Ultrastructural features of the life cycle ofAcetabularia mediterranea

Summary The portion of the life cycle ofAcetabularia mediterranea from cyst formation to gamete release is described. During maturation, the cyst nucleus undergoes a series of mitoses in which the nuclear membrane remains intact, and the spindle microtubules are confined to the intranuclear space. There is a dark requirement for the completion of cyst maturation at the end of which the nuclei are in groups and centrioles are present. The final migration of the nuclei, cleavage of the cytoplasm into gametes, and the development of flagella is cxtremely rapid, taking six to twelve hours. Cysts which fail to germinate are blocked at a late stage in the maturation process, either before or after the final nuclear division.The observations are related to mitosis in other green algae (Chlorophyceae) and to gametogenesis inBryopsis hypnoides. Changes in the amount and distribution of heterochromatin in the nuclei are discussed in relation to the life cycle ofA. mediterranea.The substance of this paper was first presented orally at the Symposium onAcetabularia held at the Max-Planck-Institut für Zellbiologie, Wilhelmshaven, Federal Republic of Germany, July 12–15th, 1972.Supported in part by Grant GM 06637 from the U.S. Public Health Service.