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Braz. J. Genet. vol. 20 no. 1 Ribeirão Preto Mar. 1997
Chromosome polymorphism in Ctenomys minutus (Rodentia-Octodontidae)
Thales Renato O. de Freitas
Departamento de Genética, Universidade Federal do Rio Grande do Sul, Caixa Postal 15053,
91501-970 Porto Alegre, RS, Brasil. E-mail: email@example.com.
A sample of 101 specimens of Ctenomys minutus was collected along its geographic range. Eight karyotypes (2n = 42, 45, 46a, 46b, 47, 48, 49 and 50) were found. The chromosome polymorphisms were due to Robertsonian rearrangements and tandem fusions. The distribution of polymorphisms indicated three population blocks: northern (2n = 49 and 50), central (2n = 46a, 47, and 48) and southern (2n = 42, 45, and 46b). These findings suggest that this species is undergoing a speciation process due to geographic isolation.
The caviomorph fossorial rodents of the genus Ctenomys form a large group of 56 living species (Reig et al., 1990). Chromosome number among species ranges from 2n = 10 to 70. Chromosome polymorphisms have been described for some species. Kiblisky et al. (1977) reported 2n = 56, 64 and 70 for C. pearsoni and 2n = 44 for C. torquatus in Uruguay, whereas Freitas and Lessa (1984) described 2n = 44 and 46 for C. torquatus in the South of Brazil. In Bolivia, C. boliviensis was described as having 2n = 36, 42, and 44 by Anderson et al. (1987). In northwestern Argentina, C. perrensis varies from 2n = 54 to 58 while C. rionegrensis in Uruguay shows 2n = 52 and 56 (Ortells et al., 1990). In the southeastern portion of Argentina, three species from the Mendocinus group, C. mendocinus, C. porteousi and C. chasiquensis, have 2n = 47/48 (Massarini et al., 1991).
MATERIAL AND METHODS
A sample of 101 specimens of C. minutus Nehring, 1887, 46 males and 55 females, was collected in the northeastern coastal plains of the States of Rio Grande do Sul and Santa Catarina, Brazil (Figure 1 and Table I). Their skulls and skins were deposited in the collection of the Departamento de Genética, Universidade Federal do Rio Grande do Sul.
|Population block||Locality||Number of collection site||Animals karyotyped|
|North ||Jaguaruna ||36 |
|Central ||Morro dos Conventos |
Passo das Torres
Praia do Barco
Palmares do Sul
|South ||Mostardas |
The chromosome slides were obtained according to the technique of Lee and Elder (1980). G- and C-bands, and the characterization of the nucleolus organizer regions (NOR) were obtained following Seabrights (1971), Sumners (1972), and Howell and Blacks (1980) techniques, respectively.
Seven diploid numbers and eight karyotypes (Figure 2) were found (2n = 42, 45, 46a, 46b, 47, 48, 49 and 50). The sex pair was the same, with a submetacentric X and an acrocentric Y-chromosome, in all karyotypes. The form with 2n = 50 (Figure 2A) consisted of 14 biarmed and 10 acrocentric pairs. Specimens with 2n = 49 (Figure 2B) had a heteromorphic pair, 14 biarmed, and eight acrocentric pairs. The 2n = 46a karyotype (Figure 2C) was composed of 16 metasubmetacentric and six acrocentric pairs. Karyotype 2n = 47 (Figure 2D) presented a heteromorphic pair while that with 2n = 48 (Figure 2E) was formed by 15 biarmed and eight acrocentric pairs. Animals with 2n = 42 (Figure 2F) showed 20 biarmed and three acrocentric pairs. Karyotype 2n = 46b (Figure 2G) had 16 biarmed and six acrocentric pairs, while the 2n = 45 karyotype (Figure 2H) differed from the 2n = 46b by the presence of a heteromorphic pair. In the forms 2n = 50, 49, 46a, 47, and 48, the smallest chromosome pair was an acrocentric dot. The nucleolus organizer region was located in the secondary constriction of the long arm of pair 8 in animals with 2n = 50, 49, 46a, 48, 42, and 46b and in pair 9 in those with 2n = 47 and 45 (Figure 3).
