New findings on sperm ultrastructure in thrips (Thysanoptera, Insecta)

Sperm ultrastructure of several species in each of the two suborders of Thysanoptera Tubulifera and Terebrantia shows a distinctive and unusual architecture. Members of the whole order share a bizarre axoneme consisting of 27 microtubular elements derived from the amalgamation of 3 (9+0) axonemes present in each spermatid at the beginning of spermiogenesis. The reciprocal shifting of these axonemes along the length of the sperm, together with their possible shortening and overlapping for short distances, could explain why in some species it is never possible to observe the complete set of 27 microtubular elements in any one cross section. Tubuliferan sperm have a small elliptical (in cross section) acrosome extending the length of the sperm. In Bolothrips insularis and Compsothrips albosignatus this structure is larger and is associated with an external, flattened vesicle throughout its length. Terebrantian sperm lack an acrosome, but display for half their length a dense body running parallel to the nucleus. The sperm, in members of this suborder, are also characterized by possession of a small mitochondrion and by the unusual bilobed outline of cross sections through the anterior sperm region, with the nucleus located in one of the two lobes. Structures serving to anchor sperm to the inner surface of the cyst cell have been observed at their anterior tips in the testes of tubuliferans. In B. insularis, an anterior appendage is formed in immature sperm and is maintained in the mature spermatozoon parallel to its long axis in the most anterior region. Such an anchoring structure has not been observed in sperm of the terebrantian species examined, probably because the testis of terebrantians contains only a single cyst of developing gametes.