My Oldest Sister Is a Sea Walnut?
Decoding of the ctenophore genome prompts reevaluation of the complexity of the metazoan
With common names such as “sea walnut,” “sea gooseberry,” and “Venus’ girdle” that reflect their
morphological diversity, the jelly-like creatures belonging to the phylum Ctenophora
that bear distinctive “combs” of cilia are not
only breathtakingly beautiful (1) but are also
key to understanding early animal evolution.
On page 1336 of this issue, Ryan et al. (2)
decode the genome of the sea walnut
Mnemiopsis leidyi, the first member of this phylum to be sequenced, and propose that ctenophores might be the earliest branch of the animal tree and the sister lineage to that of all
other animals. This paints a picture of early
animal evolution full of cell type complexity,
as well as its loss.
The ~200 ctenophore species discovered
so far live in a wide variety of marine environ-
ments and at all latitudes (3). They get their
name from the eight rows of linked tiny hairs
known as “ctenes” (Greek for combs) that run
alongside their body and propel the animals
through water. Although superficially simi-
lar to jellyfish (cnidarians), ctenophore mor-
phology is quite distinct from that of the other
three early-branching animal phyla, the porif-
erans (sponges), the largely enigmatic placo-
zoans (known solely from organisms belong-
ing to the phylum’s single genus, Trichoplax),
and the cnidarians (jellyfish, sea anemones,
and their kin). Unlike the radially symmetri-
cal jellyfish, ctenophores are biradially sym-
metrical—their main body axis is defined by
a mouth at one end and a gravity-sensing api-
cal organ at the other end. Unlike sponges and
placozoans, but like jellyfish, ctenophores
contain both muscle and nerve cells. The lat-
ter are organized as a diffuse net that appears
to be centralized at the apical organ (4).
With the exception of poriferans, whose
bodies lack tissue organization, the tissues of
the other three early-branching animal phyla
are thought to develop from two distinct
embryonic germ layers—the ectoderm (from
which the nervous system develops) and the
endoderm (the layer that gives rise to the gut).
By contrast, the tissues of all bilaterians—
Department of Biological Sciences, Vanderbilt University, Nashville, TN 37235, USA. E-mail: antonis.rokas@
formed with LCs that are remarkably sensitive to the presence of specific biological
adsorbates (9) and can serve as templates for
the synthesis of spherical and nonspherical
particles with chemical patches (10). These
advances are leading to LC-based colloidal
systems with functional properties and potential technological impact that go well beyond
traditional applications of LCs in displays.
The LCs used in the study by Turiv et
al. are low–molecular weight organic molecules that can be viewed as structured oils.
The long-range orientational ordering of
molecules in LCs, which gives rise to anisotropic viscosities and mechanical properties
not found in simple isotropic solvents (11),
is dynamic and patchy; local domains form,
consisting of molecules with similar alignment. The alignment can be described theoretically with a director, a vector that represents the local average of the molecular orientations. In the late 1990s, it was shown that
colloidal species dispersed in nematic LCs
(the simplest type of LC that has no additional
positional ordering) could locally strain the
director of LCs, as well as introduce so-called
topological defects. These defects are nano-scopic regions in which the LC orientational
order differs substantially from the bulk (7).
The presence of these strains and defects,
in combination with the anisotropic viscosities of LCs, were shown to give rise to anisotropic diffusion of colloidal particles in LCs
(12, 13). However, in these earlier studies
the MSDs followed the classical linear time
dependence, and the measurements could be
explained by construing the strain in the LC
around the colloids as being static. Turiv et
al. now demonstrate that fluctuations in the
orientation of the LC director can influence
the transfer of momentum from the LC to a
colloid, such that the diffusion of the colloid
departs from that predicted using the “static
view” of the director (see the figure).
By focusing on a class of LCs with suffi-
ciently slow fluctuations of the director, Turiv
et al. imaged the displacements of colloids on
time scales that lead to diffusive behaviors of
the colloids that are faster or slower than clas-
sical Brownian motion. The measurements
are striking examples of anomalous diffu-
sion arising from purely orientational fluctua-
tions in a solvent, and they define new ques-
tions and directions of inquiry. For example,
whereas the measurements of anomalous dif-
fusion reported by Turiv et al. occur on time
scales consistent with the orientational fluc-
tuations of the director in the LC, a detailed
description of the dynamic coupling between
the colloids and the LC is yet be elucidated.
Furthermore, the role of surface chemistry
(and, for example, colloid shape) in regulat-
ing near-particle fluctuations of the director
is yet to be fully understood.
What is clear, however, is that the observations and ideas presented by Turiv et al.
hint at new principles for manipulating colloidal transport processes. For example, one
can envisage the application of time-depen-dent external fields (electrical, magnetic, or
optical) to drive fluctuations in the director
on relevant time scales and thus influence
the exchange of momentum between colloids
and LCs that gives rise to the anomalous diffusion. Alternatively, internally generated
fields, such as those that are being explored
in the context of designs of active matter (14,
15), might plausibly be harnessed to drive orientational fluctuations in LCs and thus regulate the transport of colloids.
References and Notes
1. A. Ott, J. P. Bouchaud, D. Langevin, W. Urbach, Phys. Rev.
Lett. 65, 2201 (1990).
2. D. S. Banks, C. Fradin, Biophys. J. 89, 2960 (2005).
3. F. Höfling, T. Franosch, Rep. Prog. Phys. 76, 046602
4. T. Turiv et al., Science 342, 1351 (2013).
5. J. Planer, Ann. Chem. Pharm. 118, 25 (1861).
6. M. von Smoluchowski, Ann. Phys. 21, 756 (1906).
7. P. Poulin, H. Stark, T. C. Lubensky, D. A. Weitz, Science
275, 1770 (1997).
8. I. Musevic, M. Skarabot, U. Tkalec, M. Ravnik, S. Zumer,
Science 313, 954 (2006).
9. I. H. Lin et al., Science 332, 1297 (2011).
10. F. Mondiot, X. Wang, J. J. de Pablo, N. L. Abbott, J. Am.
Chem. Soc. 135, 9972 (2013).
11. P. G. de Gennes, J. Prost, The Physics of Liquid Crystals
(Clarendon, Oxford, ed. 2, 1993).
12. H. Stark, D. Ventzki, Phys. Rev. E 64, 031711 (2001).
13. J. C. Loudet, P. Hanusse, P. Poulin, Science 306, 1525
14. T. Sanchez, D. T. N. Chen, S. J. DeCamp, M. Heymann,
Z. Dogic, Nature 491, 431 (2012).
15. W. F. Paxton, S. Sundararajan, T. E. Mallouk, A. Sen,
Angew. Chem. Int. Ed. 45, 5420 (2006).
Acknowledgments: I thank X. Wang and D. Miller for
preparing the figure. Supported by NSF grant DMR-1121288,
Army Research Office grant W911NF-10-1-0181, and U. S.
Department of Energy grant DE-SC0004025.