A Worm Seeks Its Midpoint

Artists and architects of all eras have been inspired by symmetry in nature. This is hardly surprising, as symmetry is considered the epitome of beauty – and mirror symmetry is the absolute gold standard. Jochen Rink from the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden is seeking to discover how organisms define the mirror plane and thereby fulfill the basic prerequisite for a symmetrical body structure. To do this he studies flatworms and their astonishing ability to regenerate missing body parts.

Planarians are almost impossible to kill – at least by chopping them into pieces. A complete worm can form from even the smallest parts; all that is required is the formation of a new body axis. FOCUS_Symmetry

TEXT CATARINA PIETSCHMANN

hile most of the worms with their red wing cases with flashy try during regeneration – regardless of sit motionless at the black dots. We don’t need all that. Give whether the original animal was divid- bottom of their cul- us something else instead: eternal youth! ed into two pieces or twenty. For exam- ture dish, a few do lei- For the price of eternal youth, some ple, strips of tissue that are cut from the surely rounds through flatworm species even forego sex. The side of a flatworm initially curl up into theirW pool. With barely perceptible species that reproduce asexually are a tight corkscrew spiral and develop movements, they glide elegantly on practically immortal, because every into completely independent organ- thousands of microscopic cilia across tenth cell in the worm body is actually isms in a matter of days. This distin- the bottom of the plastic container they a stem cell that can generate all of the guishes flatworms from other regener- call home. This would be quite a majes- different cell types indefinitely. For this ation specialists, such as the axolotl: tic sight were it not for their large and reason, planarians self-renew constant- although these salamanders can regen- rather comical eyes. Planarians squint ly. For every dividing stem cell, a spe- erate missing body parts, organs and and look, not ahead, but rigidly upward. cialized cell dies, so the shape and size even parts of the brain, the cut-off piece Over 60 species of these flatworms, of the worms remains unchanged. never develops into a new animal, but which are all indeed very flat, inhabit a rather perishes. specially reserved breeding room here ONE BECOMES TWO Something the two animals do have in Dresden. They were collected from in common, though, is the symmetry all over the world and include some To reproduce, all they have to do is of their body plans. The foundations of very rare eyeless specimens from caves, hold on tight with the tip of their tail symmetry are already laid in the early and others that have a string of eyes while the head moves on. “After a few embryonic stages. Also the process of along the edge of their bodies. Flat- hours, the body tears apart and two regeneration in planarians must ensure worms are usually between 0.8 and 2.5 complete worms regenerate from the that the resulting body is symmetrical, centimeters in length, but there are also two pieces in just two weeks,” explains or that the new parts re-complete the some veritable giants that measure one Jochen Rink from the Max Planck Insti- symmetry of the existing body. “This meter in length. They usually come in tute in Dresden. This makes planarians so-called bilateral symmetry – two mir- different shades of brown or black, but one of the most sought-after test ob- ror-image body halves – is a basic struc- some also have distinctive dots or jects in stem cell research. Their ex- tural principle of higher organisms,” stripes. They are easy to care for in the traordinary regeneration capacity may says Rink. laboratory and pounce on the bits of hold the key to eternal youth and to Bilateral symmetry requires, first, an calf liver or mealworms fed to them in cures for many illnesses. axis of symmetry, known as the mid- their plastic boxes. But there is something else about line. But how does an organism actual- Given their bizarre appearance, one flatworms that fascinates Rink: not ly define a line? It is already known that might almost think they made a deal only can they build a complete organ- evolution has come up with different with nature millions of years ago: Let ism from any random tissue piece, they ways of doing this. In vertebrates, for

Photo: Eduard Solà/CC-BY-SA-3.0 others be beautiful, like the ladybirds even manage to restore perfect symme- example, the dividing cells of the early

