Reproduction: Widespread cloning in echinoderm larvae
Post on 28-Jul-2016
Widespread cloning inechinoderm larvae
Asexual reproduction by free-livinginvertebrate larvae is a rare and enig-matic phenomenon and, although itis known to occur in sea stars14 and brittlestars5,6, it has not been detected in otherechinoderms despite more than a centuryof intensive study6,7. Here we describespontaneous larval cloning in three speciesfrom two more echinoderm classes: a seacucumber (Holothuroidea), a sand dollarand a sea urchin (Echinoidea). Larvalcloning may therefore be an ancient abilityof echinoderms and possibly of deutero-stomes the group that includes echino-derms, acorn worms, sea squirts andvertebrates.
To confirm that genuine cloning wasoccurring, we reared cloning larvae and sep-arated clones individually (Fig. 1, legend).Although most sea cucumber larvae meta-morphosed normally after a month, 5 of 41(12.2%) free-swimming doliolaria larvaewere half the length of their siblings andhad a constriction around the penultimateciliary band. These buds retained a ciliaryband and were still attached by a thin tetherinto the benthic pentacula stage 12 h later(Fig. 1a). Eventually, after separation, budsdeveloped into normal auricularia larvaewith a juvenile rudiment (Fig.1b).
Most sand dollar larvae metamorphosedwithin 7 weeks, but 6 of 170 (3.5%) formed ahollow bud near the future juvenile mouth(results not shown). Once separated, theevenly ciliated buds developed into analmost solid gastrula with bilateral spicules.Within two days, a tripartite gut and furtherskeletal spicules formed, and the clonesbegan to feed.
After 4 weeks, nearly 5% of sea urchinlarvae (n500) showed constrictions attheir posterior end (Fig.1c).These eventuallyyielded uniformly ciliated buds that beganfeeding within four days, developed pairedskeletal rods within a week, and acquired anormal larval form within two weeks. Juve-nile rudiments began to form within threeweeks (Fig. 1d), accompanied by furtherlarval arms.
Larval cloning is therefore known tooccur in all classes except crinoids (featherstars and sea lilies), supporting an earlierconjecture that it might be an ancestral abilityof echinoderms8. The mechanisms by whichthis cloning occurs, however, are unexpect-edly diverse.First, clones may arise from var-ious larval body regions, including arms (seastars14, brittle stars5), the oral hood (seastars24), the posterior end (sea stars3, seacucumbers (Fig. 1a), sea urchins (Fig. 1c))and the lateral body wall (sea stars2, sand dol-lars (our results, not shown)). Second, the
developmental stage of clones at separationranges from blastulae2 to fully formed lar-vae3. Third, some clones may not separateuntil after the primary larva has begun tometamorphose5 (Fig. 1a). This indicateseither that larval cloning evolved indepen-dently on several occasions or that its mecha-nisms have diverged widely from an ances-tral mode.
Cloning can be surprisingly frequent: upto 12% in laboratory-reared sea cucumberlarvae and 1090% in samples of field-col-lected sea star larvae1,4. The fact that such acommon phenomenon should have beenoverlooked in heavily studied organismsseems remarkable. Were preconceptionsabout normal development so strong thatcloning was simply dismissed as aberrant?From a theoretical perspective, larvalcloning particularly in sea urchins challenges a central tenet of the set-asidecell theory9 because, contrary to prediction,larval body cells are not essentiallyeutelic9, but can differentiate into juvenilestructures (Fig. 1b, d). Nevertheless, suchcloning offers an opportunity to study thewell-characterized developmental regula-tory networks of sea urchins10 in a newontogenetic context.
Larval cloning represents an intriguingnew dimension to invertebrate life histo-ries. The process confers three potentialecological advantages: increased fecundityunder optimal growth conditions2,3,increased chances of settlement after aprotracted larval life1,2,5, and recycling ofotherwise discarded or reabsorbed larvaltissue5. Larval cloning may also be evolu-tionarily significant, for two reasons. First,clones may subsequently clone5, potentiallyleading to a new, entirely pelagic bauplan.Second, although it may be merely another
idiosyncrasy of echinoderm development,larval cloning might also be more taxo-nomically widespread. As nearest relativesto the echinoderms11, acorn worms offer acritical test. If their tornaria larvae clone,then ancient deuterosomes may have hadthis ability.Alexandra A. Eaves*, A. Richard Palmer*Physiology and Cell Biology Group, andSystematics and Evolution Group, Department ofBiological Sciences, University of Alberta,Edmonton, Alberta T6G 2E9, Canada, andBamfield Marine Sciences Centre, Bamfield, BritishColumbia V0R 1B0, Canadae-mail: email@example.com. Bosh, I., Rivkin, R. B. & Alexander, S. P. Nature 337, 169170
2. Jaeckle, W. B. Biol. Bull. 186, 6271 (1994).
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4. Knott, K. E., Balser, E. J., Jaeckle, W. B. & Wray, G. A. Biol. Bull.
204, 246255 (2003).
5. Balser, E. J. Biol. Bull. 194, 187193 (1998).
6. Mortensen, T. Studies of the Development and Larval Forms of
Echinoderms (Gad, Copenhagen, 1921).
7. MacBride, E. W. Nature 108, 529530 (1921).
8. Lacalli, T. C. Invertebr. Biol. 119, 234241 (2000).
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Competing financial interests: declared none.
146 NATURE | VOL 425 | 11 SEPTEMBER 2003 | www.nature.com/nature
Figure 1 Asexual budding by echinoderm larvae. a, Sea cucumber (Parastichopus californicus; Holothuroidea; Aspidochirotida) early pen-
tacula with clone attached at posterior end (arrowhead). b, Sea cucumber 18 days after separation as a complete auricularia larva, with
juvenile rudiment (arrow). c, Sea urchin (Strongylocentrotus purpuratus; Echinoidea, Echinacea) larva with a clone (arrow) constricting
from the posterior end around the robust ciliary band. d, Sea urchin clone three weeks after separation, with early juvenile rudiment
(arrow). Scale bars, 50 m. Adult P. californicus were collected sub-tidally from San Juan Island, Washington, and gametes were col-
lected by dissection. Larvae were maintained at 11 C under natural lighting in filtered sea water and were fed a red alga Rhodomonas
salina, ad libitum. Adult S. purpuratus from Prasiola Point (Vancouver Island, Canada) were spawned by intracoelomic injection of 0.55 M
potassium chloride. Larvae were maintained in a 19-h light cycle at 14 C in pasteurized sea water and were fed 106 cells per ml of a
golden brown alga Isochrysis galbana and R. salina; food and water were replaced every 72 h. To monitor their development, cloning larvae
and separated clones were isolated in 7-ml glass vials and cultured; most developed to metamorphosis. Cultures were deemed to be
healthy as no larval tissue necrosis was seen. Further details are available from the authors.
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