Karyotype and nucleolar organizer regions in Serrasalmus rhombeus (SerrasalmiNae) from Caicaradel Orinoco,Venezuela
Mauro Nirchio, Alberto Granado, Ernesto Ron and Julio E. Pérez
Mauro Nirchio. M.Sc. in Marine Sciences, Universidad de Oriente (UDO), Venezuela. Professor, UDO. Address: Escuela de Ciencias Aplicadas del Mar, Núcleo de Nueva Esparta, Universidad de Oriente. Apartado 174, Porlamar, Margarita, Venezuela. e-mail: nirchio@cantv.net
Alberto Granado. M.Sc. in Marine Sciences. Professor, UDO. Address: Instituto Limnológico, Universidad de Oriente. Caicara del Orinoco, Estado Bolívar, Venezuela. e-mail: limnológico@cantv.net
Ernesto Ron. B.S. in Marine Biology, UDO. Instructor, Escuela de Ciencias Aplicadas del Mar, Núcleo de Nueva Esparta, UDO. e-mail: ronernesto@hotmail.com
Julio E. Pérez. M.A., University of Kansas. Ph.D., Southampton University U.K. Professor, UDO. Address: Instituto Oceanográfico de Venezuela, Universidad de Oriente. Apartado 243, Cumaná, Venezuela. e-mail: jperez@telcel.net.ve
Summary
Cytogenetic studies were conducted on Serrasalmus rhombeus from Caicara del Orinoco, Venezuela. The diploid chromosome number 2n=60 was composed of 30 metacentric, 16 submetacentric, 4 subtelocentric and 10 acrocentric chromosomes (NF= 110). Four pairs of NOR-bearing chromosomes were detected. NOR are located terminally on the short arms of one pair of medium sized subtelocentric chromosomes, and on three small sized acrocentric chromosome pairs. The karyotype diversity in this species suggests that karyotype formulae and NOR number could be useful for characterization of intraspecific stocks or populations.
KEYWORDS / Karyotype / NOR / Serrasalmus rhombeus /
]]> ResumenSe estudió el cariotipo de Serrasalmus rhombeus de Caicara del Orinoco, Venezuela. El número diploide de cromosomas 2n=60 estuvo compuesto de 30 metacéntricos, 16 submetacéntricos, 4 subtelocéntricos y 10 acrocéntricos (NF=110). Fueron detectados cuatro pares de cromosomas portadores de NOR. Las NOR están localizadas en la región terminal de los brazos cortos de un par de cromosomas subtelocéntricos de mediano tamaño y en la región terminal de tres pares acrocéntricos pequeños. La diversidad del cariotipo en esta especie sugiere que la formula del cariotipo y el número de NOR podrían ser de utilidad para la caracterización de stocks o poblaciones intraespecíficas.
Resumo
Estudou-se o cariotipo de Serrasalmus rhombeus de Caicara del Orinoco, Venezuela. O número diploide de cromossomas 2n=60 esteve composto de 30 metacéntricos, 16 submetacéntricos, 4 subtelocéntricos e 10 acrocéntricos (NF=110). Foram detectados quatro pares de cromossomas portadores de NOR. As NOR estão localizadas na região terminal dos brazos curtos de um par de cromossomas subtelocéntricos de tamanho medio e na região terminal de três pares acrocéntricos pequenos. A diversidade do cariotipo nesta espécie sugere que a fórmula do cariotipo e o número de NOR poderiam ser de utilidade para a caracterização de stocks ou povoações intraespecíficas.
Received: 07/29/2002. Modified: 09/12/2002. Accepted: 10/01/2002
Introduction
"Piranhas" or "caribes", as they are commonly known, are a group of Neotropical fishes of high diversity and predatory habits, and an important part of South American freshwater communities. This group comprises 5 genera: Pygopristis, Pygocentrus, Pristobrycon, Serrasalmus and Catoprion, thoroughly distributed in South America (Machado-Allison and Fink, 1996).
In Venezuela four genera and 16 species have been identified: Pygopristis (P. denticulatus); Pygocentrus (P. cariba); Pristobrycon (P. calmoni, P. careospinus, P. maculipinnis and P. striolatus); and Serrasalmus (S. altuvei, S. eigenmanni, S. elongatus, S. gouldingi, S. irritans, S. manueli, S. medinai, S. nalseni, S. neveriensis, and S. rhombeus) (Machado-Allison and Fink, 1996). Several species have been karyotyped to date (Muramoto et al., 1968; Prado and Galetti, 1986; Cestari and Galetti, 1992; Nakayama, 1997; Nakayama et al., 2000, 2001, 2002; Centofante et al., 2002) but, as far as we know, there are no cytogenetic studies reported on these species in Venezuela.
This paper describes the diploid number, chromosome formula and NOR locations in S. rhombeus from Caicara del Orinoco, Venezuela.
