The movement pattern during cleavage in artificially activated eggs ofPimpla turionellae (Hym.)

Abstract

The early embryonic development ofPimpla is characterized by a complicated temporal and spatial pattern of ooplasmic movements detected in time-lapse films made during cleavage. The modified movements observed after the architecture of oviposited eggs had been altered artificially by centrifugation indicated that there are different dynamic systems for ooplasmic streaming, contractions, and nuclear migration. The discovery that unlaid, explanted oocytes ofPimpla can be activated by mechanical deformation provided a new way of studying alterations of egg architecture, nucleocytoplasmic interactions, and the control of morphogenetic processes during cleavage and blastoderm formation. In this article, development and ooplasmic movements in explanted oocytes with and without articifial activation are described and compared with those observed in eggs after normal oviposition. Four categories of explanted "eggs" can be distinguished: 1. Inexplanted eggs which are not activated by mechanical deformation, no movement of egg plasm can be observed, and nuclear multiplication never takes place. Thus the completion of meiosis as well as the ooplasmic movements must be triggered by deformation of the egg in the ovipositor. 2. Inartificially activated eggs with diphasic blastoderm formation, the following deviations from normal development are registered. The mixing motion at the anterior end of the egg, the transfer flow, and the forward component of the fountain flow are all absent. Instead, a homogenizing movement is observed in the ooplasm of the anterior region of the egg. The energids in this region then migrate directly to the periphery, and in due time form the blastoderm (first phase of blastoderm formation). In the posterior 2/3 of the egg, blastoderm formation is slightly retarded. The so-called mixing motion, the unipolar flow and the caudal part of the fountain flow take place as in normal development, and the energids become distributed throughout a central plasm column before they migrate radially to initiate a second phase of blastoderm formation. There are marked ooplasmic contractions at the egg poles. 3. Forartificially activated eggs with successive blastoderm formation we recorded the same deviations from normal development as in the cephalic region of eggs of category 2. Blastoderm formation also occurs in due time. In the caudal region of the egg, a "dilated" unipolar flow is found. The fountain flow is reduced and greatly delayed. Energids migrating from the anterior egg region into the posterior may be carried to the posterior egg pole in a central plasm by the fountain flow. A peripheral, ring-shaped contraction moving in a posterior direction indicates the zone where the preblastoderm gradually forms. A marked antero-posterior time gradient is evident in blastoderm formation. Development of these eggs is greatly retarded up to hatching of the larvae. 4. Ineggs without blastoderm formation after activating treatment, no energids could be found apart from the meiotic nuclei. Nevertheless, the ooplasmic movement pattern and the histological aspect of these eggs sometimes resembled those of eggs oviposited by the female. Also, formation of pseudo-pole cells could be observed. These observations demonstrate that pseudocleavage takes place in such eggs. The streaming system is apparently able to achieve the pattern of ooplasmic movements independently of nuclear multiplication. Our observations demonstrate the autonomy of the streaming systems and of energid migration. The third dynamic system, ooplasmic contractions, occurs in artificially activated eggs combined with the streaming system and/or nuclear multiplication. It may possibly act independently in the very early contractions at the egg poles; these may be comparable to events at the elevation of a fertilization membrane. The discussion concerns exogeneous and endogeneous factors which may affect the pattern of movements, and the functions of mixing motion and unipolar flows in restricting the early nuclear migration to the central plasm. Also discussed are the significance of the anterior and posterior initial regions ("Initialbereiche") and ofsuccessive blastoderm formation with respect to the relation between long-germ and short-germ egg types.