F-actin domains in the syncytial blastoderm of the dipteran Ceratitis capitata

Abstract

Laser scanning confocal microscopy on rhodamine-phalloidin-treated syncytial embryos of the dipteran Ceratitis capitata allowed us to recognize four different kinds of actin filament distribution in close spatial proximity. One domain is represented by microfilaments localized in the plasma membrane within the microprojections and membrane infoldings. At a slightly lower focal level, rhodamine-phalloidin labelling is concentrated in small irregular aggregates, which are localized around the dividing nuclei. Our results indicate that the organization of the actin aggregates follows that of the microtubules of the mitotic apparatus and suggest that the dynamic reorganization of these structures during mitosis may be microtubule-dependent. A three-dimensional network of thin actin filaments fills the whole periplasm and links the spindles together. A fourth actin domain is localized at the poles of the spindles in correspondence with the centrosomal region. The complex network of cortical filament bundles described in the present study may represent the ultrastructural basis of the tension leading to segregation of daughter nuclei at late telophase and to their lateral migration along the embryo surface.