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Photos: Max Planck Institute of Molecular Cell Biology and Genetics (4) ly halves of the future embryo. “Top”ly halvesofthefutureembryo. which correspondto the back and bel in thecellsdefine“top”and “bottom,” er cigar-shaped.Signalingsubstances aren’tIts embryos spherical,butrath - ferent approach to midline formation. voilà –thelineiscreated. cells migrateinoppositedirections,and thus: apointisdefinedfromwhichthe the right time. In short, nature proceeds by switchingcertaingenesonandoffat networks inthecellscontrolthisprocess tail form at itsoppositeends.”Signaling midline of the body, andthe head and “The reversalpointlaterturnsintothe MIDLINE STARTING POINTFOR THE cell layers. duces ahemispherecomposedoftwo flow ontheinsideofhat.Thispro- nizer, and marches back up against the at adefinedpoint,theso-calledorga subset ofthesecellsreversesdirection the yolkmasslikeatight-fittinghat.A migrateandgraduallyenvelop embryo head. the of instead formed is tail complete a (right), hedgehog much too is there If tail. no but grows, head normal availableenough hedgehog (center), a not is there If (green). strands intestinal two with tail the and (blue) brain a with head a forms usually (left) piece middle A tissue. lost regenerate to pathway signaling page PlanariansThis need the hedgehog (bottom). containers these in rapidly very reproduce can They right). (top Dresden in Institute Planck Max the of laboratory the in containers household plastic standard in live They them. for much do to have doesn’t left) (top Rink Jochen changes, water occasional and feeding from Apart page Left The fruitflyDrosophila takesadif- Planarians are easy to care for: for: care to easy are Planarians - - quences that arise when the formation quences thatarisewhenthe formation dentally discoveredthedramatic conse- in fliesandhumans. as Slit isalso active along the midline nisms throughout the animalkingdom, ably uses the same molecular mecha multiple types.Here,too,natureprob- fined, notbyasinglecelltype,but mals’ belly side. Thus, the midline is de- from theheadtotipalongani- er V-shaped stripextending alltheway along theback,andSlitmarksabroad- BAMBI isactiveonlyinanarrowstrip located exactly along the midline: both of whichareactiveonlyincells here includedtheBAMBIandSlitgenes, midline. The genes they encountered for genesthatareactiveonlyatthe and hiscolleaguesstartedbylooking emerging flybody. comes partofthemidline line alongtheback,whichagainbe signals in the endis a clearlydefined stronger, allthatremainsofthetop other, but because the belly signals are and “bottom”signalssuppresseach In theearly1960s,biologists acci And whataboutplanarians?Rink - - - signal wasonlyslightlyraised, worms place ofthehead.Andif hedgehog fine, but the pieces regenerated a tail in regeneration ofthetailwas perfectly the hedgehogsignalwaspresent, the was severelytruncated; if too muchof fect newheaddeveloped,butthetail too littleofthehedgehogsignal,aper- regenerating pieces.Ifthepiecehadfar the off the head and tail, and observed hedgehog signal in the worm body, cut his colleaguesvariedthestrengthof similar happens,”reportsRink.Heand ing pathway in planarians, something normally formedlegs. which diedsoonafterbirth,hadfour ited tothelambs’heads.Theanimals, ic cells.Surprisingly, thedefectwaslim- - hedgehog signaling protein in embryon toxin thatinhibitsrecognitionofthe eaten corn lilies. These plants contain a their head.Itemergedthattheeweshad thology, theireyesatinthemiddleof ifornia. LiketheCyclopsinGreekmy- eye werebornonafewmeadowsinCal tions. Deformedlambswithonlyone inanorganismmalfunc- of symmetry “If wedisruptthehedgehogsignal- 3 | MaxPlanckResearch 16MaxPlanckResearch FOCUS_Symmetry 21 - FOCUS_Symmetry

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with stunted and eyeless heads or, line. If the head isn’t fully formed, imal and serves as both the mouth and again, one-eyed Cyclops worms, regen- problems with the midline arise, and the anus. The midline doesn’t extend erated. Except that in the case of the result in one-eyed animals.” Clearly, to the tip of the tail until the digestive lambs, the one-eyed lambs had too lit- the signaling systems that designate the tract is fully formed. The cells seem to tle of the hedgehog signal. midline are so finely tuned that they operate like a construction team that don’t tolerate any kind of disturbance. completes the work at one location be- NEURAL PATHWAYS FORM A LOOP In order to track how a new midline fore moving on to the next. forms, the researchers then cut narrow The researchers then eagerly turned strips from the sides of some worms SIGNALING SUBSTANCES their attention to the genes that mark and used a green dye to visualize the ac- DETERMINE POSITIONING the midline. In the animals with stunt- tivity of the Slit gene in these strips. ed heads, the midline gene BAMBI was Immediately after cutting, the strip But how does the strip know that it was active right through to the tip of the tail, evidently contained no midline infor- originally located on the right-hand side but no longer in the head region. The mation. Just one day later, however, the of the flatworm’s body and that it is now stunted heads were also missing the first green cells appeared under the mi- missing its left side? And what signals nerve tracts, which reconnected into a croscope. “Not in the middle, but along the completion of regeneration and the loop below the head area. “So these an- the edge of the wound,” Rink explains. resulting growth arrest? “We still don’t imals don’t have a brain, just parts of the On the second day, two apparently know exactly how these processes work. peripheral nervous system.” And be- distinct populations of green cells had They may be controlled by the concen- cause the new head tissue lacks any mid- formed – one centered approximately tration of signaling substances in the line information, it is no longer bilater- halfway along the length of the strip, body,” says Rink. For example, if the ally symmetric, but rather radially and a second one right at its front end, concentration of a messenger substance symmetric, similar to a sphere or a cir- likely the precursors of the midline in were to decrease steadily from front to cle. In the one-eyed worms, in contrast, the head. Further, some green cells had back, a cell could identify its position in the BAMBI gene activity extends all the already settled beyond the new mid- the body based on the local concentra- way from the tail tip to the head, but the line, which eventually developed from tion. If a cut were to be made at the lo- nerve tracts are spaced much closer to- the wound edge. “We still don’t know cation of the cell, this information could gether than usual. where these cells come from.” After six again be utilized to instruct regeneration Rink concluded that “The midline days, the pair of eyes had formed. The of a head rather than a tail, or vice ver- targets the head and tail as end points. central population of green cells ex- sa. The body and wound edges, too, like- If these parts of the body are fully panded and became the pharynx, ly assume important roles in the process

