Phallacidin stains the kinetochore region in the mitotic spindle of the green algae Oedogonium spp.

We found previously that in living cells of Oedogonium cardiacum and O. don nellii, mitosis is blocked by the drug cytochaloasin D (CD). We now report on the staining observed in these spindles with fluorescently actin-labelin g reagents, particularly Bodipy FL phallacidin. Normal mitotic cells exhibi ted spots of staining associated with chromosomes; frequently the spots app eared in pairs during prometaphase-metaphase. During later anaphase and tel ophase, the staining was confined to the region between chromosomes and pol es. The texture of the staining appeared to be somewhat dispersed by CD tre atment but it was still present, particularly after shorter (<2 It) exposur e. Electron microscopy of CD-treated cells revealed numerous spindle microt ubules (MTs); many kinetochores had MTs associated with them, often lateral ly and some even terminating in the kinetochore as normal, but the usual bu ndle of kinetochore MTs was never present. As treatment with CD became prol onged, the kinetochores became shrunken and sunk into the chromosomes. Thes e results support the possibility that actin is present in the kinetochore of Oedogonium spp. The previous observations on living cells suggest that i t is a functional component of the kinerochore-MT complex involved in the c orrect attachment of chromosomes to the spindle.