Paired spermatozoa in Thermobia (Insecta, Thysanura)

Abstract

Spermatozoa from the primitive apterygote insect Thermobia domestica have been examined in the living state, in ultrathin sections, and in freeze-fracture replicas. The sperm cells contain an axoneme, two mitochondrial derivatives, and a nucleus as four parallel strands that run along most of the length of the cell. Most of them pair along the anterior region after a release from the testis to form what has been called joined (paired, double, or twin) spermatozoa. The cell junction appears in sectioned material as an attenuation of the otherwise prominent glycocalyx and as globules in the two opposing plasma membranes. These globules are in register. The distance between the opposing plasma membranes of two spermatozoa is 14 nm, which is substantially greater than the distance in a gap junction. In the freeze-fracture replicas the cell junction is seen as a straight row of prominent particles (9–10 nm), that usually are associated with the P face of the plasma membrane. In most sperm pairs the junction is located at the “ventral side” thus near the nucleus but pairing may also occur laterally. The mitochondria seem to be attached to a strand surrounding the flagellar axoneme by a set of specialized thin ribbons which in the freeze-fracture replicas are seen as prominent rows of membrane particles that are associated with the E face of the mitochondrial membrane. We could confirm that spermatozoa are motile only when appearing in pairs and that the posterior free portion can beat independently although in synchrony. The pairing may somehow be a regulating device for sperm motility.