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Supernumerary Chromosomes in Nephrops Norvegicus L. (Crustacea, Decapoda)

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Supernumerary Chromosomes in Nephrops Norvegicus L. (Crustacea, Decapoda) Heredity 76 (1996)92—99 Received 12 June 1995 Supernumerary chromosomes in Nephrops norvegicus L. (Crustacea, Decapoda) A. M. DEIANA*, E. COLUCCIA, A. MILIA & S. SALVADORI Dipart/mento di Biologia An/male ed Ecologia and tDipartimento di Biologia Sperimentale, Univers/ta' di Cagliari, Viale Poetto 1, 09126 Cagliari, Italy Supernumerarychromosomes have been identified in the chromosome complement of Nephrops norvegicus L. by C-banding and restriction endonuclease-induced banding. These chromosomes showed peculiar features of typical B chromosomes in terms of heterochromatin content, asynapsis and distorted segregation. The presence of these chromosomes can, at least partially, account for the numerical variability already observed in this species; 2n and n values determined by counting these chromosomes separately gave a less variable number of chromo- somes in the complement. 0-banding was useful to characterize some large B chromosomes with intercalary and subtelomeric bands as well as entirely positive short arms in some small submetacentric chromosomes. Nucleolar organizer regions have been identified in four pairs of chromosomes by silver staining. Keywords:Bchromosomes, chromosome banding, Nephrops norvegicus. Introduction Materials and methods Aremarkable intraspecific numerical variability of Specimenswere collected by trawling off the Sardin- the chromosome complements has been docu- ian coast at a depth of 200—700 m. Chromosome mented in Nephropidae (for reviews, see Nakamura, preparations from the testicular tissue of 10 males 1988; Corni et al., 1989). were made using the Doussau de Bazignan and The large number, small size and peculiar struc- Ozouf-Costaz technique (1985) and from embryos ture of the chromosomes present in the species according to a modified Klinkhardt technique belonging to the family make it difficult to define (1992). Briefly, ova with embryos in the first stage their karyotype. However, it seems unlikely that the were incubated in 0.02 per cent colchicine in sea variability observed among different individuals water for 3—5 h and then in a hypotonic solution of (Roberts, 1969; Hughes, 1982; Corni, 1989) is 0.075 M KC1 for 60 mm. The hypotonic medium was merely the result of a counting bias. The presence of twice replaced by fixative (3:1 methanol/acetic acid) supernumerary chromosomes and their peculiar for 20 mm and once more overnight. The chorion behaviour could better explain these variations. and the yolk were removed and the cells were In the present study on Nephrops norvegicus, we dispersed in a drop of acetic acid (40 per cent) and provide evidence of the presence of chromosomes spread over the slide surface. Embryo metaphases with some of the features and characteristic behav- were not used in determining chromosome number iour of B chromosomes (Jones & Rees, 1982a,b; because of the high loss of chromosomes resulting Jones, 1985). C-banding and DdeI-endonuclease from hypotonic treatment. digestion proved to be a useful tool in allowing a sharp differentiation of the chromosomes, a clear identification of meiotic figures and a better under- Staining standing of the nature of chromosome variability in Slideswere stained in 25 per cent Wright's stain (in this species. 0.06 M phosphate buffer, pH 6.8) for 8 mm, briefly rinsed in distilled water and air-dried. Wright's stain *Correspondence stock solution was prepared as follows: 0.25 g of 92 1996The Genetical Society of Great Britain. SUPERNUMERARY CHROMOSOMES IN NEPHROPS NOR VEGICUS 93 Wright's stain (Sigma) dissolved in 100 mL of Meiotic chromosomes methyl alcohol, stirred for 1 h, filtered and stored in In an oven at 37°C. metaphase I the bivalents, showing a very regular, circular arrangement were dumbbell-, ring- or cross- shaped; some highly condensed, chromatin bodies C-banding were identified (Fig. ib). C-banding and DdeI stain- ing showed the presence of some asynaptic chromo- Fixedchromosomes, aged 6—14 days, were treated with 5 per cent Ba(OH)2.8H20 for 3—5 mm at 50°C, somes; they appeared tightly condensed and strongly stained, the primary constriction being often difficult brieflyrinsed in 0.1 N HCI followed by a rinse in distilled water, incubated in 2 x SSC for 1 h at 60°C to identify (Figs id, 2b and 3b). They also presented distorted segregation. In fact, in metaphases II from and stained with 25 per cent Wright's stain for 7 the same meiocyte it was possible to observe that mm. these chromosomes are distributed randomly between the daughter cells (Figs 2c—e and 3c—e). On NORsilver staining and restriction endonuclease account of their peculiar structure and behaviour, banding these chromosomes could be defined as B chromo- somes whereas 'A chromosomes' indicates all the Thesetechniques were performed according to Cau other members of the complement. et al. (1988). Numericalvariability Fluorescencemicroscopy Thediploid values in spermatogonial metaphases 