Folia biologica (Kraków), vol. 57 (2009), No 1-2 doi:10.3409/fb57_1-2.13-21 UltrastructuralExaminationofSpermiogenesisandSpermatozoon UltrastructureinCongotetra Phenacogrammusinterruptus Boulenger, 1899 (Ostariophysi:Characiformes:Alestidae) AnnaPECIO Accepted September 15, 2008 PECIO A. 2009. Ultrastructural examination of spermiogenesis and spermatozoon ultrastructure in Congo tetra Phenacogrammus interruptus Boulenger, 1899 (Ostariophysi: Characiformes: Alestidae). Folia biol. (Kraków) 57: 13-21. Ultrastructural studies of spermiogenesis in Phenacogrammus interruptus using transmission electron microscopy revealed that the process is characterized by flagellum development, formation of a cytoplasmic canal, nuclear rotation, and nuclear fossa formation. Chromatin compaction proceeds during spermatid transformation within the spermatocysts as well as after spermiation within the lumen of the efferent ducts. The spermatozoon is of primitive type and exhibits characters typical for Type I aquasperm. The head consists of a spherical nucleus with highly condensed chromatin and a centrally located electron lucent area connected to a moderate-sized nuclear fossa. The nuclear fossa contain centrioles in perpendicular arrangement, surrounded by osmiophilic fibrous material. In the short midpiece, several mitochondria and vesicles are unevenly distributed in the cytoplasm forming the cytoplasmic collar at the base of the nucleus. The cytoplasmic collar surrounds theinitialpartoftheflagellum,runninginthecytoplasmiccanal.Theflagellaraxonemehasa typical pattern (9x2+2) and the flagellum contains membranous compartments in the portion immediately posterior to the termination of the cytoplasmic canal. Key words: Spermiogenesis, sperm ultrastructure, Characiformes, Alestidae, Phenaco- grammus interruptus. Anna PECIO, Department of Comparative Anatomy, Institute of Zoology, Jagiellonian Uni- versity, R. Igardena 6, 30-060 Kraków, Poland. E-mail: [email protected] Several studies in many large groups of extant gellaandaxonemestructurerevealahighdiversity fishes have shown that morphological differences at the subfamily and/or species level (BACCETTI in sperm ultrastructure can be used as independent 1991; JAMIESON 1991; MATTEI 1991; LAHNSTEI- characters of taxonomic value (MATTEI 1970, 1988; NER &PATZNER 2008). Sperm diversity can be JAMIESON 1991). Many fish taxa are represented explainedastheresultofphylogeneticinheritance, byasmallnumberofspecies (e.g.Halecomorphi– as well as adaptation to the mode of fertilization 1) and therefore the sperm morphology of one spe- (external versus internal) and environmental cir- cies may be representative of the entire group, cumstances at the time of gamete fusion (JAMIE- even at high taxonomic rank. A completely differ- SON 1991; BURNS &WEITZMAN 2005). ent situation pertains to the monophyletic Tele- Data on sperm structure in Ostariophysi, a large ostei, consisting of about 28 000 species for which group encompassing nearly 75% of all freshwater ultrastructural studies of spermatozoa have been fishes,areknownonlyforalimitednumberofspe- studied in only about 1% of extant species belong- cies, particularly in the orders Siluriformes and ing to different families (NELSON 1994; LAHN- Characiformes (JAMIESON 1991). The Silurifor- STEINER &PATZNER 2008). These studies have mes have a global distribution, whereas the Chara- revealed that the only apomorphic character com- ciformes are distributed in Africa, South and mon to the spermatozoa of all neopterygians is the Central America, and southern North America. lack of an acrosome. On the other hand, characters New World characiforms are a speciose group such as the shape of the head region, the number, comprised of 14 families and approximately 1460 shape and distribution of mitochondria, arrange- species, whereas African characiforms contain ments of centrioles and their locations within or only 208 species grouped in 4 families (CALCAG- posterior to a nuclear fossa, and the number of fla- NOTTO et al. 2005). Phylogenetic studies on 14 A.PECIO African characiforms revealed that they are para- for this study were obtained from a local aquarist. phyletic and form three separate subunits: a clade Animals were kept under a natural photoperiod in consisting of the families Distichodontidae and an aquarium (80x45x40cm) for several months Citharinidae, a clade formed by the monotypic prior to examination and fed daily ad libitum with Hepsetidae and a third clade, the Alestidae (GERY small crustaceans and Tubifex worms. Mature 1977; VARI 1979). The latter was previously in- males of total length 70-77 mm were killed by cluded into the family Characidae as the subfamily over-anaesthetization in a 1% solution of tricaine Alestinae (GREENWOOD et al. 