Spatial re-organisation of cortical microtubules in vivo during polarisation and asymmetric division of Fucus zygotes.

Fucus zygotes polarise and germinate a rhizoid before their first asymmetrical division. The role of microtubules (MTs) in orienting the first division plane has been extensively studied by immunofluorescence approaches. In the present study, the re-organisation of MT arrays during the development of Fucus zygotes and embryos was followed in vivo after microinjection of fluorescent tubulin. A dynamic cortical MT array that shows dramatic reorganization during zygote polarization was detected for the first time. Randomly distributed cortical MTs were redistributed to the presumptive rhizoid site by the time of polarisation and well before rhizoid germination. The cortical MT re-organisation occurs independently of centrosome separation and nucleation. By the time of mitosis the cortical array depolymerised to cortical foci in regions from which it also reformed following mitosis, suggesting that it is nucleated from cortical sites. We confirm previous indications from immunodetection studies that centrosomal alignment and nuclear rotation occur via MT connexions to stabilised cortical sites and that definitive alignment is post-metaphasic. Finally, we show that cortical MTs align parallel to the growth axis during rhizoid tip growth and our results suggest that they may be involved in regulating rhizoid growth by shaping the rhizoid and containing turgor pressure.