Life breaks symmetry
Life breaks symmetry
Peter Nick 0
0 Molecular Cell Biology, Karlsruhe Institute of Technology , Karlsruhe , Germany
At the dawn of the twentieth century, Ernst Haeckel's book BKunstformen der Natur^ (Art Forms of Nature, Haeckel 1904) was a major factor to convey to society the ideas of biology in general and evolutionary biology in particular. The aesthetic impression of artistically arranged and depicted life forms inspired enthusiasm for the beauty of nature and can be considered as classical masterpiece of science communication. A good deal of this overwhelming effect is due to the emphasis of symmetry, for instance when Haeckel compiles plates showing diatoms or radiolaria. Symmetry has been a central element of art over the ages, and the celebration of natural symmetry in Haeckel's work served to show nature as artist. But his, admittedly successful, propagandistic trick might be misleading: the beauty of nature is rather opposed to artistic beauty, because the ruling principle is asymmetry, not symmetry. The forced symmetry in the gardens of Versailles symbolises the subjugation of nature by civilisation, and Jacob's famous statement (L'evolution c'est bricolage) of the tinkering evolution (Jacobs 1977) reveals that life has to be asymmetric. There are different arguments for the necessity of biological asymmetry-here, just one will be pursued: to live means to develop, and development requires a deviation from equilibrium. Symmetry (and that may be one reason why it is so attractive for art) represents a state of maximal equilibrium. Real symmetry can only be a static end point, never the start of a developmental process. Life, however, is never in equilibrium; it can only oscillate around the equilibrium. For this
reason, any developmental process must start with a break of
symmetry, and a break of symmetry often heralds a
developmental process. Three contributions to the current issue
address how life breaks symmetry in plants and animals. The
comparison of these cases is revealing.
The egg of the fruitfly Drosophila melanogaster has been a
prime model to understand symmetry break in animal
development. Here, the maternal follicle imprints anteriorposterior
and dorsiventral polarity, for instance by deposition of
nontranscribed mRNA encoding the transcription factor BICOID
that lays down the anterior pole. The work by
Garbiec et al.
investigates follicle development in the neuropteran fly
Osmylus fulvicephalus. They find that a specific population of
follicle cells, the anterior centripetal cells, extend
hypertrophically forming a fold, which sets down a double
symmetry break, where not only the anterior, but also the
dorsal side is defined. Thus, there are variations to the theme
for the model worked out in the fruitfly, although this model is
widely used as paradigm for insect embryogenesis in general.
The second example for symmetry comes from plants.
Here, the formation of the stomatal apparatus proceeds from
a so-called meristemoid, where a differentiated epidermal cell
re-enters a stem-cell fate (yielding the guard cell mother cell)
and not only generates the guard cells by asymmetric division,
but also recruits asymmetric divisions of the neighbouring cell
(the subsidiary cell mother cell) to produce subsidiary cells.
The presence of a mobile signal indicates a hormonal activity.
In fact, a gradient of the phytohormone auxin at the interface
between the two mother cells has been observed. The work by
Livanos et al. (2016)
investigates this phenomenon by
different localisation techniques in combination with inhibitor
studies. Using a cerium chloride stain
(Libik-Konieczny et al.
, they can show that hydrogen peroxide accumulates in
the same location. They then test the working hypothesis that
auxin (which is secreted in a polar fashion) and apoplastic
lamellipodium as in fibroblasts, or by extension of exploratory
processes as in neurites. Despite these specific differences, the
symmetry break of mobile animal cells seems to be linked
with the dynamic polarity of the microtubules nucleated by
Compliance with ethical standards
Conflict of interest The author declares that he has no conflict of
Barker A , McIntosh K , Dawe H ( 2016 ) Centrosome positioning in nondividing cells . Protoplasma , current issue
Garbiec A , Kubrakiewicz J , Mazurkiewicz-Kania M , Simiczyjew B , Jędrzejowska I ( 2016 ) Asymmetry in structure of the eggshell in Osmylus fulvicephalus (Neuroptera: Osmylidae) . An exceptional case of breaking symmetry during neuropteran oogenesis . Protoplasma, current issue
Haeckel E ( 1904 ) Kunstformen der Natur . Wien, Leipzig
Jacob F ( 1977 ) Evolution and tinkering . Science 196 : 1161 - 1166
Libik-Konieczny M , Kozieradzka-Kiszkurno M , Desel C , MichalecWarzecha Z , Miszalski Z , Konieczny R ( 2014 ) The localization of NADPH oxidase and reactive oxygen species in in vitro-cultured Mesembryanthemum crystallinum L. Hypocotyls discloses their differing roles in rhizogenesis . Protoplasma 252 : 477 - 487
Livanos P , Galatis B , Apostolakos P ( 2016 ) Deliberate ROS production and auxin synergistically trigger the asymmetrical division generating the subsidiary cells in Zea mays stomatal complexes . Protoplasma, current issue