KARYOTYPES OF TWO MEDITERRANEAN BIVALVE SPECIES C Thiriot-Quiévreux

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C Thiriot-Quiévreux. KARYOTYPES OF TWO MEDITERRANEAN BIVALVE SPECIES. Vie et Milieu / Life & Environment, Observatoire Océanologique - Laboratoire Arago, 2004, pp.7-11. ￿hal- 03217962￿

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KARYOTYPES OF TWO MEDITERRANEAN BIVALVE SPECIES

C. THIRIOT-QUIÉVREUX Observatoire Océanologique de Villefranche, UPMC – CNRS – INSU, BP 28, 06230 Villefranche-sur-Mer, France [email protected]

BIVALVE ABSTRACT. – Chromosome preparations of two Mediterranean species were stu- MINIMUS died using an air-drying technique and conventional Giemsa staining. In Mytilaster ANOMIA EPHIPPIUM minimus (), a diploid chromosome number of 28 was found in gill tissue CHROMOSOME NUMBER KARYOTYPE and the karyotype included five metacentric and nine subtelocentric chromosome pairs. In Anomia ephippium (Anomiidae), chromosome numbers were n=6 in gona- dal tissue and 2n=12 in gill tissue. Its karyotype included one metacentric, two sub- metacentric and three subtelocentric chromosome pairs. Results are discussed within known bivalve chromosome data.

BIVALVE RÉSUMÉ. – Les préparations chromosomiques de deux espèces de Bivalves médi- terranéens ont été étudiés par une technique de suspension cellulaire et séchage à ANOMIA EPHIPPIUM l’air et une coloration standard au Giemsa. Chez Mytilaster minimus (Mytilidae), le NOMBRE DE CHROMOSOMES CARYOTYPE nombre diploïde de chromosomes est égal à 28 dans le tissu branchial et le caryo- type est composé de 5 métacentriques et de 9 paires subtélocentriques de chromoso- mes. Chez Anomia ephippium (Anomiidae), le nombre haploïde est de 6 dans le tissu gonadique et le nombre diploïde est de 12 dans le tissu branchial. Son caryo- type est composé de 1 métacentrique, 2 paires submétacentriques et de 3 subtélo- centriques de chromosomes. Les résultats sont discutés à la lumière des données connues sur les chromosomes de Bivalves.

INTRODUCTION sodium citrate in distilled water. The material was fixed in a freshly prepared mixture of absolute alcohol and acetic acid (3:1) with three changes of 20 min each. Sli- The area of Banyuls-sur-Mer (West Mediterra- des were made using an air-drying technique (Thiriot- nean, Gulf of Lion) includes many bivalve species Quiévreux & Ayraud 1982). Staining was performed with 4% Giemsa in phosphate buffer pH 6.8 and photo- (Mars 1965). In particular, the mytilid Mytilaster micrograhs of suitable metaphases were taken with a minimus (Poli, 1795) is abundant on calcareous Zeiss Photomicroscope II. substrates, such as sidewalk-like banks (Delamare Karyotype formula were obtained by statistical analy- Deboutteville & Bougis 1951). The anomiid sis of chromosome measurements. Terminology relating Anomia ephippium (Linné, 1758) is a common to centromere position follows that of Levan et al. eurybathial species from the infralittoral to the (1964). Both the arm ratio and the centromeric index are bathylittoral level (Mars 1965). Chromosome data given because each espresses centromeric position and of bivalve species have been reviewed (Patterson allows comparison with previous studies. When a cen- 1969, Nakamura 1985, Thiriot-Quiévreux 2002). tromere position was found on the borderline between This paper reports the not yet published karyotypes two categories, two chromosome categories were listed of these latter species. according to the confidence limit of the means (P=0.05).

MATERIALS AND METHODS RESULTS

Specimens of Mytilaster minimus also called Brachi- Mytilaster minimus dontes (Mytilaster) minimus were sampled from the si- dewalk-like banks near Banyuls. Specimens of Anomia ephippium were dredged at ca. 30 m in the Bay of Ba- The chromosomes of 16 metaphases taken from nyuls. 6 were counted. The diploid chromosome Live animals were incubated for 12 h in a 0.005% so- number was 28. Seven well spread metaphases lution of colchicine in sea water. Gills in Mytilaster mi- were analysed (Table I). The karyotype (Fig. 1) nimus, and gonads and gills in A. ephippium, were then consists of 5 metacentric (m), 2 subtelocentric (st) removed by dissection and treated for 45 min in 0.9% and 7 subtelocentric-telocentric (st-t) chromosome 8 C. THIRIOT-QUIÉVREUX

Table I. – Top, chromosome measurements and classification in 7 cells of Mytilaster minimus. Below, chromosome measurements and classification in 10 cells of Anomia ephippium.

