Diffuse kinetochores and holokinetic anaphase chromatin movement during mitosis in the hemipteran Agallia constricta...

Mitosis in the hemipteran Agallia constricta (leafhopper) cell line AC-20 was examined by light microscopy of living and fixed cells. During early prometaphase the numerous small (0.30-3.0-microns) chromosomes appear as discrete units that lack a primary constriction. However, by late prometaphase the chromosomes are tightly packed at the spindle equator and are no longer clearly resolvable as individuals. When viewed from the side the metaphase chromatin appears as a 2-3-microns wide band that spans the width of the spindle; when viewed from the pole it appears as a fenestrated disk. The metaphase chromatin splits at anaphase into two sister chromatin plates, each of which exhibits holokinetic poleward movement, i.e., all parts of each plate move as a single unit with the same velocity. In many early-to-mild anaphase cells the separating sister plates are connected by chromatin-containing bridges that break as anaphase progresses. Ultrastructural analyses of serial thick and thin sections from cells fixed by conventional, OsO4/KFeCN, or high pressure rapid freezing methods, reveal that by metaphase all of the chromosomes are interconnected to form a large, irregularly shaped fenestrated disk of chromatin. Similar analyses reveal that adjacent chromatids remain interconnected throughout anaphase. Each disk of metaphase and anaphase chromatin contains numerous kinetochores recessed within its pole-facing surface. Kinetochores consist of a fine, faintly staining fibrillar material arranged along the chromatin surface as thin (0.1-0.3 micron dia.) rods varying considerably (0.15-2.3 microns) in length. From these observations we conclude that the polycentric metaphase chromatin of A. constricta, and its holokinetic behavior during anaphase, arises from the aggregation or cohesion of smaller prometaphase chromosomes, each of which contains a single, diffuse kinetochore.