Spermiogenesis in Marsilea vestita: a temporal correlation between centrin expression and blepharoplast differentiation.

The motile male gamete of the water fern Marsilea vestita is a spirally shaped cell that possesses a complex cytoskeletal array of microtubules and approximately 140 cilia. Spermiogenesis in this organism is a rapid process that requires only approximately 11 h at 20 degrees C and involves the de novo synthesis of basal bodies from an organelle known as a blepharoplast. The developmental program that gives rise to the spermatozoids begins with nine mitotic divisions that occur in rapid succession during the first 5.5 h after imbibition of the dry microspore. During the next 5.5 h, the spermatids undergo a complicated differentiation process. We have asked what new proteins must be made for differentiation to proceed. Inhibitor treatments reveal that some translation is a necessary prerequisite for the differentiation and release of spermatozoids, but methionine-labeling studies demonstrate that relatively few types of proteins must be translated for this developmental program to reach completion. We have found that the dry microspores contain alpha-, beta-, and gamma-tubulin, at levels that may be sufficient for the entire developmental process. The abundance of the tubulins remains essentially constant until very late stages of spermiogenesis. In contrast to the tubulins, we show that centrin begins to increase in abundance at approximately 4 h after imbibition and that it reaches a peak at 6 h after imbibition. We also show that centrin mRNA is stored in the dry microspore, and that centrin protein abundance is regulated at the translational level. We believe that the translation of stored centrin transcripts serves as a rate-limiting step in the rapid differentiation process of spermiogenesis M. vestita. We suggest that centrin functions in the microtubule organizing centers that are required for the construction of the cytoskeleton and the motile apparatus in these structurally complicated cells.