Evidence for a direct role of nascent basal bodies during spindle pole initiation in the green alga Spermatozopsis similis

Basal body replication in the naked biflagellate green alga Spermatozopsis similis was analyzed using standard electron microscopy and immunogold loca lization of centrin, an ubiquitous centrosomal protein, and p210, a recentl y characterized basal apparatus component of S. similis. Fibrous disks repr esenting probasal bodies appear at the proximal end of parental basal bodie s at the end of interphase and development proceeds via a ring of nine sing let microtubules. Nascent basal bodies dock early to the plasma membrane bu t p210, usually present in basal body-membrane-linkers of S. similis, was a lready present on the cytosolic basal body precursors. In addition to the d istal connecting fiber and the nuclear basal body connectors (NBBC) of the parental basal bodies, centrin was present on the fibrous probasal bodies, in a linker between probasal bodies and the basal apparatus, in the connect ing fiber between nascent basal bodies and their corresponding parent, and, finally, a fiber linking the nascent basal bodies to the nucleus. This NBB C probably is present only in mitotic cells. During elongation a cartwheel of up to seven layers is formed, protruding from the proximal end of nascen t basal bodies. Microtubules develop on the cartwheel indicating that it te mporarily functions as a microtubule organizing center (MTOC). These microt ubules and probably the cartwheels, touch the nuclear envelope at both side s of a nuclear projection. We propose that spindle assembly is initiated at these attachment sites. During metaphase, the spindle poles were close to thylakoid-free lobes of the chloroplast, and the basal bodies were not in t he spindle axis. The role of nascent basal bodies during the initial steps of spindle assembly is discussed.