Chromosome analysis of Proteocephalus osculatus (Cestoda: Proteocephalidea)

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The cestode order Proteocephalidea Mola, 1928 is dominated by the cosmopolitan genus Proteocephalus Weinland, 1858, which includes about 40% of its nominal species. Proteocephalus spp. are found in freshwater fishes, amphibians, and reptiles (Freze V.I. 1965: Proteocephalideans – Tapeworm Helminths of Fish, Amphibians and Reptiles. Essentials of Cestodology, Vol V. Publ. House Nauka, Moscow, 540 pp., in Russian; Brooks D.R. 1978: Syst. Zool. 27: 312-323; Brooks D.R., Hoberg E.P., Weekes P.J. 1991: Proc. Biol. Soc. Wash. 104: 651-668). There is presently a lack of agreement on the taxonomic status of many Proteocephalus species. Brooks et al. (Brooks et al. 1991, op. cit.) have suggested that various species currently assigned to Proteocephalus may be more closely related to a variety of different groups than to each other. The recent comparative studies based on morphological features and molecular systematic analysis have demonstrated a general uniformity among most Proteocephalus spp. in Europe (Scholz T., Hanzelova V. 1998: Tapeworms of the Genus Proteocephalus Weinland, 1858 (Cestoda: Proteocephalidae), Parasites of Fishes in Europe, Academia, Praha, 118 pp.; Zehnder M.P., Mariaux J. 1999: Int. J. Parasitol. 29: 1841-1852). On the other hand, a high degree of intraspecific morphological variability has recently been found to occur within Palaearctic Proteocephalus spp. (Hanzelova V., Spakulova M. 1992: Folia Parasitol. 39: 307-316; Anikieva L.V. 1993: Parazitologiya 27: 260-268; Anikieva L.V. 1995: Parazitologiya 29: 505-510; Hanzelova V., Snabel V., Kral’ova I., Scholz T., D’amelio S. 1999: Can. J. Zool. 77: 1450-1458). Due to the presence of high intraspecific variability and the scarcity of taxonomically useful morphological characters, cytogenetics may be a useful tool to differentiate species and clarify their phylogenetic relationships. Proteocephalus osculatus (Goeze, 1782) is a specific parasite of the European wels (Silurus glanis L.) and is distributed throughout its host’s Eurasian range (Freze 1965, op. cit.; Dubinina M.N. 1987: Class Cestoda Rudolphi, 1808. In: O.N. Bauer (Ed.), Key to the Parasites of Freshwater Fishes. Vol 3. Publ. House Nauka, Leningrad, pp. 5-76, in Russian). This study provides the first description of the karyotype of P. osculatus based on the analysis of colchicinetreated mitotic chromosomes. Ten specimens of P. osculatus used for karyological exami-nation were recovered from the gut of European wels (S. glanis) caught in Curonian Bay near Ventė settlement (Lithuania) in June 1999. Whole living specimens were placed in 0.01% colchicine in physiological solution for 3-4 h at room temperature. Fixation involved three changes (20 min each) of a freshly prepared solution of ethanol-acetic acid (3 : 1). Each slide was made from a single individual using a cellular suspension air-drying technique (Petkeviciūtė R., Ieshko E.P. 1991: Int. J. Parasitol. 21: 11-15). Slides were stained directly with 4% Giemsa in phosphate buffer (pH 6.8) for 40 min. Metaphase plates, suitable for karyological analysis, were photographed, and photomicrographs were used for construction of karyotypes. Seven well-spread metaphase plates were chosen for karyometric analysis. The classification of chromosomes followed that of Levan et al. (Levan A., Fredga K., Sandberg A. 1964: Hereditas 52: 201-220). When a centromere position was on the borderline between two categories, two chromosome categories were listed. From 10 specimens of P. osculatus, 79 mitotic plates were obtained where the chromosomes were spread well enough to be counted, and of these, 70 contained 18 chromosomes. All other values were lower than 18 and were attributed either to aneuploidies or, more likely, to loss of chromosomes during slide preparation. The chromosomes of P. osculatus were relatively small. Their mean absolute length ranged from 1.02 μm to 3.26 μm (Table 1), and the mean total length of the haploid set (TCL) was 17.49 μm. The first pair of homologues was noticeably larger than the remaining pairs which decreased in size fairly gradually (Fig. 1). According to centromeric index values, chromosomes 1, 7, and 9 were subtelocentric, pairs 2, 5, and 8 submetacentric, pair 3 metacentric, 4 subtelo-submetacentric, and 6 submeta-metacentric. Within the order Proteocephalidea, chromosome morphology has been described for three Proteocephalus spp.: P. percae, P. exiguus, and P. macrocephalus (Spakulova M., Hanzelova V. 1992: Folia Parasitol. 39: 324-326; Petkeviciūtė R. 1993: Biology (Vilnius) 1: 47-48; Hanzelova V., Snabel V., Spakulova M., Kral’ova I., Fagerholm H-P. 1995: Can. J. Zool. 73: 1191-1198; Scholz T., Spakulova M., Snabel V., Kral’ova I., Hanzelova V. 1997: Syst. Parasitol. 37: 1-12). The same chromosome number (2n = 18) has been found, but with some differences in chromosome morphology. A different chromosome number (2n = 14) was reported for another proteocephalidean, Acanthotaenia multitesticulata, but without any details of karyotype morphology (Vijayaraghavan S., Subramanyam S. 1980a: Z. Parasitenkd. 63: 65-70). The diploid chromosome numbers in Cestoda range from 6 in Microsomacanthus spp. (Hymenolepididae) (Petkeviciūtė R., Regel K.V. 1994: J. Helminthol. 68: 53-55) to 28 in Nematotaenia dispar (Nematotaeniidae) (Vijayaraghavan S., Subramanyam S. 1980b: Riv. Parassitol. 41: 371-375). ChroFOLIA PARASITOLOGICA 48: 159-161, 2001