formed, then there is a complete mid- which sits right in the middle of the an- of regeneration, as the cells from the Photos: Max Planck Institute of Molecular Cell Biology and Genetics

22 MaxPlanckResearch 3 | 16 Photos: Max Planck Institute of Molecular Cell Biology and Genetics

Photos: Max Planck Institute of Molecular Cell Biology and Genetics tion.” Theabilitytosimplysplit intwo healing atall,butthatof reproduc ians doesn’t thepurposeofwound serve regeneration capacityofplanar- dinary ually. “Sowesuspect thattheextraor- can no longer regenerate reproduce sex- other striking correlation: species that now like to find out why this is the case. different locations,” Rink says. He would ly, thishappenedseveraltimesandin pathway changedinthem.Interesting- course ofevolution.“Asinglesignaling able tobutlostthecapacityover that cannolongerregeneratewereonce scientists haveevidencethatthespecies as theircapacityforregeneration.The cies intheInstitute’s collection,aswell relationships among the flatworm spe- timeitsuffersasmalltear.”head every Rink. “Thewormshouldn’t growanew ry. “Thisisimportant,”saysJochen it canestimatetheseverityofinju- more rapidly. nal thatcompelsallstemcellstodivide worm withaneedleraisesanalarmsig- cells ofthebody, sinceprickingthe some signalsmustbeabletoreachall sue isabletorecognizethis.Moreover, back andbellyhalvesmeethere.Thetis- has formed from it again. it from formed has mirror-symmetricalworm a days, 14 After parts. missing the reformed entirely almost has day,strip tenth the the On brown). (light strip the of edge former the on formed already has tissue new of lot a days, six healed. After has cut the corkscrew.and a again in unfolds tissue Twolater,daysthe up curls initially strip the graphic), the in (gray animal complete a of side the from off cut is strip narrow a If midline: the of Regeneration page Left midline. the becomes finally it until middle the toward more and more shifts it areas), (blue line this above cells new of integration tissue. the Through(blue) new and (violet) old the between forms line a days, few next the Over edge. wound the at but tissue, the of middle the in Above The scientists also discovered an- The systemissowellcontrolledthat Rink’s teamalsostudiedthegenetic A narrow strip of lateral tissue doesn’t usually contain any midline cells. After just one day, the first midline cells (green) form – not, however,not, – form (green) cells midline day,first one the just cells. After midline any contain usually doesn’t tissue lateral of strip narrow A Day Day 0 Day Day 1 - a basisfortestingthecollective theory. nome ofplanarians.Thisnow provides search grouprecentlydecodedthege- Planck InstituteinDresden, Rink’s re- Together with colleaguesfromtheMax this ideafurther. Acollective,asitwere. ual, butmany,” saysRink,developing the wormisnolongerasingleindivid- pendently ofoneanother. Somaybe many stemcelllinesthatmutateinde- tion, theanimalshouldconsistof vision carriesaninherentriskofmuta- an animal that doesn’t age, and each di- “When stem cells constantly divide in organism’s high number of stem cells. sible uptonowisprobablyduethe facilitate research considerably. thegrowthofmidlinewould serve into the worm’s genome and simply ob- to insert a gene for a fluorescent protein modifications in flatworms. The ability is notyetpossibletomakegenetic Unlike inthefruitflyandzebrafish,it STEM CELLS ONE INDIVIDUAL, MANY up. dle thatwillsoondry vantage ifyouhappentoliveinapud- trouble oflookingforapartnerisanad rather thanhavingtogothroughthe The factthatthishasnotbeenpos- Day Day 2 - Day Day imal species,includinghumans.“This last commonancestorofallextantan- Flatworms areearlydescendantsofthe leaves alsohaveaclearaxis ofmirror nient. Ontheotherhand,most plant of wingsorlegsaresimilarly inconve- swim incircles.” Unequaldistributions ger thanyourleftone,you canonly es,” saysRink.“Ifyourright finisbig- began tomoveusinglateralappendag- plan becameessentialonceorganisms “Maybe abilaterallysymmetricbody dictates thedirectionofmovement. “right.” Thus,thehead-tailaxisalso “front” and“back,”“left”or prerequisite for the definition of more thanastructuralfeature.Itisthe a variationofthetheme. afterthatwasmerely Rink. Everything years agointheCambrianperiod,”says grams weredeveloped500million biology. Theunderlyingmolecularpro- day ismirror-symmetrical. structure ofmostlivingorganismsto- umn andfins,legsorwings.Thebody animals developedarigidspinalcol existedbeforethefirst ror symmetry therefore appearthatthisformofmir- been bilaterally symmetrical.” It would ancestor musthavehadaheadand 4 Bilateral symmetry, however, is “Symmetry is very deeplyrootedin isvery “Symmetry 3 | MaxPlanckResearch 16MaxPlanckResearch Day Day 6 23 - FOCUS_Symmetry

