant animal rela-tives4. Although most comb jellies use onlysmooth muscles, one species, Euplokamis,has a specialized, coiled feeding structurethat it flicks at high velocities by contrac-tion of striated muscles5. Could it be thatthe molecular fingerprint of the striated

muscles ofEuplokamis will confirm yet

another case of convergent evolution?Steinmetz and colleagues study also raises

another intriguing question: how many otherclaims of cell-type similarity that have beenbased on detailed morphological studies will beoverthrown by new molecular data? Anotherexample of this lies in the presumed evolution-arily ancient split of invertebrate and vertebratephotoreceptor cells, which has been proposedon the basis of the cells structural differences.However, the recent discovery of a vertebrate-type photoreceptor in the eyes of larvae of alamp shell (brachiopod) an invertebrate challenges previous thoughts about the evolu-tionary origins of these cell types6.

The coming years will almost certainly bringanswers to these questions, as genome miningand gene-expression analysis are extended toother evolutionary lineages. In the meantime,Steinmetz and colleagues have taught us a

familiar lesson that which looks similar innature is not necessarily similar by descent.

Andreas Hejnol is at the Sars InternationalCentre for Marine Molecular Biology,University of Bergen, 5008 Bergen, Norway.e-mail: andreas.hejnol@sars.uib.no

1. Schmidt-Rhaesa, A. The Evolution of Organ Systems(Oxford Univ. Press, 2007).

2. Seipel, K. & Schmid, V. Dev. Biol.282, 1426 (2005).3. Steinmetz, P. et al. Nature487, 231234 (2012).4. Edgecombe, G. D. et al. Organisms Diversity Evol.11,

151172 (2011).5. Mackie, G., Mills, C. & Singla, C.Zoomorphology107,

319337 (1988).6. Passamaneck, Y. J., Furchheim, N., Hejnol, A.,

Martindale, M. Q. & Lter, C. EvoDevo2,6 (2011).

Figure 1 | Movers and shakers. a, The striated muscles in the cod fishGadus morhua and the jellyfishAurelia aurita show a very similarpattern when observed under a fluorescence microscope after stainingwith a dye that binds to the muscle protein actin. (Photos: B. Vellutini,Sars Centre). b, This evolutionary tree depicts the genetic relationships4between the animal lineages studied by Steinmetz and colleagues3. Theirresults show that striated muscle cells (indicated by the striated muscle

symbol) have evolved independently (stars) in both the cnidarian (jellyfish)lineage and the lineage that leads to the protostomes and deuterostomes,which includes all bilaterally symmetrical animal species. Striatedmuscle is absent in most other animal groups, except the speciesEuplokamis. Whether this comb jellys striated muscle is related to thatof jellyfish or vertebrates, or represents another convergent evolution event,remains to be determined.

a bCod

Jellysh

Deuterostomia(sh, frogs,urchins)

Protostomia(insects,snails)

Cnidaria(jellysh)

Placozoa(Trichoplax)

Porifera(sponges)

Ctenophora(comb jellies)

Choanozoa(unicellularagellates)

?

Only Euplokamis

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