Materials and Methods
]]> Sixteen individuals (9 males and 7 females) of white piranha Serrasalmus rhombeus (Linneaus, 1766; Characidae, Serrasalminae) were captured with seine nets in the lowland flood plains near Caicara del Orinoco, Bolivar State, Venezuela, and transported to the Instituto Limnológico of the Universidad de Oriente. Voucher specimens were deposited at the Ichthyology Collection of the Escuela de Ciencias Aplicadas del Mar, Universidad de Oriente.Each specimen was injected with 0.1% colchicine i.p. (1ml/100g fish weight). The fish were maintained in a well aerated aquarium and after 2hr they were sacrificed. The kidneys were removed and placed in a hypotonic solution of 0.4% KCl. Each kidney was minced with fine forceps and a fine cellular suspension was obtained then by repeated aspiration and forced release with a glass syringe. After 30min in the hypotonic solution, the cellular suspension was centrifuged at 1,000rpm for 3min. The hypotonic solution was discarded and the cellular pellet was suspended and washed 3 times in a methanol-acetic acid mixture 3:1 (V:V).
One droplet of the cellular suspension was dropped on a clean microscope slide, previously chilled in a freezer, from a height of 45cm. The slides were briefly passed over a flame and then allowed to air-dry.
For conventional karyotype the preparations were stained during 20min with 5% Giemsa in phosphate buffer pH 6.88. Detection of Nucleolar Organizer Regions (NOR) followed the silver staining method (Howell and Black, 1980). Staining with AgNO3 was performed sequentially to identify the correspondence among NOR bearing chromosomes after staining them with Giemsa.
Figure 1. Karyotype of Serrasalmus rhombeus. NOR-bearing chromosomes revealed after sequential staining are in squares.
The mitotic figures were photographed using a green filter and ASA 50 film. The resulting photographs were scanned (1200dpi) and stored as tif images. Long arm (L), short arm (S) and total chromosome length were measured from each chromosome to the nearest 0.01mm using the measuring tool of Adobe Photoshop Software v.5.0. The relative length (RL%) of each chromosome pair in relation to the total length of the haploid chromosome length was obtained from these measurements. Chromosomes were identified by the arm ratio criteria (Levan et al., 1964). Metacentric (M), submetacentric (SM) and subtelocentric (ST) chromosomes were considered to be biarmed, and acrocentric (A) chromosomes were considered to be uniarmed.
Results and Discussion
All the specimens of S. rhombeus analyzed were characterized by a modal karyotype 2n=60, obtained in 79% of all cells examined (182 cells). The hypomodal and hypermodal counts, on the whole, hardly reach 21% of all the cells recorded, and probably result from preparation-caused defects such as chromosome loss, overlap, miscounting and additional chromosomes from another spread. The karyotype was composed of 30 M, 16 SM, 4 ST and 10 A chromosomes with an arm number (Nombre Fondamentale) of NF=110 (Figure 1). No heteromorphic sex chromosomes were observed. The difference in relative length (RL%) between the two first chromosome pairs was 0.4%. Ag-stained metaphases (Figure 1) showed four pairs of NOR-bearing chromosomes, located terminally on the short arms of one pair of medium sized ST chromosomes (Nº 25), and on three small sized A chromosome pairs (Nº 26, 27, 30).
Published data on cytogenetic features in the piranhas group show that the variation in the diploid chromosome number ranges from 2n=58 to 2n=64 (Porto et al., 1991). In the genus Serrasalmus, some species have been karyotyped, and all have shown multiple NOR localized on the short arms of ST-A chromosomes, varying in number and staining intensity (Galetti et al., 1985; Cestari and Galetti, 1992; Nakayama, 1997).
]]> In the particular case of S. rhombeus, karyological studies performed in nominal forms of the species from Central Amazon near Manaus, Brazil, revealed two cytotypes that occur sympatrically at Catalão and Camaleão lakes, without presence of intermediates between the cytotypes, suggesting that each cytotype represents a different fish species. Cytotype 1 consists of 2n=60, 20M + 24SM + 6ST + 10A and had a difference of less than 0.5% in relative length (RL%) between the first two chromosome pairs. Cytotype 2 consists of 2n=58, 30M + 16SM + 2ST+10A and had 2.1% of difference in RL% between the first and second chromosome pairs, thus the largest pair is about twice the size of chromosome pair 2. Multiple NORs in short arms of 5 to 12 ST-A chromosomes were detected in both cytotypes, and number, size and intensity of NORs were variable intraindividually (Nakayama et al., 2001).The karyotype of S. rhombeus from Caicara del Orinoco, falls within the chromosomal pattern of the piranhas, in number and NOR multiplicity. While these agree with those of Cytotypes 1 from Catalão and Camaleão lakes in Brasil (Nakayama et al., 2001) both in chromosome number as in relative length between the first two chromosome pairs, it differs in the number of each type of chromosomes in 10M, 8SM and 2ST, revealing that the sample of the population analyzed herein possesses an additional Cytotype for the species. The presence of karyotype diversity in S. rhombeus suggests that karyotype formulae and NOR number could allow the characterization of intraspecific stocks or populations.
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