Abstract
Individual samples from five populations of the characid fish Astyanax scabripinnis (Jenyns, 1842) from the Tietê and Paranapanema river basins (Brazil) were studied. All individuals analyzed presented 50 chromosomes but three different karyotypic forms were observed. Additionally, some individuals of one karyomorph presented a macro supernumerary chromosome. No chromosomal differentiation was observed between males and females in any sample analyzed. C-banding revealed two distinct distribution patterns in these populations in which strong terminally located heterochromatic blocks were detected in the long arms of some chromosomes of the specimens from Cascatinha, Cintra, and Funari streams, whereas populations of the Capão Bonito stream and the Barbosa waterfall revealed less evident heterochromatic blocks on the chromosomes. The comparative study of the 18S rDNA localization by fluorescent in situ hybridization, and detection of active nucleolus organizing regions by silver stain (Ag-NORs) showed differences in rDNA content and expression of this gene site among the individuals analyzed. Chromosome mapping of the 5S rDNA revealed the presence of four sites located in two distinct chromosomal pairs, with no apparent differences among karyomorphs. Based on the biogeographical distribution and specific biological characteristics of the species, the data focus on the chromosome differentiation mechanisms involved in the speciation process acting on the populations of the Astyanax scabripinnis species complex.
Highlights
Characiformes is the most important group of fish in number of individuals and species in the Neotropics (ARTONI et al 2000)
Considered as a complex of species by MOREIRA-FILHO & BERTOLLO (1991), A. scabripinnis shows great morphological diversity (SOUZA et al 1996), mainly in terms of body length, body height and snout length (MOREIRA-FILHO & BERTOLLO 1991). Cytogenetic data on this species reveal a variable karyotype with diploid numbers ranging from 2n = 46 to 2n = 50 chromosomes (MOREIRA-FILHO & BERTOLLO 1991, SOUZA & MOREIRA-FILHO 1995, MAISTRO et al 1998, among others)
Karyological analyses of specimens of A. scabripinnis have revealed the occurrence of supernumerary chromosomes, with different types of B chromosomes identified in different populations based on their morphology and constitutive heterochromatin distribution patterns (SALVADOR & MOREIRA-FILHO 1992, MIZOGUCHI & MARTINSSANTOS 1997, NÉO et al 2000)
Summary
Specimens of A. scabripinnis (90 females and 83 males) from the largest hydrographic system of the Alto Paraná river, Brazil, were studied According to their origin, samples were distributed as follows: Tietê river basin – 72 individuals (39 females and 33 males) from the Cascatinha stream, 22°59’23”S 48°25’31”W; 52 (29 females and 23 males) from the Cintra stream, 22°52’35”S 48°28’56”W; 17 (9 females and 8 males) from the Funari stream, 22°53’09”S 48°29’31”W, and Paranapanema river basin – 22 (10 females and 12 males) from the Capão Bonito stream, 22°54’33.91”S 48°30’54.92”W, and 9 (3 females and 6 males) from the Barbosa waterfall, 22°55’22”S 48°32’40”W. Chromosomal mapping of the 18S and 5S rDNA sites was carried out using fluorescent in situ hybridization (FISH), according to the procedure established by PINKEL et al (1986), with adaptations. Chromosome morphology was determined based on the ratio of arms length (according to LEVAN et al (1964), and chromosomes were classified as metacentrics (m), submetacentrics (sm), subtelocentrics (st) and acrocentrics (a) and arranged in karyotype in descending order of size
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