NOTES Synaptonemal complex analysis in spermatocytes and oocytes of turbot, Scophthalmus maximus (Pisces, Scophthalmidae) N. Cuñado, J. Terrones, L. Sánchez, P. Martínez, and J.L. Santos Abstract: A surface-spreading synaptonemal complex (SC) technique was used to analyze spermatocytes and oocytes of turbot (Scophthalmus maximus) to visualize the process of chromosome synapsis. The total SC length was 205 ± 12 μm in males and 172 ± 29 μm in the only female analyzed. A representative SC karyotype of turbot was obtained. Each SC showed lateral elements of equal length. No bivalent exhibiting atypical synaptic behaviour that could be as- sociated with heteromorphic sex chromosomes was observed, either in males or in the female. The DNA content of turbot was evaluated in eight individuals of both sexes by flow cytometry analysis. The 2C mean DNA content of tur- bot (1.308 ± 0.009 pg/cell) was among the lowest observed within fishes. No statistical differences in DNA content were revealed between the sexes [Wilcoxon/Mann–Whitney test; P(W x = 0.243)]. The SC/DNA content ratio observed in turbot was the highest reported to date in bony fishes (Osteichthyes). Key words: Scophthalmus maximus, fish, DNA content, meiosis, synaptonemal complex. Résumé : Une technique d’étalement en surface des complexes synaptonémiques (CS) a été employée pour étudier les spermatocytes et les oocytes du turbot (Scophthalmus maximus) afin d’y visualiser le processus de synapse chromoso- mique. La longueur des CS totalisait 205 ± 12 μm chez les mâles et 172 ± 29 μm chez la seule femelle examinée. Un karyotype CS représentatif du turbot a été obtenu. Chaque CS montrait des éléments latéraux d’égale longueur. Aucun bivalent montrant un comportement synaptique atypique pouvant être dû à des chromosomes sexuels hétéromorphes n’a été observé chez les mâles et la femelle. Le contenu en ADN du turbot a été évalué chez huit individus des deux sexes par analyse de cytométrie en flux. La valeur 2C moyenne pour le turbot (1,308 ± 0,009 pg/cellule) figure parmi les plus faibles observées chez les poissons. Aucune différence statistique n’a été révélée entre les sexes [test de Wilcoxon/Mann–Whitney; P(W x = 0,243)]. Le ratio CS / contenu en ADN observé chez le turbot est le plus élevé parmi toutes les valeurs rapportées à ce jour parmi les poissons osseux (Osteichthyes). Mots clés : Scophthalmus maximus, poisson, contenu en ADN, méiose, complexe synaptonémique. [Traduit par la Rédaction] Notes 1147 Introduction Unlike studies on higher vertebrates, cytogenetic studies on fishes have traditionally been hampered by the small chromosome size of this group and the difficulty of obtain- ing serial bands (Medrano et al. 1988). These problems are even greater in flatfishes because their cellular DNA content is among the lowest within fishes, only 20% compared with that of mammals (Ventakesh et al. 2000). Nevertheless, their chromosome number lies around the modal number ob- served within Teleostei (2n = 48; Klinkhardt et al. 1995). Thus, only about 40 of the 570 species of Pleuronectiformes (Nelson 1994) have been karyotyped to date and C banding, Ag staining, and FISH (fluorescence in situ hybridization) have been applied only in a few cases (Bouza et al. 1994; Pardo et al. 2001). Adoption of microspreading techniques, originally devel- oped for the study of synaptonemal complexes (SCs) in spermatocytes (Counce and Meyer 1973), has allowed the acquisition of high resolution data on the ultrastructure of meiotic chromosome synapsis in several groups of plants and animals (e.g., von Wettstein et al. 1984; Gillies 1989). In addition, the visualization of whole SCs in fishes has re- sulted in a more accurate chromosome structure, because surface-spread pachytene chromosomes are several times Genome 44: 1143–1147 (2001) © 2001 NRC Canada 1143 DOI: 10.1139/gen-44-6-1143 Received April 10, 2001. Accepted August 2, 2001. Published on the NRC Research Press Web site at http://genome.nrc.ca on November 7, 2001. Corresponding Editor: W. Traut. N. Cuñado and J.L. Santos. Departamento de Genética, Facultad de Biología, Universidad Complutense, 28040- Madrid, Spain. J. Terrones, L. Sánchez, and P. Martínez. 1 Departamento de Biología Fundamental, Area de Genética, Universidad de Santiago de Compostela, 27002-Lugo, Spain. 1 Corresponding author (e-mail: paumarpo@lugo.usc.es).