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Gayana (Concepción)

versión impresa ISSN 0717-652Xversión On-line ISSN 0717-6538

Gayana (Concepc.) v.64 n.2 Concepción  2000 



Pedro Jara-Seguel2, Santiago Peredo1, Claudio Palma-Rojas2, Esperanza Parada1 y Gladys Lara1

1Departamento de Ciencias Biológicas y Físicas, Facultad de Ciencias, Universidad Católica de Temuco,
Casilla 15-D, Chile. E-mail:
2Laboratorio de Citogenética, Departamento de Biología, Universidad de La Serena, Casilla 599, La Serena, Chile.


The karyotype of a freshwater population of Diplodon chilensis located in La Poza area of the Villarrica Lake (39º18'S; 72º05'W), Southern Chile, was studied. The chromosomes were obtained by squash of cleaving embryos, previously treated with colchicine and fixed in ethanol-acetic acid 3:1 at 4ºC and stained using the Feulgen reaction. Diplodon chilensis showed a diploid karyotype of 2n = 34, with metacentric and submetacentric chromosomes. This work is the first approach to cytogenetic characterization of Chilean populations of this species.

Keywords: Hyriidae, Diplodon chilensis, karyotype, freshwater bivalves, Southern Chile.


Se estudió el cariotipo de una población dulceacuícola de Diplodon chilensis ubicada en el sector La Poza del Lago Villarrica (39º18'S; 72º05'O), sur de Chile. Los cromosomas fueron obtenidos por aplastado de embriones en segmentación, pretratados con colchicina, fijados en etanol-ácido acético 3:1 a 4ºC y teñidos mediante la reacción de Feulgen. Diplodon chilensis presenta un cariotipo diploide 2n = 34, constituido por cromosomas metacéntricos y submetacéntricos. Este trabajo es el primer aporte a la caracterización citogenética de poblaciones chilenas de esta especie.

Palabras claves: Hyriidae, Diplodon chilensis, cariotipo, bivalvos dulceacuícolas, sur de Chile.


Diplodon chilensis (Gray 1828) is a freshwater bivalve, widely distributed in lakes and rivers from central and southern Chile.

However, information of this species are restricted to ecological studies reported by Peredo & Parada (1986), Parada et al. (1987, 1989 a, b, 1990), and Parada & Peredo (1994). Therefore, it is funda- mental to understand processes such as evolution, phylogenetic relationships and the current taxonomic classification, which are still under discussion in D. chilensis.

In Chile and the Argentinean Patagonia have been described 25 species of Diplodon Spix 1827, named as D. chilensis by Haas (1930-31). This author (1969) used shell features to recognise two subspecies: D. chilensis chilensis distributed from Valparaíso to Chiloé and D. chilensis patagonicus inhabiting in Austral Región (Chile) and the Argentinean Patagonia with Futalauquén Lake as meridional argentinean border. Moreover, evolutive relationships of the Chilean hyriids was proposed by Parodiz (1977) based on morphology, shell features of fossils and biogeographical data. At present, no accurate and reliable genetic information has been published.

Karyotypes are known for most marine bivalve mollusks (Palma-Rojas et al. 1997). However, in freshwater bivalves, only have been reported the karyotipes of the unionids Unio pictorum and U. tumidus and Dreissena polymorpha (Dreissenidae)(Barsîenè 1994).

This paper was carried out to investigate karyotypical characteristics of a freshwater population of D. chilensis from Villarrica Lake, Southern Chile. The aim is to improve our knowledge on genetic data of Chilean and South American hyriids.


During January 1998, adult specimens of D. chilensis were collected in La Poza area, Villarrica Lake (39º18'S; 72º05'W), Southern Chile.

Cleaving embryos were removed from the inner demibranchs of gravid females and used to study the karyotype. Chromosomes were obtained from embryos using the squash method, previously treated with colchicine at 0.05% for 3 hours. The samples were fixed in ethanol-acetic acid 3:1 at 4ºC, and stained using the Feulgen reaction (Navarrete et al. 1984) and acetocarmine.

