ORIGINAL PAPER The spermatogenesis and sperm structure of Timema poppensis (Insecta: Phasmatodea) Marco Gottardo • David Mercati • Romano Dallai Received: 15 February 2012 / Revised: 2 April 2012 / Accepted: 4 April 2012 / Published online: 5 May 2012 Ó Springer-Verlag 2012 Abstract The ultrastructure of spermatogenesis and spermatozoa was studied in Timema poppensis Vickery & Sandoval, 1999, a putative basal taxon of Phasmatodea. The apical portion of testis follicles consists of spermato- gonial cells with polymorphic nuclei. Primary spermato- cytes display very short primary cilia originating from the peripheral centrosomes. Early spermatids develop a con- spicuous ‘‘nebenkern’’ consisting of fused mitochondria. They have a single peripheral centriole with microtubular triplets, which expresses a 3.6-lm-long cilium featuring a 9 ? 2 axonemal pattern. In a later stage, the centriole and the ciliary shaft displace toward the inner part of the cytoplasm by an infolding of the plasma membrane. Mature spermatids exhibit a derived centriole with micro- tubule doublets devoid of dynein arms, which is equipped with a dense arc-like outer structure. Ciliary degeneration was not observed during spermiogenesis. Spermatozoa are short flagellate cells about 55–60 lm in length. They are characterized by a three-layered acrosomal complex. The distinctive bell-shaped morphology of the acrosome vesicle is likely an autapomorphic trait of Timema. The flagellum has a 9 ? 9 ? 2 axoneme, two accessory bodies, two flattened cisterns, and two elongated mitochondrial derivatives. Results support the hypothesis that Phasmato- dea, comprising Timema ? Euphasmatodea, form a mono- phyletic group. The presence of 17 protofilaments in the wall of accessory microtubules and the flattened configu- ration of the flagellum are potential apomorphic groundplan features of the order. Within Phasmatodea, a key evolu- tionary divergence was from the conventional insect sper- miogenesis and sperm structure of Timema, to the unusual spermiogenetic process and peculiar sperm structure of Euphasmatodea. As a result, Timema retains more sperm character states found in the polyneopteran ground pattern, while Euphasmatodea have evolved outstanding sperm au- tapomorphies, like the loss of mitochondria and flattened cisterns, and the presence of strongly expanded accessory bodies. Keywords Timema Á Euphasmatodea Á Spermatogenesis Á Ciliary shaft Á Sperm flagellum Á Insect phylogeny Introduction Stick and leaf insects (Phasmatodea) are a phytophagous group of hemimetabolous hexapods (Bedford 1978). The order comprises over 3,000 described species, which often display elaborate morphological and behavioral adapta- tions to resemble part of plants, like twigs, leaves, or barks. Phasmatodeans mostly inhabit the tropical and subtropical regions of the world, where they are associated with the vegetation of a wide range of forest ecosystems. From a systematic point of view, the group is well nested within the comprehensive clade Polyneoptera, which includes also the extant lower neopteran groups Blattodea, Dermaptera, Embioptera, Grylloblattodea, Isoptera, Mantodea, Manto- phasmatodea, Orthoptera, Plecoptera, and Zoraptera (Terry Communicated by A. Schmidt-Rhaesa. M. Gottardo Á D. Mercati Á R. Dallai (&) Department of Evolutionary Biology, University of Siena, Via Aldo Moro 2, 53100 Siena, Italy e-mail: romano.dallai@unisi.it M. Gottardo e-mail: gottardo@unisi.it D. Mercati e-mail: david.mercati@unisi.it 123 Zoomorphology (2012) 131:209–223 DOI 10.1007/s00435-012-0158-z