Molecular and genetic analyses of nuclear migration in Saccharomyces cerevisae, Aspergillus nidulans and Neurospora crassa have revealed a conserved mechanism for fungal nuclear migration during interphase that depends upon the presence of intact microtubules

Nuclear migration is essential for the movement of pronuclei during fertilization, for normal mitotic and meiotic cell division and for a variety of interphase functions (Morris et al., 1995). For example, interphase nuclear migration is essential for axis determination, embryogenesis and eye morphogenesis in Drosophila melanogaster and the morphogenesis of other epithelial structures, such as the vertebrate brain (reviewed by Morris et al., 1995). Molecular and genetic analyses of nuclear migration in Saccharomyces cerevisae, Aspergillus nidulans and Neurospora crassa have revealed a conserved mechanism for fungal nuclear migration during interphase that depends upon the presence of intact microtubules (Morris et al., 1995; Oakley and Morris, 1980; Sullivan and Huffaker, 1992). The minusend directed microtubule motor dynein and components of the dynein-associated dynactin complex are required for interphase nuclear migration (reviewed by Morris et al., 1995; Steinberg, 1998). Many proteins of unknown function are also important for fungal nuclear migration. Much less is known about the mechanism of nuclear migration during metazoan development. In syncytial Drosophila embryos that lack nuclei because DNA synthesis and nuclear division have been blocked with aphidicolin, centrosomes undergo the movements of the normal nuclearcentrosomal complex, indicating that the force that produces nuclear movement could act through the centrosome (Raff and Glover, 1989). Pharmacological studies have implicated microtubules in a variety of metazoan nuclear migrations (Reinsch and Gonczy, 1998). Furthermore, dynein and dynactin appear to act in Drosophila nuclear migration: a dominant-negative allele of a dynactin component, Glued1, causes defects in nuclear migration in the Drosophila eye (Fan and Ready, 1997). Mutations in another Drosophila gene, marbles/klarsicht, also cause defects in nuclear migration (Fischer-Vize and Mosley, 1994; Welte et al., 1998); a molecular characterization of marbles has yet to be reported. To address further the control of nuclear migration during animal development, we have undertaken the study of two sets of nuclear migrations that occur during C. elegans development. The first involves the embryonic formation of the dorsal hypodermal syncytium, hyp7. During the morphogenesis stage of embryogenesis, 17 of the 23 hyp7 cells initiate elongation by extending over the dorsal midline to a contralateral position (Fig. 1A). The cell continues to change shape until it forms an elongated strip over the dorsal surface of the embryo. The nucleus then migrates to the contralateral position within the cytoplasm (Sulston et al., 1983). The 3171 Development 126, 3171-3181 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 DEV5304