Fig. 1. – Karyotype of Mytilaster minimus. Scale bar = 5 µm KARYOTYPES OF TWO BIVALVE SPECIES 9 pairs. The Figure 2 shows the ideogram constructed Three species of Mytilaster were studied, i.e. from relative length and centromeric index values. Mytilaster (Brachidontes) minimus with n=15 (Rasotto et al. 1981), with 2n=26 and a karyotype including 6 metacentric, Anomia ephippium 4 submetacentric, 1 subtelocentric and 1 telocentric chromosome pairs, and Mytilaster solidus with The chromosomes of 25 metaphases taken from 2n=28 and 5 metacentric, 3 submetacentric, 2 8 animals were counted. The diploid chromosome subtelocentric and 4 telocentric chromosome pairs number was 12. Sixteenth meioses with n=6 were (Libertini et al. 1996). The chromosome number of also observed in the gonad (Fig. 3). Ten well Mytilaster minimus, here studied (n=14), is differ- spread metaphases were analysed (Table I). The ent from the one reported (n=15) by Rasotto et al. karyotype (Fig. 3) consists of 1 m, 1 sm, 1 sm-st, 1 (1981) but similar to Mytilaster solidus (Libertini st-sm, 2 st. et al. 1996) and to (Vitturi et al. 2000). Its karyotype formula (5 m, 9 st) in- cludes more subtelocentrics than the other Mytilaster species even if several pairs are on the DISCUSSION bordeline of subtelocentric-telocentrics. Chromo- somal rearrangements have been suggested as a source of variation at the level among Chromosome data of the Mytilidae family have mytilid species (Libertini et al. 1996). Further been reported by Libertini et al. (1996) and more banding techniques would be necessary to analyse recently by Torreiro et al. 1999 and Vitturi et al. the role of chromosomal rearrangements which 2000. The diploid chromosome numbers range may have occurred during the evolutionary from 2n=22 to 32. One species of Brachidontes, speciation process. B. recurvus showed 2n=30 with 15 submetacentric Among Anomiidae, Rasotto et al. (1981) re- chromosome pairs (Diopotex-Chong et al. 1978) ported haploid number of n=12 in Anomia while Brachidontes pharaonis had 2n=28 with 6 ephippium while Ieyama & Inaba (1974) and metacentric-submetacentrics and 8 subtelocentrics Ieyama (1984) gave 2n=14 in gill tissue cell of (Vitturi et al. 2000) and Brachidontes rodriguezi Anomia chinensis with a karyotype including 3 2n=32 with 2 metacentrics, 12 subtelocentrics and metacentric and 4 subtelocentric chromosomes and 2 telocentric-subtelocentrics (Torreiro et al. 1999). 2n=26 in Monia umbonata. Our study showed n=6

Fig. 2. – Ideograms constructed from relative length and centromeric index values. A, Mytilaster minimus;B,Anomia ephippium. 10 C. THIRIOT-QUIÉVREUX

Fig. 3. – Anomia ephippium. A, meiosis; B, mitotic metaphase; C, karyotype. Scale bar = 5 µm.

