Spermatogenesis And Sperm Ultrastructure Of Mecoptera

Mecoptera is unique in Holometabola in that the most species possess a pair of prominent compound eyes in their larval stages. However, the phylogenetic position of Mecoptera within Holometabola is still controversial. Additional characters are desperately needed to solve the phylogenetic problem of Mecoptera.Spermatozoa are highly specialized male gametes in sexually reproductive animals. The spermatozoa of insects are characterized by patterns of rapid and divergent evolution, varying in morphology, especially the ultrastructure, to the extent that variation in sperm morphology provides valuable characters for the assessment of evolution and the reconstruction of phylogenies in various groups of insects. However, sperm structure is still poorly documented in Mecoptera to date, and only seven species have been investigated in three families. With regard to the spermatogenesis of Mecoptera, only Panorpidae and Bittacidae have been briefly involved in the spermiogenesis during the investigations of sperm ultrastructure. The detailed process of spermatogenesis has not been studied.In this thesis, the male reproductive system, spermatogenesis and sperm ultrastructure of Panorpidae, Bittacidae, and Panorpodidae were investigated using light and electron microscopy. The spermatogenesis of Mecoptera was studied in detail; the morphology of the male reproductive system and the sperm ultrastructure were described and compared in different groups. In addition, the phylogenetic analysis of Mecoptera was conducted based on the sperm ultrastructure for the first time. The main results are as follws:The male reproductive system of Mecoptera consists of paired symmetrical testes with testicular follicles, two epididymides, two partitioned vass deferentia, a pair of specialized seminal vesicles, a pair of mesadenia, one ejaculatory sac together with ectadenia(except Neopanorpa of Panorpidae and Panorpodidae), and aedeagus. The seminal vesicles of mecopterans are peculiar compared with most other insects, since they mainly serve a secretory function rather than storing sperm. The morphology of the male reproductive system is diverse significantly on the family level, such as the number of testicular follicles, the relative postion of testis and epididymis, the length of the postvesicular vasa deferentia, the size of mesadenia, and the presence of ectadenia. However, the morphology is similar within the same family.The spermatogenesis of Mecoptera was studied for first time. The results show that it is similar in Mecoptera. Spermatogonium undergoes seven successive divisions in each cyst,and generates 128 spermatids that transform later into spermatozoa, forming a sperm bundle with number normally less than 128. The acrosome is generated by the proacrosomal granule material, which originates from Golgi apparatus. The elongated nucleus is transformed through nucleus elongation and streamlining with a simultaneous condensation of chromatin.The axoneme is generated from the basal body. The mitochondrial derivatives are transformed from the conspicuous interlocked structure nebenkern, which is formed from numerous mitochondria aggregation and fusion in the cytoplasm of spermatids. Two lateral laminae are present alongside the lateral grooves of nuclear envelope in the spermatids during spermiogenesis. However, they disappear in the mature spermatozoa. In addition, in a few spermatid axonemes several microtubules were seen to be positioned outside the nine axonemal doublets. However, they disappear quickly, assuming a 9 + 2 axoneme pattern with nine doublets and two central microtubules in the flagellum.Spermatozoa are elongate in Mecoptera, longer than 1000 ?m, and each consist of a short head, a neck region, and a long flagellum. In addition, spermatozoa show a prominent and regular glycocalyx seen as longitudinal ridges that project from the plasma membrane. The head is composed of the apical bilayered acrosome with acrosomal vesicle and perforatorium,and the elongated nucleus with two lateral grooves. The neck region mainly consists of centriole and centriolar adjunct. The centriole, composed of nine doublets, inserts into one side of the posterior extremity of the nucleus. The centriolar adjunct is an electron-dense,sheath-shaped structure, which lies in the periphery surrounding the tip of the flagellum. The flagellum comprises an axoneme, two mitochondrial derivatives, one or two accessory bodies,and two peculiar extra-axonemal accessory structures. The two mitochondrial derivatives differ in length and diameter. Along the flagellum one smaller mitochondrial derivative gradually diminishes, and eventually disappears with only one larger mitochondrial derivative retained in the remaining length of the flagellum. The accessory body originated from centriolar adjunct, is shorter than the axoneme and mitochondrial derivatives. Two accessory structures are thin, assuming triangular shape in cross-section, and extend along the entire length of the flagellum. At the terminal portion of the flagellum, the mitochondrial derivative decreases in size and the microtubules of axoneme become disorganized because the doublets lose dynein arms and radial spokes.Sperm ultrastructure is diverse significantly on the family level of Mecoptera, such as the apperance of nucleus, the size of lateral grooves, the pattern of axoneme, the number of accessory body, and the cross-section shape of mitochondrial derivative. However, spermultrastructure is similar within the same family. In addition, the 9 + 9 + 2 axoneme of Panorpodidae was observed in Mecoptera for first time. And the 9 + 2 axoneme of Panorpidae and Bittacidae is likely to be acquired from the 9 + 9 + 2 axoneme by the evolution reversal.The phylogenetic relationship of Antliophora was analyzed on the basis of sperm ultrastructure with maximum parsimony. It is the first time to conduct the phylogenetic analysis of Mecoptera based on sperm ultrastructure. The results show that Mecoptera is a monophyly group, and the sister group of Mecoptera is Siphonaptera. It also supports that the sister taxon of Panorpodidae is Panorpidae rather than Bittacidae within Mecoptera.