Sperm structure and spermiogenesis in Coletinia sp. (Nicoletiidae, Zygentoma, Insecta) with a comparative analysis of sperm structure in Zygentoma

Abstract

The spermatozoon of Coletinia sp. has a bilayered acrosome, a short nucleus (4μm) and a relatively short sperm tail with two mitochondrial derivatives. The chromatin is uniformly dense except for several electron-lucid channels or strands which permeate the nucleus and which originate in the spermatid as invaginations of the nuclear envelope. The invaginations occur mostly or exclusively along two meridians of the spermatid that are also characterized by the presence of a longitudinal rod of medium electron density. The two rods (designated as ‘mid-spermatid rods’) evidently are instrumental in the formation of the electron-lucid channels. The significance of this elaborate system of intranuclear channels is not understood. The sperm tail has a 9+9+2 axoneme with each of the nine microtubular doublets accompanied by an accessory microtubule; scant intertubular material can also be distinguished. Hence, the tail axoneme resembles that of many pterygote insects. Each of the two mitochondrial derivatives contains a crystalline inclusion that has periodically spaced layers going in different directions on either side of the midline. Two synapomorphic traits appear to be shared by Ateluridae and Nicoletiidae, namely the invaginations of the nuclear membrane along two meridians of the nucleus and the shape of the crystalline inclusions of the mitochondrial derivatives. Four species from the family Lepismatidae were also examined as to their sperm ultrastructure. Three of them, Allacrotelsa kraepelini, Ctenolepisma longicaudata and Ctenolepisma sp., were found to be very similar to the two previously examined lepismatids, Thermobia domestica and Lepisma saccharina. On the other hand, spermatozoa of Tricholepisma aurea were aggregated in small groups rather than pairwise joined as seen in the other lepismatids. Sperm characters are also used to reconstruct a phylogenetic hypothesis which suggests a close relationship between Ateluridae and Nicoletiidae.