Download Evolution of mechanical forces controlling spindle positioning and

Evolution of mechanical forces controlling spindle positioning and spindle elongation in
nematode embryos
Marie Delattre, LBMC, ENS de Lyon
We are exploring the essential mechanisms of mitotic spindle positioning and spindle
elongation. To this end, we use the large one-cell embryo of the nematode C. elegans as a
model system. In this cell, the mitotic spindle is centrally located in metaphase and becomes
posteriorly pulled by molecular motors, asymmetrically enriched at the posterior cortex
during anaphase. As the spindle gets posteriorly displaced, it also elongates and undergoes
rigorous transverse oscillations. We recorded the first embryonic division of ~40 different
nematode species. We found that despite the conserved asymmetric position of the spindle at
the end of mitosis, spindle motion and spindle elongation were very different from one
species to the other. Thus, several-equally good-combinations of parameters can lead to the
same final phenotype. We propose that the regulation of cortical pulling forces, central
spindle stiffness and microtubule dynamics may have changed across evolution leading to the
combination of phenotypes observed in the wild. Using laser microsurgery experiments on
microtubules, spindle and centrosomes; we are first evaluating the contribution of different
mechanical forces on the mitotic spindle, in the different species. Second, we are correlating
the different phenotypes observed to molecular changes of key molecular components, such
as force generators or MAPs. Genome sequences of many worm species are available, and
protein replacement experiments can be easily performed in the C. elegans embryo.
Therefore, we can test in vivo if molecules have acquired new functions across nematode
evolution. Conversely, this evolutionary comparison will lead to a better description of
conserved protein functions.