© 2004 The Japan Mendel Society Cytologia 69(3): 261–273, 2004 Cytogenetic Studies on Metaphase Chromosomes of Six Bivalve Species of Families Mytilidae and Veneridae (Nucinelloidea, Mollusca) Abdel-Basset M. Ebied1 and Fayza M. Aly2,* 1 Cytogenetic Laboratory, Zoology Department, Faculty of Science (Qena), South Valley University, Egypt 2 Zoology Department, Faculty of Science (Qena), South Valley University, Egypt Received December 8, 2003; accepted April 3, 2004 Summary Chromosomes of 6 bivalve species were studied from mitotic metaphases using cell suspension techniques. Among Family Mytilidae, Modiolus barbatus (Linnaeus 1758) has a diploid chromosome number of 2nϭ32 with 5 metacentric, 2 submetacentric, 5 subtelocentric and 4 telocen- tric chromosome pairs, Septifer excisus (Wiegmann 1837) has 2nϭ26 with 9 metacentric, 2 submeta- centric, and 2 telocentric chromosome pairs and Septifer bilocularis (Linnaeus 1758) also has 2nϭ26 with 8 metacentric, 2 submetacentric and 3 subtelocentric chromosome pairs. Among Family Veneridae, Ruditapes decussatus (Linnaeus 1758) has 2nϭ38 with 6 metacentric, 5 submetacentric, 3 subtelocentric and 5 telocentric chromosome pairs, Circe scripta (Linnaeus 1758) has 2nϭ38 with 6 metacentric, 6 submetacentric, 4 subtelocentric and 3 telocentric chromosome pairs and Venus ver- rucosa (Linnaeus 1758) has also 2nϭ38 with 7 metacentric, 4 submetacentric, 4 subtelocentric and 4 telocentric chromosome pairs. These results are reported for the first time in Egypt. Key words Chromosomes, Karyotypes, Ideograms, Mytilidae, Veneridae, Bivalvia. The bases of cytotaxonomy were laid down when the principle of chromosome individuality was accepted and the chromosome theory of heredity became established. It was noticed that the karyotype characterized by the chromosome number, size and morphology, is a definite and con- stant feature of each species. Since 1992, new karyological data have been added for 65 bivalve species among which the first karyological data are given for 49 species. Within the bivalve class, the most frequent chromosome number of 2nϭ38 was reported by Nakamura (1985) and Thiriot- Quievreux (1994). Chromosome studies have been reported on 14 species of Family Veneridae: the chromosome number 2nϭ38 is most frequently (Menzel 1968a, Gerard 1978, Rasotto et al. 1981, Wang et al. 1998), while chromosome number and morphology (Koulman and Wolff 1977, Corni and Trentini 1990), karyotype (Ieyama 1990) and karyometric analysis (Ieyama 1985, Borsa and Thiriot- Quievreux 1990) are described. The diploid chromosome number (2n) of 30 bivalve species (Family: Mytilidae) range from 22 to 32. This diploid chromosome complements have all types of chromosome, and the total diploid chromosome length has been reported mostly from the Mytilidae and shows an increase as 2n in- crease. The karyotype variety is very apparent in the Mytilidae (Ahmed 1974, Diupotex-Chong et al. 1978, Ieyama 1984, Thiriot-Quievreux 1984, Cornet 1993, Pasantes et al. 1996, Holland et al. 1999, Vitturi et al. 2000, Thiriot-Quievreux 2002). Previous data are compiled in Table 1. This report presents the chromosome numbers, karyotypes and morphometric measurements of chromosomes in 3 species of Family Mytilidae, Modiolus barbatus, Septifer excesis and Septifer bilocularis and 3 species of Family Veneridae, Ruditopes decussatus, Circe scripta and Venus ver- rucosa, specially in Egypt. These outcomes are compiled in Table 2. * Corresponding author 262 Abdel-Basset M. Ebied and Fayza M. Aly Cytologia 69(3) Table1. Interspecific comparisons of karyological data Species/Family Origin 2n Karyotype Reference Mytilidae Septifer bilocularis Japan 26 3m, 2m/sm, 7st, 1t Ieyama 1983b Septifer excisus Japan 26 2m, 1m/sm, 2sm, 7st, 1t Ieyama 1983b Septifer keenae Japan 28 3m, 2m/sm, 2sm, 4st, 2t/st, 1t Ieyama 1983b Septifer virgatus Japan 28 4m, 1m/sm, 2sm, 4st, 1t/st, 2t Ieyama 1983b Modiolus barbatus Italy 32 6m, 8sm, 2st Libertini et al. 1996 Modiolus barbatus Italy 32 Rasotto et al. 1981 Modiolus arcuatulus Egypt 24 5m, 7sm Ghobashy et al. 1995 Modiolus arcuatulus Japan 32 6m, 6sm, 3st, 1t Ieyama 1984c Mytilus edulis France 28 6m, 8sm/st Martinez-Lage et al. 1996 Mytilus edulis France 28 2m, 6sm, 6st Thiriot-Quievreux and Ayraud 1982 Mytilus galloprovincialis Spain 28 6m, 8sm/st Martinez-Lage et al. 1994 Mytilus galloprovincialis France 28 1m, 6sm, 7st Thiriot-Quievreux and Ayraud 1982 Mytilus trossulus Canada 28 Gonzalez-Tizon et al. 2000b Mytilus trossulus Canada 28 7m, 7sm Martinez-Lage et al. 1997 Mytilus californianus Canada 28 7m, 7sm Martinez-Lage et al. 1997 Mytilus coruscus Japan 28 5m, 3sm, 6st Ieyama 1984c Mytilus desolations Indean 28 5m, 2m/sm, 4sm/st, 3st Thiriot-Quievreux 1984 Brachidontes minimus W Medit 28 5m, 9st Thiriot-Quievreux, unpublished erranean Brachidontes pharaonis S Medit 28 6m/sm, 8st Vitturi et al. 2000 erranean Brachidontes