Lurdes Foresti de Almeida-Toledo

Conservation of the 5S-bearing chromosome pair and co-localization with major rDNA clusters in five species of Astyanax (Pisces, Characidae).

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 local- ization 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.

Sex chromosome evolution in fish: the formation of the neo-Y chromosome in Eigenmannia (Gymnotiformes)

Abstract.Chromosomes of a species of Eigenmannia presenting a X1X1X2X2:X1X2Y sex chromosome system, resulting from a Y-autosome Robertsonian translocation, were analyzed using the C-banding technique, chromomycin A3 (CMA3) and mithramycin (MM) staining and in situ digestion by the restriction endonuclease AluI. A comparison of the metacentric Y chromosome of males with the corresponding acrocentrics in females indicated that a C-band-positive, CMA3/MM-fluorescent and AluI digestion-resistant region had been lost during the process of translocation, resulting in a diminution of heterochromatin in the males. It is hypothesized that the presence of a smaller amount of G+C-rich heterochromatin in the sex chromosomes of the heteromorphic sex when compared with the homomorphic sex may be associated with the sex determination mechanism in this species and may be a more widely occurring phenomenon in fish with differentiated sex chromosomes than was initially thought.

Sex chromosome evolution in fish. II. Second occurrence of an X1X2Y sex chromosome system in Gymnotiformes

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.

Fluorescence in situ hybridization with rDNA probes on chromosomes of two nucleolus organizer region phenotypes of a species of Eigenmannia (Pisces, Gymnotoidei, Sternopygidae)

Nucleolus organizer regions (NORs) were analysed in two related and geographically close populations ofEigenmannia sp. 1 (Pisces, Gymnotoidei, Sternopygidae) using silver staining and fluorescencein situ hybridization (FISH). The two populations differed in their Ag-NOR phenotypes, displaying fixed differences in the NOR-bearing chromosome pairs. FISH with rDNA probes showed that these differences were due to the location of rDNA cistrons. This finding, showing fixed NOR differences between two populations belonging to the same species in a connected river system, is highly significant in terms of evolutionary change, possibly indicating an initial step of genetic differentiation. This result also has important implications from the cytosystematic point of view, as NORs usually have a very constant karyotypic location in fish species and have been used as species-specific chromosome markers.

Cytogenetic and DNA content in six genera of the family Callichthyidae (Pisces, Siluriformes)

SUMMARYCytogenetic studies involving conventional Giemsa staining, C-banding analysis and silver staining of NORs were performed on nine species belonging to six genera of the family Callichthyidae. The diploid number ranged from 2n = 44 to 2n = 100, the number of chromosomal pairs with NORs ranged from 1 to 4 and constitutive heterochromatin was mainly distributed in the centromeric and/or pericentromeric position of the chromosomes. The DNA content of erythrocytes from six species studied ranged from 1.18±0.07 to 2.77±0.22 pg/nucleus. The extensive variability in karyotypes and in nuclear DNA content detected are in accordance with the initial hypothesis that chromosome rearrangements and polyploidy have played a significant role in the evolutionary history of Callichthyidae.

Karyotypic evolution in Neotropical freshwater fish

The largest freshwater fish fauna of the world is found in the Neotropical region and includes 2500 to 2700 known species [1], and a total number of species that may actually reach 5000 [2]. Until the late seventies not much was known about the cytogenetic characteristics of these fish and the cytogenetic data available about the Neotropical fish fauna was limited to knowledge of the chromosome number of about 252 species, most of them obtained from specimens of unknown geographical origin, as listed by Almeida-Toledo [3]. Cytogenetic studies focusing on the Neotropical fish species have greatly developed within the last twenty years, mainly in Brazil. The number of species already karyotyped has reached 706, belonging to 207 genera and 38 families [4]. These data include not only chromosome number and formulae, but also data about the presence of differentiated sex chromosomes, supernumerary B-chromosomes, and the location of nucleolus organizer regions (NORs). For a growing number of species, restriction banding analysis, in situ localization of rDNA sequences in the chromosomes [5, 6], of satellite DNA probes (obtained from heterochromatic regions either from pericentromeric sequences [7] or from B-chromosomes [8]) or of telomeric sequences and, more recently, immunocytogenetic analyses [9] are being carried out.

High intraspecific karyological conservation in four species of Gymnotus (Pisces: Gymnotiformes) from Southeastern Brazilian basins

SUMMARYFour species of the genus Gymnotus from Southeastern Brazilian basins were extensively studied in relation to their karyology. A high intraspecific conservatism was detected and the cytotypes were species-specific, in relation to diploid number, NOR-bearing chromosomes and C-band pattern. No geographic chromosome races were found. The differences detected in the distribution of NOR associated heterochromatin were considered as a good evidence of the heterochromatin loss that seems to have occurred in the evolutionary story of this group of fishes. Gymnotus is a complex of species which the variation is still poor understood. The cytogenetic data here described allowed the reinterpretation of previous karyological data already available in literature and is an attempt to contribute to a better understanding of this group of Neotropical freshwater fishes.

Restriction fragment length polymorphisms of mitochondrial DNA among five freshwater fish species of the genus Astyanax (Pisces, Characidae)

Restriction fragment length polymorphism (RFLP) analysis of mitochondrial DNA (mtDNA) was employed to characterize species and populations of Astyanax, a Neotropical freshwater fish genus. Samples of five species, A. altiparanae, A. fasciatus, A. lacustris, A. scabripinnis paranae and A. schubarti, from the Upper Parana and Sao Francisco river basins were analyzed. Two out of the ten restriction enzymes employed generated species-specific mtDNA patterns for each of the five species. MtDNA exhibited considerable polymorphism within and among populations. All populations sampled showed relatively high values of haplotype diversity. Geographically localized haplotypes were detected for A. altiparanae and A. fasciatus from the Upper Parana and Sao Francisco basins. The relationships between populations are discussed.

Karyotype and NOR conservatism with heterochromatin reorganization in Neotropical Bryconids

SUMMARYCytogenetic studies on Bryconid fishes have shown the occurrence of karyotype and NOR conservatism and heterochromatin reorganization. The present data on three species representative of the migratory genus Brycon corroborate the hypothesis according to which conservative patterns of karyotypic evolution can be related to high levels of vagility and large populational size in Neotropical freshwater fishes.

The contrasting role of heterochromatin in the differentiation of sex chromosomes: an overview from Neotropical fishes

During the evolutionary process of the sex chromosomes, a general principle that arises is that cessation or a partial restriction of recombination between the sex chromosome pair is necessary. Data from phylogenetically distinct organisms reveal that this phenomenon is frequently associated with the accumulation of heterochromatin in the sex chromosomes. Fish species emerge as excellent models to study this phenomenon because they have much younger sex chromosomes compared to higher vertebrates and many other organisms making it possible to follow their steps of differentiation. In several Neotropical fish species, the heterochromatinization, accompanied by amplification of tandem repeats, represents an important step in the morphological differentiation of simple sex chromosome systems, especially in the ZZ/ZW sex systems. In contrast, multiple sex chromosome systems have no additional increase of heterochromatin in the chromosomes. Thus, the initial stage of differentiation of the multiple sex chromosome systems seems to be associated with proper chromosomal rearrangements, whereas the simple sex chromosome systems have an accumulation of heterochromatin. In this review, attention has been drawn to this contrasting role of heterochromatin in the differentiation of simple and multiple sex chromosomes of Neotropical fishes, highlighting their surprising evolutionary dynamism.