in meioses and 2n=12 in mitoses of Anomia REFERENCES ephippium. This is the smallest chromosome num- ber reported among bivalve species studied up to now where the most frequent haploid chromosome number is n=19 (Thiriot-Quiévreux 2002). Delamare Deboutteville C, Bougis P 1951. Recherches sur le trottoir d’Algues calcaires effectuées à Banyuls Looking among close taxonomically bivalve pendant le stage d’été. Vie Milieu 2: 161-181. families, the Ostreidae also showed low haploid Diopotex-Chong MA, Rodriguez-Romer F, Uribe-Alco- chromosome numbers (n=10 for most of the spe- cer M, Laguarda-Figueras A 1978. Karyotypic cha- cies and n=9 in Dendostrea folium, Ieyama 1990) racters of Brachidontes recurvus Rafinesque, 1820, as the Gryphaeidae with n=9 in Pycnodonta co- (Pelecypoda: Mytilidae). An Centro Cienc del Mar y chlear (Vitturi et al. 1985). The interpretation of Limnol, Univ Nal Auton Mexico 5: 55-58. chromosome number as an indicator of evolution- Giribet G, Wheeler W 2002. On bivalve phylogeny: a ary status must be viewed with caution and is not high level analysis of the () based necessarily congruent with bivalve phylogeny on combined morphology and DNA sequence data. based on combined morphology and DNA se- Invertebr Biol 121: 271-324. quence data (Giribet & Wheeler 2002). The Sub- Ieyama H 1984. Chromosomes of six species in three fa- class Pteriomorpha is considered monophyletic. milies of Pteriomorpha. Venus 43: 106-110. However chromosome numbers range from n= 6 to Ieyama H 1990. Chromosomes of the oysters Hyatissa 19 within this Subclass. Even within families of imbricata and Dendostrea folium (Bivalvia: Pterio- this Subclass, chromosome numbers varied which morpha). Venus 49: 63-68. do not imply an evolutionary trend. Looking at the Ieyama H, Inaba A 1974. Chromosome numbers of ten families studied by Giribet & Wheeler (2002) and species in four families of Pteriomorpha (Bivalvia). following the same phylogenetic rank, one finds Venus 33: 129-137. n=11 to 16 in the Mytilidae, n=14 to 19 in the Levan A, Fredga K, Sandberg AA 1964. Nomenclature Arcidae, n=13-14 in the Pteriidae, n=16 to 19 in for centromere position on chromosomes. Hereditas 52: 201-220. the Limidae, n=9-10 in the Ostreidae, n=13 to 19 in the Pectinidae, and n=6-14 in the Anomiidae (ref- Libertini A, Boato A, Panozzo M, Fogato V 1996. Ka- ryotype and genome size in some species of Myti- erences quoted in Nakamura 1985, Libertini et al. lidae (Bivalvia, Mollusca). Kromosomo II 82: 1996, Thiriot-Quiévreux et al. 1991, Thiriot- 2819-2827. Quiévreux 2002). This chromosomal variability Mars P 1965. Faune marine des Pyrénées-Orientales. Vie does not infer any evolutionary trend. Gene se- Milieu 15 (suppl 4), 156 p. quences and chromosome profiles may not have Nakamura HK 1985. Review of molluscan cytogenetic evolved in parallel fashions. information based on the CISMOCH-Computerized index system for Molluscan chromosomes. Bivalvia, ACKNOWLEDGMENTS. – This work was carried out du- Polyplacophora and Cephalopoda. Venus 44: 193- ring the summer 1985 at the Laboratoire Arago, Ba- 225. nyuls-sur-Mer. Thanks are due to the collaboration of J Patterson CM 1969. Chromosomes of molluscs. Proc Soyer, who was the Director of the Laboratoire Arago at 2nd Sympos Mollusca. Mar Biol Ass India 2: 635- that time, and to P Chang for English editing. 689. KARYOTYPES OF TWO BIVALVE SPECIES 11

Rasotto M, Altieri D, Colombera D 1981. I cromosomi Torreiro A, Martínez-Expósito, Trucco MI, Pasantes JJ spermatocitari di 16 specie appartenenti alla classe 1999. Cytogenetics in Brachidontes rodriguezi d’Orb Pelecypoda. Atti 5th Congr Soc Malac Ital: 113-127. (Bivalvia, Mytilidae). Chromosome Res 7 : 49-55. Thiriot-Quiévreux C 2002. Review of the literature on Vitturi R, Carbone P, Catalano E 1985. The chromoso- bivalve cytogenetics in the last ten years. Cah Biol mes of Pycnodonta cochlear (Poli) (Mollusca, Pele- Mar 43: 17-26. cypoda). Biol Zbl 104: 177-182. Vitturi R, Gianguzza P, Colomba MS, Riggio S 2000. Thiriot-Quiévreux C, Albert P, Soyer J 1991. Karyoty- Cytogenetic characterization of Brachidontes pharao- pes of five subantarctic bivalve species. J Moll Stud nis (Fisher P., 1870): karyotype, banding and fluores- 57: 59-70. cent in situ hybridization (FISH) (Mollusca: Bivalvia: Thiriot-Quiévreux C, Ayraud N 1982. Les caryotypes de Mytilidae). Ophelia 52: 213-220. quelque espèces de Bivalves et de Gastéropodes ma- Reçu le 29 avril 2003; received April 29, 2003 rins. Mar Biol 70: 165-172. Accepté le 16 juin 2003; accepted June 16, 2003 12

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