Turning Inside Out

During development, embryos of the multicellular green alga Volvox carteri form a spherical monolayer of cells that must turn inside out in order to produce the mature spheroid alga in which reproductive cells are enclosed inside a somatic cell layer. To perform this feat, a tight curve must be produced, involving dramatic rearrangements of cell shape and position. Nishii et al. identified the gene invA, which was required for inversion. The InvA protein belongs to a family of microtubule-dependent molecular motors known as kinesins. In mutant alga lacking InvA, cells changed shape as usual, but inversion failed because of the inability of cells to move with respect to the intercellular cytoplasmic bridges. It is this movement that is needed to generate and maintain the fold in the cell layer required for inversion. — SMH