Axis establishment and microtubule-mediated waves prior to first cleavage in Beroe ovata

The single axis (oral-aboral) and two planes of symmetry of the ctenophore Beroe ovata become established with respect to the position of zygote nucleus formation and the orientation of first cleavage. Bisection of Beroe eggs at different times revealed that differences in egg organisation are established in relation to the presumptive oral-aboral axis before first cleavage. Lateral fragments produced after but not before the time of first mitosis developed into larvae lacking comb-plates on one side. Time-lapse video demonstrated that waves of cytoplasmic reorganisation spread through the layer of peripheral cytoplasm (ectoplasm) of the egg during the 80 minute period between pronuclear fusion and first cleavage, along the future oral-aboral axis. These waves are manifest as the progressive displacement and dispersal of plaques of accumulated organelles around supernumerary sperm nuclei, and a series of surface movements. Their timing and direction of propagation suggest they may be involved in establishing cytoplasmic differences with respect to the embryonic axis. Inhibitor experiments suggested that the observed cytoplasmic reorganisation involves microtubules. Nocodazole and taxol, which prevent microtubule turnover,blocked plaque dispersal and reduced surface movements. The microfilament-disrupting drug cytochalasin B did not prevent plaque dispersal but induced abnormal surface contractions. We examined changes in microtubule organisation using immunofluorescence on eggs fixed at different times and in live eggs following injection of rhodamine-tubulin. Giant microtubule asters become associated with each male pronucleus after the end of meiosis. Following pronuclear fusion they disappear successively, those nearest the zygote nucleus shrinking first, to establish gradients of aster size within single eggs. Regional differences in microtubule behaviour around the time of mitosis were revealed by brief taxol treatment, which induced the formation of small microtubule asters in the region of the nucleus or spindle during both first and second cell cycles. The observed wave of change may thus reflect the local appearance and spreading of mitotic activity as the zygote nucleus approaches mitosis.