Active MPF triggers a cascade of events including nuclear envelope breakdown, chromosome condensation, mitotic spindle assembly and cyclin destruction by the anaphase promoting complex (APC). Cyclin destruction in turn results in MPF inactivation, thereby terminating mitosis

Maternal regulation is a key feature of early development with many components necessary for embryogenesis being deposited by the mother into the oocyte. In zebrafish, the genome is silenced until the ninth cleavage cycle (Kane and Kimmel, 1993), indicating that meiosis, mitosis and many other essential processes required for embryonic development are regulated by maternal factors until this stage. Meiosis, the process by which haploid germ cells are formed, consists of two consecutive nuclear divisions in the absence of DNA replication (Nebreda and Ferby, 2000). During metazoan oogenesis, mature oocytes are arrested at specific stages of meiosis prior to fertilization. Oocytes from fish, amphibians and most mammals arrest at metaphase II of meiosis (Austin, 1965; Streisinger et al., 1981; Selman et al., 1993). Activation of the oocyte is in most species induced by sperm entry, resulting in a wave of cytosolic calcium at the moment of fertilization (Stricker, 1999) thereby causing a decrease in cyclin-dependent kinase activity, and inducing the oocyte to resume meiosis (Kline and Kline, 1992; Sensui and Morisawa, 1996; McDougall and Levasseur, 1998; Levasseur and McDougall, 2000). This leads to one set of sister chromatids being extruded to produce a haploid oocyte nucleus. Following the completion of meiosis the haploid maternaland paternal-derived genomes combine to create a diploid zygote. Studies inDrosophila, mouse and human show that the pronuclei migrate towards each other (congression) and subsequently enter mitosis (Schatten et al., 1985; Simerly et al., 1995; Callaini and Riparbelli, 1996). The mixing of the maternal and paternal chromosomes therefore occurs at the first mitosis. In species such as sea urchin, Caenorhabditis, Xenopus , and as demonstrated here the zebrafish, a diploid nucleus forms through the fusion of two haploid pronuclei before the first mitosis (Longo and Anderson, 1968; Ubbels et al., 1983; Strome and Wood, 1983). Entry into mitosis is induced by the activation of maturation promoting factor (MPF), also referred to as cyclin-dependent kinase (Cdk1) (Evans et al., 1983; Murray and Kirschner, 1989; Murray et al., 1989). Active MPF triggers a cascade of events including nuclear envelope breakdown, chromosome condensation, mitotic spindle assembly and cyclin destruction by the anaphase promoting complex (APC). Cyclin destruction in turn results in MPF inactivation, thereby terminating mitosis (Murray and Hunt, 1993; Zachariae and Nasmyth, 1999). Early embryonic, maternally regulated vertebrate cell cycles iffer substantially from somatic cell cycles. Early embryonic cells have rapid synchronous cell cycles with alternating M (mitosis) and S (DNA synthesis) phases and lack G (gap) phases. The rapid repeated cleavages result in cells with successively smaller volumes. In many organisms the cleavages of early embryos appear to proceed irrespective of various perturbations (Nagano et al., 1981; Gerhart et al., 1984; 3907 Development 130, 3907-3916 © 2003 The Company of Biologists Ltd doi:10.1242/dev.00606