Sixty metaphase plates were counted, and the best 10 plates were photographed. Chromosome morphology was established according to the centromeric index (CI) Levan et al. (1964). Total chromo- some relative length, as a percentage, and the relative lengths of short arm (SA) and long arm (LA) over total length of the haploid complement, were also obtained. SA and LA lengths were used to elaborate a karyo-ideogram to analise chromosome differences within as well as between karyotypes (Spotorno 1985).


Cleaving embryos of Diplodon chilensis showed a diploid karyotype 2n = 34, with 9 pairs of metacentric chromosomes (m) and 8 pairs of submetacentric chromosomes (sm). Two chromoso- mes (pairs 1 and 2) are twice more longer as pairs 16 and 17 (Figures 1-3). Chromosomes lengths range between 2.5 and 6.5 µm (Table I).

Figure 1. Metaphase plate from embryos of Diplodon chilensis (Feulgen reaction stain).

Figure 2. Karyotype of Diplodon chilensis, from La Poza area, Villarrica Lake, Southern Chile. ).

Figure 3. Karyo-ideogram of Diplodon chilensis. (t = telocentric; st = subtelocentric; sm = submetacentric; m = metacentric; LA = long arm; SA = short arm; CS = chromosome size.

Table I. Relative mean length in um (+ confidence interval (CI) at 95%) of the short arm (SA) and long arm (LA) of each chromosome pair, chromosome size (CS), and chromosome shape (SH) of Diplodon chilensis, located in La Poza area Villarrica Lake, Southern Chile (n = 10).


Chromosome SA CI LA CI CS SH

1 5.37 0.60 5.90 0.19 6.5 m
2 3.20 0.28 5.10 0.63 4.8 m
3 3.25 0.35 3.92 0.37 4.1 m
4 2.55 0.52 3.80 0.25 3.7 m
5 2.40 0.58 3.70 0.51 3.5 m
6 2.47 0.26 3.25 0.54 3.3 m
7 1.85 0.31 3.62 0.32 3.2 sm
8 1.90 0.45 3.45 0.51 3.1 sm
9 2.00 0.46 3.12 0.51 3.0 m
10 2.05 0.20 3.10 0.16 3.0 m
11 1.85 0.48 3.15 0.47 2.9 m
12 1.72 0.32 3.22 0.14 2.9 sm
13 1.65 0.36 3.10 0.31 2.8 sm
14 1.35 0.20 3.40 0.16 2.8 sm
15 1.70 0.32 2.92 0.44 2.6 sm
16 1.34 0.29 3.20 0.29 2.6 sm
17 1.25 0.25 3.07 0.17 2.5 sm

The mean relative arm length of the short arm (SA) and long arm (LA) for each chromosomic pair together with its confidence intervals are summarized in Table I.


Diplodon chilensis presents symmetrical karyotype according to Stebbins (1971). Likewise, morphometric characteristics agrees with previous reports for other species such as Unio pictorum and U. tumidus (Barsîenè 1994). However, the diploid chromosome number of D. chilensis (2n = 34) is lower than Unio species (2n = 38).

Due to the staining method used in this study it was not possible to show any secondary constrictions within the chromosomes that could indicate the amount and location of nucleolar organizer regions (NOR) within the karyotype. Nevertheless, in interphasic embryonic cells it was observed a number of nucleolei that ranged between 1 and 2 per nucleus, which, probably, indicate two active NOR in the karyotype of this species.

Karyotype characteristics of D. chilensis, such as the number and size of chromosomes, and symmetry, emphasise the relevance to increase cytogenetic studies in the genus. Parada et al. (1990) and Parada & Peredo (1994) proposed that variability on morphological and life strategies in D. chilensis should be caused by environmental factors. Future studies on chromosome banding such as C- banding, silver staining of NORs and G banding procedures in lentic and lotic populations will provide cytological information to dilucidate whether the variability on morphological and life strategies in D. chilensis are related to differences of genetic type and also to understand chromosomal divergence and evolutionary significance in the genus Diplodon.


This work was supported by Dirección de Investigación, Catholic University of Temuco-Chile (Proyect: 99-4-03). The authors wish to thank Dr. Elisabeth von Brand for the critical review of the manuscript.


Fecha de recepción: 03.01.2000
Fecha de aceptación: 20.04.2000


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