Microtubule organizing centers and the origin of centrioles during spermatogenesis in the pteridophyte Phylloglossum

Spermatogenesis in the lycophyte Phylloglossum is characterized by profound ultrastructural changes that involve complex microtubule arrays and discre te microtubule organizing centers (MTOCs). The first visible MTOC is an ele ctron-opaque acentriolar centrosome that; organizes the mitotic spindles in late spermatogenous cells. In the spermatid mother cell, centrioles arise de novo within the pericentriolar matrix of the MTOC. Approximately 20 cent rioles, which ultimately function as basal bodies, originate in each of two branched "blepharoplasts." Constituent centrioles of each organelle radiat e from a central region where they are interconnected by cartwheel cylinder s, each with nine-fold symmetry. The development and structure of this nove l centriolar-generating organelle suggests a direct evolutionary link with the bicentriole of other lycophytes, and are consistent with the concept th at multiflagellated spermatozoids in Phylloglossum evolved independently of those in other pteridophytes. During spermiogenesis, two additional struct urally defined MTOCs organize the cytoskeleton and locomotory apparatus, wh ich comprises 20 staggered flagella over a multilayered structure, An MTOC that overlies the multilayered structure and consists of a cloud of electro n-opaque material is involved in repositioning basal bodies and generating flagella. The spline, a band of up to 200 microtubules, provides the archit ectural framework for development and maintenance of cell shape and is orga nized by the lamellar strip, a highly structured MTOC. Microtubule arrays d uring spermatogenesis in Phylloglossum are diverse and include mitotic, cyt okinetic, cytoskeletal, and locomotory assemblages. MTOCs responsible for t he nucleation and organization of these arrays are among the most elaborate and morphologically distinct of any described in land plants. (C) 2000, Wi ley-Liss. Inc.