Congress Abstracts - Society for Developmental Biology

Additionally, using ISH and immunofluorescence we determine the expression pattern of zen and localization of phosphorylated Mad
(pMad), main regulators of the dorsal ectoderm patterning in Drosophila. Our results suggest that the genetic network triggered by
Mad and Zen in dorsal domains of Drosophila are conserved in other cyclorrhaphan flies and CG6234 might be originated prior
Drosophila radiation as part of the network involved in amnioserosa formation. Fondecyt 3110129 & 1120254
Program/Abstract # 276
The arthropod segmentation clock and what it tells us about the origin and evolution of segmented body plans
Peel, Andrew (Univ of Leeds, UK); Sarrazin, Andres (Pontificia Universidad Catolica de Valparaiso, Chile); Averof, Michalis
(Institut de Genomique Fonctionnelle de Lyon, France)
Many biological processes occur under the control of 'molecular clocks'. One such process is the rhythmical formation of mesodermal
somites in an anterior-to-posterior progression along the primary body axis of vertebrate embryos. Somites are blocks of tissue that
give rise to segmented structures; e.g. vertebrae and their associated muscle. In collaboration, Andrés Sarrazin, Michalis Averof and
myself have recently provided rigorous proof that the body units (segments) of an arthropod, the beetle Tribolium castaneum, also
form via the activities of a segmentation clock. We have shown that homologues of Drosophila pair-rule genes (odd-skipped and evenskipped)
exhibit oscillatory expression, with a period of 95 minutes (at 30°C), within cells of the posterior growth zone during the
formation of Tribolium abdominal segments. This finding suggests that vertebrate somites and arthropod segments form using similar
developmental principles. Given the evolutionary distance separating vertebrates and arthropods this finding might also imply that a
segmentation clock played an ancient and ancestral role in animal development. However, vertebrate segmentation clocks consist of a
complex network of 40-100 oscillating genes, none related to the two known Tribolium oscillating genes. It is therefore too early to
conclude that the arthropod and vertebrate segmentation clocks are evolutionarily related. I will report our recent work and discuss
what it does, and does not, tell us about the origin and evolution of segmented body plans. More generally, I will discuss what recent
studies on arthropod segmentation mechanisms have taught us about the evolution of developmental gene networks.
Program/Abstract # 277
Actin-based cytokinetic twist breaks Left-Right symmetry in C. elegans
Tiongson, Michael; Bao, Zhirong (Memorial-Sloan Kettering Cancer Center, USA)
In C. elegans, the establishment of the left-right body plan can be traced back to the third cleavage of embryogenesis. At the end of the
4-cell stage, sister cells ABa and ABp initiate division synchronously, with their spindles aligned to the LR axis. As their contractile
rings ingress, ABa and ABp each engage in a whole cell twisting movement such that by the end of the division, the left side
daughters are moved anterior relative to the right side daughters. At this point, the ABa and ABp daughters intercalate to solidify their
cellular positions. As previous micromanipulations showed, this resulting positional bias is sufficient to specify the handedness of the
animal. Using micromanipulation to force the right AB daughters more anterior to the left is enough to reverse all normal L\/R
asymmetries of the worm. In order to generate a cellular twist, a molecular motor must be implemented to generate such a force.
Observing that the twist seemed to be coupled to the cytokinesis ring contraction, we directed our investigation towards actomyosin.
We found that 45% of adult worms homozygous for mutant alleles of act-2 exhibitted situs inversus totalis (reversal of left-right body
plan asymmetry), indicating near randomization of handedness choice. Using high-resolution time lapse fluorescent microscopy, we
discovered that act-2 homozygous embryos exhibited similar reversal rates during the 4-6 cell division. In addition, disrupting actin
polymerization through pfn-1 RNAi also resulted in near randomization of handedness at the 4-6 cell division (~40%). In these
embryos, ABa and ABp cellular twist is absent and symmetry is not broken until the daughters intercalate randomly at the end of
division. Finally, we have observed handed actin structures that correlate with the LR symmetry breaking event. We are
currently using quantitative image analysis to investigate how disrupting actin polymerization affects these handed structures and LR
symmetry breaking.
Program/Abstract # 278
Non-stochastic assignment of asymmetry in the vertebrate ancestral brain
Boutet, Agnès (Centre de Biochimie, France); Lagadec, Ronan; Laguerre, Laurent; Godart, Benoît; Mazan, Sylvie (CNRS UPMC,
France)
L/R asymmetry exists in the vertebrate epithalamus but its evolutionary origin is largely unknown.
This subdivision of the dorsal diencephalon composed of the habenula and the pineal organ/parapineal nucleus displays stricking
anatomical asymmetries in many species. In actinopterygii, an asymmetric Nodal (cyc) expression precedes the differential L/R
epithalamus morphogenesis. However in zebrafish Nodal signalling is not controlling the habenular asymmetry per se but the
migration of the parapineal organ toward the left habenular nucleus. From these results it has been proposed that Nodal signalling was
not breaking symmetry in the brain but rather directing laterality. On the other hand, since the asymmetric nodal expression has not
been reported in any other vertebrate taxon, it was thought that the asymmetry was stochastic at the basis of the vertebrate lineage. In
this work we have characterized components of the Nodal pathway from a chondrychthe and an agnatha and found them all expressed
in the embryonic diencephalon. In addition this expression was always reported on the left in both species. Clear molecular
asymmetries were reported in the habenula of catsharks in spite of the absence of parapineal nucleus. Lampreys are not devoid of
parapineal but the organogenesis of this structure starts clearly after habenular asymmetry set up suggesting that the differential
morphogenesis of the habenula is not depending on a nodal-dependant migration toward the left of the parapineal as it is the case in
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