0-bandingwas obtained following Casperson et a!. ranged from 131 to 140. Variability was present (1969). among different individuals as well as among cells of the same individual (Tables 1 and 2). In metaphase Results I, counting of bivalents after conventional staining was of ambiguous interpretation because the asynap- Mitotic chromosomes tic B chromosomes could not be easily distinguished The chromosome complement of N norvegicus from bivalents. After C and DdeI treatment, which comprises metacentric, submetacentric and acrocen- facilitated the identification of asynaptic chromo- tric chromosomes and shows a great variability of somes, we obtained a modal number of 64 bivalents the 2n and n values, within and among individuals and 8 B chromosomes in metaphase I. Congruent (Farmer, 1974; Corni et al., 1989). A few metacen- values were obtained in metaphase II for the A tric and submetacentric chromosomes, often without chromosomes whereas the number of B chromo- homologues, are remarkably larger than all the somes ranged from three to five. others (Fig. la). All centromeric regions showed C-positive Morphologyof B chromosomes reaction (Fig. lc,d). Moreover, entirely C-positive chromosomes, variable in number within and among Accordingto their different features three groups of individuals, were observed. Similar results were B chromosomes were identified. obtained with the restriction endonuclease DdeI with 1 The first group corresponds to the largest chromo- sharper resolution (Figs 2 and 3). some of the complement which is only partially Quinacrine identified some AT-rich clusters local- heterochromatic; after banding it was easily identifi- ized on the short arms of some small submetacentric able because of large, positive paracentromeric chromosomes. Moreover, some of the large chromo- blocks of chromatin (Fig. 2a). In meiotic metaphase somes showed intercalary and subtelomeric bands on I, it was asynaptic and doubled up, often forming a both arms (Fig. 4a). univalent ring. For this reason, it could be regarded Ag-staining revealed four to eight nucleoli in the as an isochromosome. In some specimens this interkinetic nucleus of the embryos (Fig. 4e). In chromosome was without homologues, both in the mitotic metaphases, five to eight Ag-positive regions spermatogonial and in the meiotic metaphases (Fig. were identified, localized in the short arms of three 2a—e). In other cases two homologues were distin- pairs of small acrocentric chromosomes as well as in guishable which were asynaptic, doubled up in the centromeric region of a small pair of metacentric meiotic metaphases and showed distorted segrega- chromosomes (Fig. 4d). tion (Fig. 3a—e). The Genetical Society of Great Britain, Heredity, 76, 92—99. (e), thelargeBchromosomes of(d)showtheirunivalentnatureafterashorterphotographic exposure. metacentric, submetacentric and acrocentricchromosomes(arrowed);(3)smallchromosomes, mainlyacrocentric(+). chromosomes are:(1)apair of largechromosomeswithanextensive,paracentromeric positiveregion(*);(2)large C-banding; thecentromericregions ofallchromosomesandthewholeB areC-positive.TheidentifiableB able fromtheothermembers ofthecomplement.(c)Spermatogonialmetaphaseand (d)meioticmetaphaseIafter Fig. 94 A.M.DEIANAETAL. I (a)Embryonalmetaphaseand (b)meioticmetaphaseIafterWright'sstaining.Bchromosomes areindistinguish- tbt4 t%d t :. S S. p #0 S a I(, 40 eta %V I' •44v 'I #* 1. V;1'St '• a. , a The GeneticalSociety ofGreatBritain,Heredity, 76, 92—99. a, S a;.4. S ''II, 1%tS 'S a a •0 1±.- * - —..4l. ,s.. C S • a 4 • t '1(4 a— a. •1 4It, !t%. - 'at,, t . .. S S In S SUPERNUMERARY CHROMOSOMES IN NEPHROPS NOR VEG/CUS 95 *4 a b S 4 (. V S 4* I. a ti I, it * C, to I' ''p4 t :'a 4t p-I, S. 5 • ..a A' "(Crc t b:l e 0 a a *0 * ¼ S.÷ • a- IsS S e'1% s A %'4 * •1 ,..E •e "44, S II 9 t;0\ e S 5t + b SI a -'•0 w L's' I.-. +a Si.4p ' *0 S.-4. 'p aS Fig.2 DdeI-induced banding. Specimen I. (a) Spermatogonial mitosis; the identifiable B chromosomes are: (1) a single, large chromosome with an extensive, paracentromeric positive region (*); (2) large metacentric, submetacentric and acrocentric chromosomes (arrowed); (3) small chromosomes, mainly acrocentric (+).(b)Meiotic metaphase I. B chromo- somes are recognizable as they are strongly stained and asynaptic. The large chromosome (*) makes a peculiar univalent ring. (c—e) Pairs of two metaphases II; each of them comes from the same meiocyte I; it is evident that B chromosomes undergo a distorted segregation. The Genetical Society of Great Britain, Heredity, 76, 92—99. 96 A. M. DEIANAETAL. * a b * ci a, F' ..1r,%9 C•' 4 'St1S •.tIø .4 ., S a% * ejs, :; S , $ icr * S 0 &p 5 4I . 4•Wjtvet q. S ajr p 4 4.. V. a. t*r S4j •• I * S '4 p talc 4Pt ,:1SI'.. ae aiv Fig.3 DdeI-induced banding. Specimen Ii. A pair of chromosomes (*) is present in this specimen. They are easily recognizable in spermatogonial metaphase (a) as well as in metaphase I (b). Metaphases II with two (c), none (d) or one (e) of these chromosomes evidence their distorted segregation. 2 The second group comprises completely hetero- uted in metaphase II (Figs 2c—e and 3c—e).
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