RESUMEN
A comparison of R-banding patterns obtained by 5-bromodeoxyuridine incorporation was made between the chromosomes of two fish species of the genus Astyanax (Characiformes: Tetragonopterinae), A. altiparanae with 2n = 50 chromosomes, and A. schubarti with 2n = 36 chromosomes. The two species present the highest and the lowest chromosome numbers found in this fish genus, respectively, for which the modal chromosome number is 50. R-band homeology was detected, involving eleven chromosomes of A. schubarti and seventeen chromosomes of A. altiparanae, indicating a close chromosomal relationship between the two species, in spite of their great difference in chromosome number. A chromosome fusion in the past history of the group was hypothesized as a possible cause of the discrepant chromosome numbers of the two species.
Asunto(s)
Bromodesoxiuridina/química , Cromosomas/química , Cromosomas/genética , Evolución Molecular , Peces/genética , Animales , Brasil , Bandeo Cromosómico/métodos , Análisis Citogenético/métodos , Femenino , Genética de Población/métodos , Masculino , Mitosis/genética , Ríos , Homología de Secuencia de Ácido Nucleico , Especificidad de la EspecieRESUMEN
New data are presented on the sex chromosomes of the fish species Eigenmannia virescens (Gymnotiformes, Sternopygidae). A new finding, involving the occurrence of ZZ/ZW sex chromosomes, is described in specimens sampled from the São Francisco and Amazon river basins in Brazil. All individuals had a chromosome number of 2n = 38. The homologs of the sex chromosome pair from the São Francisco river basin sample differed only in their morphology, while those from the Amazonian sample differed both in morphology and heterochromatin pattern. A possible model for the evolution of the sex chromosomes in E. virescens is proposed, including data from populations from the Paraná (Brazil) river basin, in which male heterogamety has already been described. The occurrence of different sex chromosome systems in species and populations of the neotropical freshwater fish fauna is discussed.
Asunto(s)
Cromosomas/genética , Evolución Molecular , Gymnotiformes/genética , Animales , Femenino , Cariotipificación , Masculino , Cromosomas Sexuales/genéticaRESUMEN
An early stage of sex chromosome differentiation is reported to occur in the electric eel Eigenmannia virescens (Pisces, Sternopygidae) from populations of two tributaries of the Paraná river system (Brazil). Cytogenetic studies carried out in the two populations showed that the Mogi-Guaçu population is characterized by 2n = 38 chromosomes and undifferentiated sex chromosomes and the Tietê population presents 2n = 38 both for males and females and an XX:XY sex chromosome system. The X-chromosome is acrocentric, easily recognized by the presence of a conspicuous heterochromatin block in its distal portion; the Y-chromosome is probably one of the medium sized acrocentrics present in the male karyotype. BrdU induced R-bands of the two populations did not reveal any difference in the euchromatic regions of the chromosomes. AluI and HaeIII restriction enzyme digestion patterns and chromomycin A3 staining of the X-chromosome are presented. The possible role of heterochromatinization in the evolution of sex chromosomes in fish is discussed.
Asunto(s)
Anguilas/genética , Heterocromatina/genética , Cromosoma X/genética , Cromosoma Y/genética , Animales , Colorantes Azulados , Brasil , Bromodesoxiuridina , Bandeo Cromosómico , Evolución Molecular , Femenino , Cariotipificación , MasculinoRESUMEN
A multiple sex chromosome system of the X1X1X2X2:X1X2Y type is reported to occur in the fish species Brachyhypopomus pinnicaudatus (Gymnotiformes, Hypopomidae), being the second occurrence of this sex chromosome system in Gymnotiformes and the fifth among Neotropical freshwater fish. The possible origin of this system was hypothesized to be a centric fusion, which occurred in an ancestral form, of two medium-sized acrocentrics, giving origin to the metacentric neo-Y. Heterochromatic DAPI-positive regions were visualized in the pericentromeric region of all the chromosomes, including the Y-chromosome. In-situ hybridization with (TTAGGG)n (all-human-telomeres probe) did not detect any telomeric interstitial regions (ITS), indicating a possible loss of terminal segments of the chromosomes involved in the neo-Y formation.
Asunto(s)
Evolución Molecular , Peces/genética , Peces/fisiología , Cromosomas Sexuales , Animales , Femenino , Colorantes Fluorescentes/farmacología , Hibridación in Situ , Hibridación Fluorescente in Situ , Cariotipificación , Masculino , Modelos Genéticos , Cromosomas Sexuales/ultraestructuraRESUMEN
Major and 5S ribosomal genes have been localized in chromosomes from five fish species, genus Astyanax, using in situ hybridization (FISH) with 28S and 5S rDNA probes. In situ signals for the major rDNA co-localized with the 5S rDNA clusters in the pericentromeric region of one marker chromosome in all five species analyzed. The conserved localization of these two rDNA clusters in the five related Astyanax species was considered as indicative of a close relationship among them. The use of these molecular markers for elucidating evolutionary relationships among closely related taxa is discussed.