1966). After an methasulphonate (MS-222). The gonads were re- evaluation of fossils and phylogenetic analysis, it moved, cut into small fragments, and fixed in 3% wasshownthatthefamilyCharacidaeiscomposed glutaraldehyde in a phosphate buffer (pH 7.4) of only neotropical genera, but the Alestidae form overnight. The material was postfixed in 1% os- a trans-Atlantic family within the Characiformes mium tetroxide in the same buffer and dehydrated (GERY 1977; ZANATA &VARI 2005). The mem- in graded alcohol. A small testis sample was em- bers of the African alestids evolved independently bedded in Epon 812. Ultra-thin sections were con- in Africa for 90-112 million years and developed trasted with uranyl acetate and lead citrate and into the most diverse group of characiforms, in- were later examined under a JEOL JEM-100SX cluding 105 species, ranging from the diminutive transmission electron microscope. species of Virilia to the large tiger fish, Hydro- cynus goliath (ZANATA &VARI 2005). Sperm structure among characiforms has now Results been studied in several genera belonging mostly to the neotropical fish fauna, e.g. in Erythrynidae (QUAGIO-GRASSIOTTO et al. 2001a), Curimatidae Spermiogenesis (QUAGIO-GRASSIOTTO et al. 2003), Anostomidae (PECIO 2003) and several species of Characidae, The stages of spermatids in P. interruptus were both from subfamilies with external and internal identified as early, medium or advanced according fertilization (BURNS et al. 1995, 1998; PECIO & to the state of chromatin, location of the centriolar RAFIÑSKI 1999; ROMAGOSA et al. 1999; ANDRADE complex in relation to the nucleus, the formation et al. 2001; PECIO et al. 2005, 2007; VERISSIMO- of the flagellum and rotation of the nucleus. The SILVEIRA et al. 2006). Among African characifor- nuclei of early spermatids possess heterogeneous mes, spermatozoon structure has been described in chromatin and are surrounded by a wide zone of one Citharinus sp. of the Citharinidae (MATTEI et cytoplasm containing spherical to elliptical mito- al. 1995) and in Alestes dentex of the Alestidae, in chondria and centrioles in a perpendicular ar- which the spermiogenesis was analyzed (SHAHIN rangement (Fig. 1). 2006a). For other alestids, Micralestes sp. and The next stage is characterised by the beginning Phenacogrammus interruptus, the similarity to of flagellum formation and the dislocation of the A. dentex spermatozoon in centrioles arrangement centriolarcomplexfromtheperipheryofthecellto in nuclear fossa was indicated by BURNS et al. (in a position adjacent to the nucleus. At this time, a press). depression begins to appear in the nuclear contour The main purpose of the present study is to pro- near the proximal centriole forming the future nu- vide information on spermiogenesis and sperm clear fossa. The distal centriole differentiates into structure in the externally fertilizing species P. in- the basal body and remains associated with the terruptus for a comparison with sperm structure of plasma membrane. The developing flagellum runs A. dentex and other species of Characiformes. This in the cytoplasmic canal, i.e. the space between the information will contribute to the growing data- flagellar membrane and plasmalemma of the canal base of ultrastructural characters that are being (Fig. 2). The chromatin is finely fibrous in nearly used in hypothesizing phylogenetic relationships the entire volume of the nucleus except for the among characiform species (WEITZMAN &MALA- electron lucent area (Fig. 2). BARBA 1998). At the medium stage, the flagellum and cyto- plasmic canal undergo extension and the nucleus starts to rotate anterior to the centriolar complex, Material and Methods formingadeepindentationfortheproximalcentri- ole (Fig. 3). Nuclear chromatin starts to condense The natural population of the Congo tetra, irregularly,withelectronlucentandelectrondense Phenacogrammus interruptus Boulenger, 1899, areas appearing within the nucleus. Most mito- inhabits large areas of the Congo river system. chondria are distributed at the basal pole of the nu- This species is one of the most popular, colorful cleus, where the main mass of cytoplasm surrounds and commonly available African tetra. The males the cytoplasmic canal. In the course of differentia- SpermiogenesisandSpermatozoonUltrastructurein Phenacogrammusinterruptus 15 Figs 1-4. Spermiogenesis stages in early spermatids of P. interruptus. Fig. 1. Early spermatids have nuclei with heterogeneous chromatin; mitochondria and the centriolar complex are located in a wide layer of cytoplasm. Fig. 2. A spermatid at the beginning of flagellum and cytoplasmic canal formation. The nucleus possesses an electron lucent area. Fig. 3. A spermatid during rotation of the nucleus above the centriolar complex. The