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Supernumerary B chromosomes restricted to males in Astyanax scabripinnis (Pisces, Characidae)

Erotides Alice Rocon Stange^I; Lurdes Foresti de Almeida-Toledo^II

^IDepartamento de Biologia, CEG, Universidade Federal do Espírito Santo, Vitória, ES, Brasil
^IIInstituto de Biociências, Universidade de São Paulo, Caixa Postal 11461, 05422-970 São Paulo, SP, Brasil. Send correspondence to L.F.A.T.

ABSTRACT

Specimens of Astyanax scabripinnis (36 males and 19 females) from the Jucu river, Vitor Hugo county, State of Espirito Santo, Brazil, were analyzed cytogenetically and a system of supernumerary B chromosomes restricted to males was detected for the first time in fish. The basic chromosome number and karyotype for both males and females was 2n - 50 (NF - 64), with six metacentrics, eight submetacentrics and 36 acrocentrics. In testis cells, the basic modal number was 25 bivalents. Supernumerary microchromosomes ranging in number from zero to four were observed in somatic cells of 34 of the 36 males; in three males the B chromosomes were restricted to the germ cells and in two males and all females they were absent.
Nucleolus organizer regions were identified by silver staining and the NORs varied in number, size and location, both inter and intraindividually. The maximum number of active NORs per cell was 15, but 13 morphologically distinct chromosome pairs may be involved in the nucleolus organization in this population.
Small heterochromatin blocks were detected in the pericentromeric regions of all chromosomes and faint bands were detected in the telomeric regions of some chromosomes. The Bs presented intermediate staining when submitted to C banding.
The presence of supernumerary B chromosomes restricted to males in the A. scabripinnis complex strongly suggests a role for these chromosomes in the evolutionary history of this group.

Keywords: Supernumerary B chromosomes; Astyanax scabripinnis; Pisces.

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