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Localization of Ribosomal Genes in Pleuronectiformes Using Ag-, CMA3-Banding and in Situ Hybridization

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Localization of Ribosomal Genes in Pleuronectiformes Using Ag-, CMA3-Banding and in Situ Hybridization Heredity 86 (2001) 531±536 Received 10 May 2000, accepted 2 October 2000 Localization of ribosomal genes in Pleuronectiformes using Ag-, CMA3-banding and in situ hybridization BELEÂ N G. PARDO, CARMEN BOUZA, JAIME CASTRO, PAULINO MARTIÂNEZ & LAURA SAÂ NCHEZ* Departamento de BiologõÂa Fundamental, Area de GeneÂtica, Facultad de Veterinaria, Universidad de Santiago de Compostela, 27002 Lugo, Spain In this paper we present the analysis of nucleolar organizer regions (NORs) in ®ve species of the order Pleuronectiformes (Scophthalmus maximus, Scophthalmus rhombus, Platichthys ¯esus, Solea solea and Solea lascaris), a group of ®sh poorly studied cytogenetically. In spite of the small chromosome sizes, which characterize the karyotypes of this group, we implement both classical (Ag- and CMA3- staining) and molecular (in situ hybridization with a major rDNA probe) techniques for NOR location. NORs were localized in a single chromosome pair in all species studied and showed size variation within each species. Evidence of a NOR-site polymorphism was detected in Platichthys ¯esus and Scophthalmus maximus. NOR location was apparently the same within families (S. rhombus vs. S maximus, and S. solea vs. S. lascaris) but clearly diered among families, and supports the phylogenetic relationships proposed for the order Pleuronectiformes. Keywords: in situ hybridization, NOR location, NOR polymorphism, phylogeny, Pleuronectiformes. Introduction Pleuronectidae, Soleidae, Scophthalmidae, Bothidae and Cynoglossidae) have been subjected to karyotype Flat®shes constitute a group of ®sh not well known analysis. In the majority of these studies only Giemsa cytogenetically. Hinegardner (1968) demonstrated that staining and four types of banding analysis (C-, Q-, RE- some ®sh, in particular the Tetradontidae, have very banding and Ag-staining) have been reported (Kikuno small genomes. The cellular DNA contents of ¯at®sh are et al., 1986; Fan & Fox, 1991; Vitturi et al., 1993; Bouza among the lowest found in ®sh (being 20% of that of et al., 1994). Mammals). In fact, they have very small chromosomes, Nucleolar organizer regions are usually detected in and their chromosome number lies around the modal ®sh by silver nitrate and GC-speci®c ¯uorochrome value described within Teleosts, 2n 48 acrocentric staining (Galetti et al., 1984). Silver treatment stains chromosomes, this karyotype being considered as the only active NORs (Hubbell, 1985), and therefore is more primitive one in this group (Ohno, 1970). The phylogeny appropriate for the study of NOR activity than for of the order Pleuronectiformes is rather controversial NOR location. Molecular techniques oer new oppor- and some authors (Vernau et al., 1994), comparing tunities for ®sh cytogenetics. In situ hybridization (ISH) molecular and morphological data, have suggested that has proved to be useful for the localization of repeated the family Soleidae could be the ancestral one within the gene families. In situ hybridization with rDNA probes order. In contrast, Le Grande (1975) proposed the directly detects the location of ribosomal RNA genes, Pleuronectidae as the ancestral family, using cytogenetic regardless of their activation status. data. However, there still remain many unsolved ques- In the present paper we describe the location of major tions in connection with chromosomal evolution in ribosomal genes in several species of the order Pleuro- Pleuronectiformes (Sakamoto & Nishikawa, 1980; Patro nectiformes, a group of ®sh of high interest for ®sh & Prasad, 1981; Kikuno et al., 1986; Vitturi et al., 1993). farming. Five species belonging to three dierent fam- Out of the more than 500 species of the order ilies were analysed using conventional cytogenetic Pleuronectiformes, only 37 (belonging to the families techniques (Ag- and CMA3-staining) and in situ hybrid- ization with an 18S rDNA probe: Scophthalmidae: *Correspondence. E-mail: [email protected] turbot (Scophthalmus maximus) and brill (Scophthalmus Ó 2001 The Genetics Society of Great Britain. 531 532 B. G. PARDO ET AL. rhombus); Pleuronectidae: ¯ounder (Platichthys ¯esus); Post-hybridization washes were carried out in 2 ´ SSC and Soleidae: sole (Solea solea) and sand sole (Solea for 5 min, three times in 50% formamide with 0.05% lascaris). Karyotype analysis is useful for studying of Tween 20 for 5 min, twice in 2 ´ SSC for 5 min phylogenetic relationships among related species, and (all these washes at 42°C) and twice in TNT (0.1 M also as a tool for genome mapping, a primary goal for Tris-HCl, 0.15 M NaCl, 0.05% Tween-20) for 3 min at genetic improvement in ®sh culture. room temperature. Hybridization probes were detected according to the method of Kerstens et al. (1995), using the TSA-Indirect kit (NEN Life Science Products). Five Materials and methods to 10 metaphases were checked in each individual. Samples and cell cultures Results and discussion