The eight karyotypes were distributed along a 330-km transect of the coastal plain of Rio Grande do Sul and Santa Catarina (Figure 1). In Jaguaruna beach (localities 33 and 36), 2n = 49 and 2n = 50 were found. Karyotype 2n = 46a extended from the southern banks of the Ararangua River (locality 37) to Emboaba Lake (locality 19), a distance of about 135 km. A hybrid population with 2n = 46a, 47, and 48 was found along the eastern banks of Barros Lake (locality 35). Diploid numbers of 2n = 47 and 48 were found along the banks of the Fortaleza Lake (locality 29). In Passinhos (locality 34) and Palmares do Sul (locality 30) only karyotype 2n = 48 was observed. Cytotype 2n = 42 was found in Mostardas (locality 38), while 2n = 45 and 46b were observed in the south, in Tavares (locality 46).
G-banding patterns were analyzed to study chromosome rearrangements. The 2n = 50 karyotype was found in the northern part of C. minutus distribution (Figure 4A) and this form was used as a standard for comparing the other karyotypes found in this species (Figure 4B). The metacentric pair 2 and the acrocentric pairs 16, 17, 19, 20, 22, 23, and 24 from 2n = 50 were found in other karyotypes as chromosomes or chromosome arms. The variant from 2n = 50 was 2n = 49 which had the 20/17 fusion in heteromorphic form. The diploid number 2n = 46a presented two fusions, 20/17 and 23/19. The karyotype 2n = 48 presented the same fusions, plus a fission in chromosome 2 (2p and 2q) that was responsible for the heterozygote 2n = 47. The diploid number 2n = 42 had the fusions 20/17 and 23/19 and the metacentric pair 2. However, among pairs 16, 24, and 22, two different rearrangements occurred forming a new chromosome: a tandem fusion between chromosomes 24 and 16, followed by a centric fusion with chromosome 22. Finally, the 2n = 46b form was the most divergent, having 1) 2q as an acrocentric chromosome, 2) 2p as a metacentric chromosome due to a pericentric inversion (2pinv), and 3) chromosome 22 and a new one formed due to the tandem fusion 24/16 as two isolated chromosomes.
The constitutive heterochromatin of C. minutus showed a highly variable pattern. Animals with 2n = 46a and 48 had a positive autosomal C-band, only in the pair with the secondary constriction (Figures 5A and B). The X did not have positive C-bands (Figure 5A), while the Y had two blocks of positive C-bands in its long arm (Figure 5B). Specimens from Jaguaruna beach (localities 33 and 36) showed constitutive heterochromatin in the centromeric region of 54% of the autosomes and in the X, with the long arm of the Y-chromosome being entirely heterochromatic (Figure 5C).
Figure 5 - Heterochromatin variation in Ctenomys minutus. A, Female with 2n = 48. B, Male with 2n = 48: the Y-chromosome presents a large heterochromatic block. C, Karyotype with 2n = 50 with heterochromatic blocks in almost all the chromosomes. The bar is 10 m. Arrows indicate C-bands.
The results of the present report confirm the large karyotypic variability of the genus Ctenomys (Reig and Kiblisky, 1969; Ortells et al., 1990). Based on our findings we conclude that eight different karyotypes of Ctenomys minutus have originated by Robertsonian rearrangements and tandem fusions. Similar findings have been reported for other subterranean rodents, Spalax ehrenbergi and Thomomys (Thaeler Jr., 1974; Patton, 1980; Wahrman et al., 1985; Nevo, 1986). There is a hybrid zone, as found in other subterranean species, e.g., Spalax ehrenbergi and Thomomys talpoides (Thaeler, 1974; Patton et al., 1979; Nevo, 1986).
The relationship between the geographical distribution of the specimens and the chromosome rearrangements showed the existence of three population blocks. The northern block was characterized by 2n = 49 and 50. The 20/17 and 23/19 fusions in 2n = 46a and the 2p-2q fission in 2n = 47 and 48 were characteristic of the central block. The southern block was mainly characterized by the presence of the new submetacentric chromosome (22, 24 and 16) and an inversion of 2pinv. In the central and southern blocks, chromosomes with arms 20/17 and 23/19 were biarmed chromosomes, while in the northern block they changed to acrocentric. Chromosomes 2 and their arms also changed from the central to the southern block. Fissions were recorded from the central to the northern block, whereas fusions were found from the central to the southern block. Each of the three population blocks of C. minutus had a predominant karyotype, suggesting mainly intra-block gene flow, with geographic barriers hindering gene flow between blocks. The Ararangua River separates the northern from the central block, and marshes and wet lands isolate the central from the southern block.
