| One key step in the evolution from unicellular to multicellular organisms in the family Volvocaceae is the so called "inversion", during which volvocacean embryos turn themselves completely right-side out. This morphogenetic process is driven by cell shape changes and cell movements of somatic cells relative to cytoplasmic bridges and serves as a model for cell sheet curling in metazoans. Investigations in Volvox carteri show that microtubuli drive the cell shape changes and a microtubuli-dependent motor protein is involved in cell movements during inversion. Although all Volvox species are wrong-side out at the end of cleavage, there are different tactics of inversion e.g. in V. carteri versus V. globator. Because previous studies dealt mainly with the species V. carteri, we studied inversion in V. globator to allow for comparison. Cell shape changes during inversion were analyzed by light microscopy, TEM, SEM and cLSM. In the course of inversion, cells of invert
ing embryos changed their form successively from pear shaped to teardrop-, spindle-, flask similar-, pencil-shaped and finally became columnar. Phalloidin-TRITC labelled F-actin was detectable throughout inversion. In the teardrop-shaped cells f-actin showed an apical location. Based on our results, we have developed a preliminary model for the inversion process in V. globator. |