Most individuals analysed were collected from natural populations, 10 specimens of each of the ®ve species Scophthalmus rhombus showed a diploid number of 44 studied (Scophthalmus maximus, Scophthalmus rhombus, chromosomes and 48 chromosome arms (NF), compri- Platichthys ¯esus, Solea solea, and Solea lascaris) being sing two metacentric, one submetacentric and 19 acro- taken from this source. In addition, 12 specimens of centric chromosome pairs (Fig. 1). Ag-staining, which S. maximus from a ®sh farm were analysed. reveals active nucleolar organizer regions (NORs) in the Chromosome spreads were obtained from spontane- previous interphase (Howell & Black, 1980; Hubbell, ously dividing cells of kidney and spleen, as previously 1985), localized active NORs in the short arms of a large described by Bouza et al. (1994). submetacentric pair (Fig. 2d). CMA3-staining, which reveals the presence of GC-rich heterochromatin, is usually associated with NOR regions in ®sh and Ag-staining Amphibia (Schmid, 1982; Phillips & Ihssen, 1985; Mayr Ag-staining was performed according to Howell & et al., 1986). In S. rhombus, NOR regions were CMA3 Black (1980). Around 20 metaphases were checked in positive (Fig. 2e), and no other region showed positive each individual for analysing the Ag-NOR pattern. staining with this technique. rDNA-ISH con®rmed the presence of ribosomal genes in the short arms of the same Ag- and CMA positive submetacentric pair CMA -staining 3 3 (Fig. 2f). This karyotypic constitution appears very Slides were treated with 0.5 mg/mL CMA3 for 2 h and similar to that reported previously in S. maximus counterstained with 0.1 mg/mL distamycin A (DA) for (Bouza et al., 1994), the location of rDNA cistrons in 15 min, as described by Schweizer (1979). Ten meta- pair number 3 being con®rmed after applying Ag- and phases were checked in each individual. Photographs CMA3-staining, and rDNA-ISH (Fig. 2a±c). were made using Kodak plus X Pan 125 ASA ®lm in an A rather incomplete description of the karyotypes of Olympus Vanox microscope with the standard blue Solea solea (2n 42; Barker, 1972), and S. lascaris excitation ®lter. (2n 42, NF 50; Vasil'ev, 1978) has been previously reported only with Giemsa staining. The karyotypes of these species, as revealed in our work, showed 2n 42 rDNA in situ hybridization (ISH) and NF 56±58, comprising 4 metacentric, 4±5 sub- The pB plasmid, which includes a long fragment of the metacentric and 12±13 acrocentric chromosome pairs. human rDNA transcription unit (the promoter and The variation in the number of chromosome arms and, most of the 18S gene; Wilson et al., 1978) was used accordingly, in the number of submetacentric and as a probe to detect rDNA clusters in the individuals acrocentric chromosomes, was due to the length poly- analysed. The probe was labelled by nick-translation morphism of the NOR-bearing chromosome pair with biotin 16-dUTP (Boehringer). Prior to hybridiza- observed in both species. This pair ranged from acro- tion, RNAse-pretreated slides were denatured for 2 min centric to submetacentric, and resulted in NFs of 56 at 70°C with 70% formamide in 2 ´ SSC and dehydra- (both acrocentric), 57 (acrocentric/submetacentric) or 58 ted through a graded ethanol series at )20°C, and then (both submetacentric) chromosome arms, in dierent 1air dried. After 10 min denaturation at 100°C, 30 lLof individuals. Our observations for S. lascaris clearly the hybridization mixture containing 50% formamide in disagreed with the karyotype described for this species 2 ´ SSC, 10% dextran sulphate, 100 ng/mL of herring by Vasil'ev (1978), who only reported four biarmed sperm DNA and 60 ng of biotinylated probe were chromosome pairs (1 M + 3 SM). This incompatibility applied to each slide under a coverslip. Hybridization could in part be due to the polymorphism observed in was performed overnight at 37°C in a humid chamber. the NOR-bearing pair, but also to misclassi®cation of Ó The Genetics Society of Great Britain, Heredity, 86, 531±536. RIBOSOMAL GENES IN PLEURONECTIFORMES 533 Fig. 1 Standard karyotype of Scophthal- mus rhombus (brill) obtained with Giemsa staining. The NOR-bearing chromosomes are shown in the insets after Ag- and CMA3-staining. Bars 1 lm. several biarmed chromosomes due to low technical 1996). In Pleuronectiformes, Vitturi et al. (1993) des- resolution. After Ag- (Fig 2j, m), and CMA3-staining cribed the existence of NOR size variation in B. podas. (Fig 2k, n), and rDNA-ISH (Figs 2l, o), NOR regions By contrast, in P. olivaceus (Kikuno et al., 1986) no appeared localized in the short arm of a medium variation with regard to NOR size was detected. In the submetacentric pair in both species. The karyotypes of present study, a NOR size heteromorphism was detected these two species are very similar, with NORs appar- with Ag-staining in the majority of the individuals of the ently located in the same chromosome pair. ®ve species analysed (Fig. 2). This variation was con- Platichthys ¯esus exhibited a karyotype of 2n 48, ®rmed with CMA3-staining and rDNA-ISH by using a close to the ancestral one of Teleosts (Klinkhardt et al., human 18S rRNA probe (Fig.
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