School of Botany - Theses
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ItemStudies on the flagella and cytoskeleton of pleurochrysis carterae (prymnesiophycae) and mallomonas splendens (synurophyceae)(University of Melbourne, 1989)The flagellar apparatus of motile, coccolith-bearing cells of Pleurochrysis carterae (Braarud & Fagerlund) Christensen (Prymnesiophyceae) and that of Mallomonas splendens (G.S.West) Playfair (Synurophyceae) are described at interphase and during cell division. The interphase arrangement of the flagellar apparatus in Pleurochrysis carterae is similar to that in Pleurochrysis sp. (Inouye & Pienaar 1985) but I report new information on microtubular root 1 (R1) and describe in detail the transition region of the flagellar axoneme. The basal body of the longer flagellum is shown to be that which is associated with R1. A distinctive membrane decoration is noted on specific areas of the peripheral endoplasmic reticulum (PER) and I discuss the role of the PER in scale secretion and endocytosis. The basal bodies of P. carterae duplicate before mitosis and the crystalline roots (CRs), which arise from R1 and root 2 (R2), disassemble at prophase as their component microtubules elongate towards the future spindle poles. By late prometaphase the basal bodies have segregated semi-conservatively and each pair displays diminutive flagellar roots for the future daughter cells. The two parental basal bodies now bear R1s, indicating that the basal body that produced a short flagellum in the parental cell has transformed into one that will produce a long flagellum in the daughter. All flagellar roots assume their interphase appearance by early cytokinesis. Amputation of the flagella of P. carterae results in their immediate regeneration according to deceleratory kinetics. In the presence of 1?g/ml cycloheximide flagella regenerate to only c. half-length. Immunofluorescence microscopy shows that CRs diminish in size as flagella regenerate and, in the presence of cycloheximide, are depleted when flagella cease to elongate. These data indicate that a pool of presynthesized precursors is available for flagellar regeneration in P. carterae and that part of this pool consists of tubulin from the CRs. Cells of Mallomonas splendens bear a single emergent flagellum (F1) and a non-functional basal body (F2). Direct observations on dividing cells show that the FIs of daughter cells arise from newly formed basal bodies as the parental F1 retracts and thereby transforms into a new F2; the parental F2 remains as such for successive generations. These results are confirmed with thin-sections. The flagellar apparatus of M. splendens at interphase displays numerous features that are novel in the Synurophyceae: the basal bodies are surrounded by a distinctive fibrous capsule; this is continuous with a thick, fibrous band that constitutes the anterior portion of the rhizoplast; the posterior of the rhizoplast forms a cone of numerous, striated straps over the nuclear apex; a single (three-membered) microtubular root (R1) is present which forms a loop around the basal bodies and descends to terminate on the rhizoplast near the nuclear apex; the descending portion of R1 has at least two orientations with respect to the basal bodies. The development of these structures is described from dividing cells. The R1s and rhizoplasts for daughter cells are formed de novo in association with new basal bodies at early mitosis as the parental structures disassemble. The new rhizoplasts are the organizing centres for the mitotic spindle. Mitosis in M. splendens is compared to that in the Chrysophyceae. The final chapter deals with the deployment of the four posterior bristles in M. splendens. Bristles articulate at their flexed basal ends, via an attached fibrillar complex, on specialized body scales (base-plate scales). Bristles are formed independently of their base-plate scales and I describe how they are united outside the cell. Mature posterior bristles are secreted onto the plasma membrane at late interphase and then extruded, basal ends first, from beneath the layer of body scales. Once bristles are fully extruded they are drawn back to the posterior apex of the cell with their basal ends leading - thus a 180� reorientation of the bristles has been effected outside the cell. A cytoplasmic protuberance which contains at least one microtubule accompanies the bristles throughout this reorientation and I propose that it is intimately involved. The fibrillar complex is formed in situ on the bristles and appears to mediate the deployment of bristles onto new base-plate scales.