rodriguezi Argentina 28 2m, 12st, 2t Torreiro et al. 1999 Veneridae Circe scripta Japan 38 19m Ieyama 1980 Venus verrucosa Italy 38 Rasotto et al. 1981 Ruditopes decussatus Spain 38 Rodriguez-Juiz et al. 1996 Ruditopes decussatus France 38 9m/sm, 10st/t Borsa and Thiriot 1990 Ruditopes philippinarum Canada 38 Gonzalez-Tizon et al. 2000b Ruditopes aureus France 38 17m/sm, 2st/t Borsa and Thiriot 1990 Venerupis pullastra Spain 38 3m, 8sm, 8st Insua and Thiriot-Quievreux 1992 Venerupis rhomboides Spain 38 4m, 8sm, 4st, 3t Insua and Thiriot-Quievreux 1992 Chione cancellata USA 38 Menzel 1968a Irus mitis Japan 38 15m, 4sm Ieyama 1980 m: metacentric, sm: submetacentric, st: subtelocentric, t: telocentric. Table2. Summary of karyological data of the present study Karyotype Family/Species 2n Reference m m/sm sm st st/t t Mytilidae Septifer bilocularis 26 3 2 7 1 Ieyama 1983b Septifer bilocularis 26 8 2 3 Present study Septifer excisus 26 2127 1 Ieyama 1983b Septifer excisus 26 9 2 2 Present study Modiolus barbatus 32 6 8 2 Libertini et al. 1996 Modiolus barbatus 32 5 2 5 4 Present study Veneridae Circe scripta 38 19 Ieyama 1980 Circe scripta 38 6 6 4 3 Present study Ruditopes decussatus 38 9 10 Borsa and Thiriot 1990 Ruditopes decussatus 38 6 5 3 5 Present study Venus verrucosa 38 Rasotto et al. 1981 Venus verrucosa 38 7 4 4 4 Present study m: metacentric, sm: submetacentric, st: subtelocentric, t: telocentric. 2004 Cytogenetic Studies on Metaphase Chromosomes 263 Materials and methods Samples of the different species investigated were collected from different localities along the coast of the Red Sea from Hurghada to Quseir City. The collected specimens were transferred di- rectly to the laboratory and kept alive in tanks of aerated seawater. The whole animals were prepared by the warm-dry method of Kligerman and Bloom (1977) with some modification made by Nakamura (1986) as follows: The bivalves were kept in 0.005% colchicine solution for 2–6 h before being sacrificed, and then the gonads and gills were removed, cut into small pieces and soaked in 0.075 m KCl hypotonic solution. These pieces were fixed in freshly mixed Carnoy’s fixative (3 : 1 methyl alcohol : acetic acid) with 3 changes of 20 min dura- tion. Tissues were then minced gently in 50% acetic acid to prepare a cell suspension. A drop of the cell suspension was pipetted out and placed on heated clean glass slides. For conventional kary- otypes, slides were stained directly with Giemsa (4% in phosphate buffer, pH 6.8) for 20 min. The best spread chromosome configurations were cut out from the photographs and chromo- somes were paired on the basis of centromeric position and size for karyotyping. The following morphological features were used to compare the karyotypes: the relative length (Rl), the arm ratio (Ar) and the centromeric index (Ci). Fundamental number (FN) was calculated on the basis that metacentric, submetacentric and subtelocentric having a value of four and acrocentric has a value of two. Nomenclature of chromosome type adopted by Levan et al. (1964) were used in the present study. Results and discussion Modiolus barbatus The chromosome number from the species Modiolus barbatus was determined from gills, (a) (b) Fig. 2. Ideograms of the chromosomes of Modiolus bar- Fig. 1. Mitotic metaphase spread (a) and karyotype (b) of batus constructed with respect to the relative length (a) Modiolus barbatus 2nϭ32. and the centromeric index (b). 264 Abdel-Basset M. Ebied and Fayza M. Aly Cytologia 69(3) oogonial and spermatogonial germ line cells; gills offered better results for chromosomal analysis than gonads. The photograph of the cell spread and the karyotype of this species showed that it has a diploid chromosome number of 2nϭ32 with fundamental number (FN) of 56 (Fig. 1a). An ideogram (Fig. 2a, b) was constructed from the relative length and centromeric indices with the cen- tromere drawn at the same level to make the visual comparison easier. The karyotype of Modiolus barbatus show 4 different groups of chromosomes (Fig. 1b). Group A is composed of 5 metacentric chromosome pairs with an arm ratio ranging from 1.09 to 1.64, relative length from 6.02% to 10.42% and centromeric index variying from 37.93 to 47.78. Group B has 2 submetacentric chromosome pairs with an arm ratio ranging from 1.88 to 2.00, rela- tive length from 5.21% to 6.65% and centromeric index variying from 33.33 to 34.78. Group C consists of 5 subtelocentric chromosome pairs with an arm ratio ranging from 3.12 to 4.17, relative length from 4.05% to 6.95% and centromeric index variying from 19.35 to 24.27 and group D con- tains 4 telocentric chromosome pairs with arm ratio ranging from 7.67 to 9.60, relative length from 4.51% to 6.13% and centromeric index variying from 9.43 to 11.54 (Table 3). The diploid chromo- some number of Modiolus barbatus 2nϭ32 is in agreement with that reported by Libertini et al.