KARYOTYPE AND NUCLEOLUS ORGANIZER REGIONS OF PYRRHULINA CF AUSTRALIS (PISCES, CHARACIFORMES, LEBIASINIDAE)

The mitotic chromosomes and nucleolus organizer regions (NORs) of the Upper Parana Basin fish Pyrrhulina cf. australis were studied. The karyotype (2n = 40; 6st + 34a) is characterized by the presence of four chromosome pairs with NORs, one of them with NORs in both terminal regions. Comparison of this karyotype with those of other Characiformes, revealed a strong similarity to the family Erythrinidae.

CYTOGENETIC AND DNA CONTENT STUDIES OF ARMORED CATFISHES OF THE GENUS CORYDORAS (PISCES, SILURIFORMES, CALLICHTHYIDAE) FROM THE SOUTHEAST COAST OF BRAZIL

Cytogenetic and DNA content studies were done on six nominal species of Corydoras from the southeast coast of Brazil. The data show that several nominal species present local populations with differences in karyotype or DNA content. There are at least two groups of Corydoras species with similar karyotypic structure in this region: the first composed by C. ehrhardti, C. nattereri and C. paleatus and the second composed of C. barbatus, C. macropterus and C. prionotos. These two groups of species are probably not derived directly from the same ancestral line. The speciation process of Corydoras species from the southeastern coast of Brazil is discussed.

CYTOGENETIC AND DNA CONTENT IN 6 GENERA OF THE FAMILY CALLICHTHYIDAE (PISCES, SILURIFORMES)

Cytogenetic 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.

SYNAPTONEMAL COMPLEX-ANALYSIS IN SPERMATOCYTES OF TILAPIA, OREOCHROMIS-NILOTICUS (PISCES, CICHLIDAE)

Some adaptations of the synaptonemal complex (SC) whole-mounting technique first used in plants permitted its application to meiotic studies in tilapia, Oreochromis niloticus. Direct observation of the chromosome pairing process and bivalent structure during the meiotic prophase of this fish species by light and electron microscopy permitted the analysis of SCs in autosomes and the possible identification of sex chromosomes. The analysis of SCs in spermatocytes of 0. niloticus revealed that all 22 bivalent chromosomes completely paired, except for the occurrence of a size heteromorphism in the terminal region of the largest bivalent associated with the presence of an incompletely paired segment during the synapsis process, which may be the cytological visualization of an XX/XY sex chromosome system in this species.

Spontaneous triploidy and NOR activity in Eigenmannia sp. (Pisces, Sternopygidae) from the Amazon basin

The occurrence of a natural triploid among diploid individuals of Eigenmannia sp from the Amazon basin is described. The diploid individuals had 2n=31 and 32 chromosomes, the difference in chromosome number being due to a Robertsonian rearrangement. The triploid individual presented 2n=46 and had two fused acrocentrics. NOR activity was present in the three homologues of triploid and a NOR polymorphism was detected in the triploid and diploids. © 1985 Dr W. Junk Publishers.

New occurrence of a macro B-chromosome in Astyanax scabripinnis paranae (Pisces, Characiformes, Characidae)

Cytogenetic studies performed on 17 specimens (11 females and six males) of Astyanax scabripinnis paranae from the Cascatinha stream showed that this population has 2n = 50 chromosomes (8M + 22SM + 10ST + 10A), two chromosome pairs with NORs and conspicuous C-band positive blocks in the terminal position of the long arm of five chromosome pairs. Three females presented 2n = 51 chromosomes and the extra chromosome was a large metacentric similar in size and morphology to the first chromosome pair in the karyotype. This accessory chromosome was entirely heterochromatic in C-banded metaphases, which permitted its classification as a supernumerary chromosome. Some aspects related to the morphology of such macro B-chromosomes are discussed.

Cytogenetic studies of three species of Glandulocaudinae (Pisces, Characiformes, Characidae)

Cytogenetic studies of three species of the subfamily Glandulocaudinae showed that Mimagoniates microlepis presents 2n = 52 chromosomes (6M + 20SM + 18ST + 8A), Mimagoniates lateralis presents 2n = 52 chromosomes (6M + 20SM + 16ST + 10A) and Glandalocauda melanogenys presents 2n = 52 chromosomes (4M + 12SM + 22ST + 14A). The number of NOR-bearing chromosome pairs ranged from two to four. Differences in number and position of NORs at the species and population levels were detected, suggesting that several chromosome rearrangements occurred in the evolutionary process of this group. Some aspects related to the chromosome evolution of the Glandulocaudinae are discussed.

Synaptonemal complex formation in spermatocytes of the autotriploid rainbow trout, Oncorhynchus mykiss (Pisces, Salmonidae)

Meiotic cells of triploid male rainbow trouts were analyzed by a surface-spreading SC technique in order to show the process of chromosome synapsis. At zygotene the formation of SCs involved, almost exclusively, two sets of lateral elements (LEs), and the remaining set of LEs presented different synaptic configurations involving one, two, three, or four LEs. The absence of SCs involving more than two LEs from mid- to late pachytene indicates that the multivalents produced by synapsis involving more than two LEs, are eliminated before the end of pachytene; thus, the mechanism of chromosome pairing in triploid male rainbow trouts produces, almost exclusively, bivalents with a probable extensive nonhomologous synapsis involving the extra set of chromosomes.

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 of Eigenmannia sp.1 (Pisces, Gymnotoidei, Sternopygidae) using silver staining and fluorescence in situ hybridization (FISH). The two populations differed in their AS-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.