Sperm ultrastructure and spermiogenesis in the relic species Tricholepidion gertschi Wygodzinsky (Insecta, Zygentoma)

The silverfish Tricholepidion gertschi is of interest in that it is the most basal representative of Zygentoma. An ultrastructural study of its spermiogenesis was performed to find out whether there are traits which resemble those of other, more advanced insects. This was found to be the case; spermiogenesis can be considered to be of a common insectan type, leading to the formation of elongated sperm cells with acrosome, nucleus, neck region and a tail with axoneme and two mitochondrial derivatives. Total cell length, 50 microm, is short for an insect. There are some specializations, which probably represent autapomorphies. The acrosome has a posterior canal or cleft that makes a U-turn. The centriole adjunct forms a prominent post-nuclear ring surrounding the centriole and have a posterior extension, and further originates nine intertubular fibers with a longitudinal periodicity and two accessory bodies. The mitochondrial derivatives have five rows of regularly spaced cristae within a crystalline matrix. The axoneme has accessory tubules consisting of 16 protofilaments, formed at the B-tubules of the doublets and placed at some distance from them in the posterior part of the sperm tail.