Chromosomal variation in fossorial rodents has been reported as having two origins: population structure (Patton, 1985) or ecological adaptation (Nevo 1986; Wahrman et al., 1969, 1985). The results reported here show that C. minutus is a very complex species. Chromosome variation is an important evolutionary process in this species. Geographic barriers established populational blocks, which in turn gave rise to three chromosome races. The 2n = 46a and 2n = 50 group is formed by polymorphic populations. The 2n = 42 group is more complex than the others because of rearrangements due to tandem and centric fusions. The rearrangements found in each block may represent meiotic barriers similar to those found by Capanna et al. (1976), Capanna (1984), and Corti and Ciabatti (1990) in the genus Mus. Since geographic barriers are also known in C. minutus, it would be interesting to perform a barrier efficiency meiotic test among these cytotypes.
These findings show the absence of a relationship between similarity at the gene level and chromosome variation in C. minutus. Moreira et al. (1991) made a biochemical analysis of the same animals studied herein. No correlation was detected between the enzymatic variability (based on 12 loci for all populations) and the geographic distances among demes. The S values ranged from 0.96 to 0.66. The highest similarity index (0.96) was found between populations 35 and 29, that are far away (35 km) from each other and that presented 2n = 46a, 47 and 48, and 2n = 48 and 47, respectively. Populations 31 (2n = 46a) and 35 had a low degree of similarity (0.84) though they are close geographically (about 10 km). Other examples are populations 36 (2n = 50) and 37 (2n = 46a), chromosomally different, separated by the Ararangua River, but showing high genetic similarity.
The genus Ctenomys has different C-banding patterns, and thus different amounts of constitutive heterochromatin. Reig et al. (1990) classified the species of Ctenomys, using C-banding, into three groups: 1) negative C-bands in some pairs; 2) positive C-bands in the centromeric region, and 3) strongly positive C-bands in the short arms and centromeric regions of the chromosomes. The species studied herein can be classified into the first two groups. Karyotypes 2n = 49 and 50 have C-bands in centromeric regions, while the other forms have positive C-bands in the chromosome with the secondary constriction and in the Y only. Constitutive heterochromatin varies in relation to the distribution of karyotypes: 2n = 50, in the north, presents a greater amount of constitutive heterochromatin than other populations in the south. This variation was also found in C. flamarioni (Freitas, 1990, 1994) in which the amount of constitutive heterochromatin shows a gradient from north to south along the geographic distribution. Redi et al. (1990) and Garagna et al. (1993) suggest that in Mus m. domesticus there is a relation between heterochromatin amount variation and frequency of chromosome rearrangements. This fact is also observed in C. minutus. Two events occur at the same time: chromosome rearrangements and variation in quantity of constitutive heterochromatin. Molecular studies would explain the relation between chromosome rearrangements and constitutive heterochromatin variation.
Thanks are due to Drs. Margarete S. Mattevi, Rivo R. Fischer, Enrique P. Lessa, Alfredo Langguth, Ives Sbalqueiro, and Francisco M. Salzano for valuable comments, and also to Dr. Joseph Cook for suggestions that improved the manuscript. Thanks are also due to the Centro de Estudos Costeiros Limnológicos e Marinhos (UFRGS). Research supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico (No. 409272/87).
Uma amostra de 101 exemplares de Ctenomys minutus foi coletada ao longo de sua distribuição geográfica e apresentou oito cariótipos (2n = 42, 45, 46a, 46b, 47,48, 49 e 50). Os polimorfismos cromossômicos deveram-se a rearranjos Robertsonianos e fusões in tandem. A distribuição dos polimorfismos evidenciou três blocos populacionais: ao norte (2n = 49 e 50), no centro (2n = 46a, 47 e 48) e ao sul (2n = 42, 45 e 46b). Estes dados sugerem que esta espécie está passando por um processo de especiação devido ao isolamento geográfico.
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(Received May 3, 1995)