SYMMETRY IN NATURE

Symmetry exists in both animate and inanimate nature. Sugar and amino acid molecules can have a mirror plane or behave like image and mirror image – a phenomenon known as chirality. A different spa- tial arrangement in an otherwise chemically identical molecule can alter its characteristics consider- ably: with mirror-image scent mol- ecules, it can give rise to a grassy or citrus smell, and with drugs like thalidomide, it can result in the drug being either effective cal. However, they can behave like image and mirror im- or harmful. Most minerals also arrange their atoms in sym- age with each other – like the right and left hand. metrically structured crystal lattices, like the double pyra- Organisms generally show a preference for one direc- mids in diamonds and the cubes in table salt. Even snow- tion of rotation: in snail shells, the right-left ratio is flakes always have a six-fold axis of rotation. 20,000 to 1. The field bindweed plant, in contrast, spi- Nature presents a special kind of symmetry in the rals exclusively to the left. structure of spirals. These arise in all orders of magnitude, While the mirror plane is usually the only symmetri- from the DNA double helix to the arrangement of seeds in cal element found in the animal kingdom, plants fre- a sunflower head, animal horns, and galaxies. Because spi- quently have a combination of mirror planes or multiple rals don’t have a mirror plane, they are per se asymmetri- axes of rotation or rotary mirror axes.

symmetry even though they never move. The deeper reason for symme- TO THE POINT try in biology may therefore lie in transport or growth processes. l The BAMBI and Slit genes are involved in determining the middle of the body Overall, symmetry must have a pur- in flatworms – and possibly also in other organisms. pose. Nevertheless, it is never perfect – l Concentration gradients indicate to cells where they are located. This allows just good enough for said purpose. them to “remember” their position and orientation in the body if they become separated from the rest of the organism. Missing parts are then newly formed “The right arm will never have the ex- in the correct position. act same number of cells as the left l The capacity of planarians to regenerate entire body parts may not serve the one,” says Rink. purpose of wound healing, but instead represent an extreme form of asexual So was symmetry a basis for com- reproduction. plexity? The answer is probably yes, but there is an exception. We know of one single multicellular organism that is GLOSSARY not symmetrical: Trichoplax adhaerens, which translates approximately as Bilateral symmetry: This is the predominant form of symmetry in the animal kingdom. Over 95 percent of animal species are bilaterians, including humans. The middle of the “hairy sticky plate.” It looks like a mis- body constitutes the axis of symmetry, at which every point on the right and the left can shapen pancake. Thanks to muscle cells be reflected and brought into congruence with its opposite point. Some groups, such as and a carpet of eyelash-like hairs on its the sea urchins and starfish, are bilateral as larvae but later become radially symmetrical. belly side, this tiny, three-millime- Hedgehog: A signaling protein that controls embryonic development in most animals. ter-long animal can move like an amoe- The protein is part of a signaling pathway that consists of various enzymes and receptors ba. “It’s a fascinating case, as it shows within cells. The signaling pathway must have emerged at a very early stage in evolution, as it arises in almost all animal groups. In insects, it controls the formation of the body that symmetry isn’t absolutely essential segments and wings. In vertebrates, the hedgehog signaling pathway is responsible for multicellular life.” Also, Trichoplax for bilateral symmetry and the arrangement of limbs. Damage to this signaling pathway may have once been symmetrical and leads to significant malformations and can cause cancer.

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