Contribuição citogenética à análise da biodiversidade em Astyanax fasciatus (Pisces, Characidae).
Resumen
Astyanax fasciatus is characterized as a cytogenetically diverse species. Sympatric and
syntopic occurrence of distinct cytotypes corroborates the hypothesis that A. fasciatus might
represent a species complex sharing a common denomination. In this work, specimens from
three collection sites along Mogi-Guaçu River, on Southeastern Brazil, were examined: (1)
close to headwaters (Ouro Fino MG), (2) in the mean river portion (Cachoeira de Emas,
Pirassununga SP, characterized by the presence of a dam) and (3) close to river mouth at
Pardo river (Barrinha SP). Two karyotypes bearing perfectly paired chromosomes, named
standard cytotypes, were identified; one of them with 2n=46 and another one with 2n=48
chromosomes. The cytotype 2n = 48 was found in all collection sites, whereas the cytotype 2n
= 46 was restricted to Barrinha and Cachoeira de Emas. In this latter locality, the cytotype
2n=46 was predominant, but variant karyotypical forms were also reported, bearing 2n=45
and 47 chromosomes, besides a structural variant with 2n=46. A variant with 2n=47
chromosomes was also found in Ouro Fino. The Ag-NORs and 18S and 5S rDNA sites
showed a conserved distribution among cytotypes, as well as the constitutive heterochromatin,
preferentially located at terminal region on the long arms of submetacentric, subtelocentric
and acrocentric chromosomes and terminal region on short arms of a submetacentric pair.
This latter region showed to be GC-rich after chromomycin A3 staining and it corresponds to
the location of a Nucleolar Organizer Region. Sites bearing the satellite DNA As51 were
detected at terminal region on the long arms of several chromosomes, distributed over 4
submetacentric pairs, 3 subtelocentric pairs and one acrocentric pair in the standard cytotype
2n=46, and over 3 submetacentric pairs, 4 subtelocentric pairs and one acrocentric pair in the
standard cytotype 2n=48. The variant karyotypical forms also presented other chromosomes
bearing such satellite DNA, remarkably at a large metacentric chromosome bearing a terminal
site on the long arms (found in two variant karyotypes), two subtelocentric pairs bearing
additional interstitial site (found in one variant karyotype), and one submetacentric pair
bearing a subterminal site on the long arms (found in one variant karyotype). Data based on
RAPD (Random Amplified Polymorphic DNA) were poorly informative to analyze the
reported diversity, indicating a high number of migrants per generation among cytotypes. On
the other hand, data from ISSR (Inter-Simple Sequence Repeats) showed a low structuring,
mainly between two standard cytotypes from Barrinha, where a Nm value of 0,4301 was
observed, with a genetic identity of 0,6862 and genetic distance of 0,3765. The values of
genetic distance (0,3219) and genetic identity (0,7248) between cytotypes with 2n=48 from
Barrinha and Ouro Fino also evidenced a slight differentiation, indicating that the dam at
Cachoeiras de Emas is probably a barrier to gene flow among populations located upstream
and downstream the dam. The obtained results with molecular markers do not discard the
possibility of inbreeding among the cytotypes of A. fasciatus, as a source of the diversity
found. Hypothetically, the standard cytotype with 2n=48 might be the resident form at Mogi-
Guaçu River, while the cytotype with 2n=46 would represent an invasive form, showing
recent divergence. Although the variant karyotypes present a karyotypical structure similar to
the cytotype with 2n=46, there are evidences that chromosomes typical from the cytotype
with 2n=48 have been incorporated, suggesting that such variants may be derived from viable
crossings among standard cytotypes, and/or their offsprings, which share some homologies, as
demonstrated by chromosomal markers. The presence of a higher number of As-51 sites in
some variants reinforces their inbreeding origin. The As-51 sites, which showed to be specific
for some variant forms, might be originated by complementary chromosomal rearrangements,
propitious to new locations of this satellite DNA on karyotypes.