Occurrence of euchromatic B chromosomes in natural populations of Moenkhausia bonita and M. forestii (Pisces: Characidae)

Moenkhausia is a highly specious genus among the Characidae, composed of 96 valid species. Only twelve species have a known karyotype. Thus, here are presented the first cytogenetic data of two allopatric populations of Moenkhausia bonita and one of M. forestii, both belonging to the upper Paraná River basin (PR) with discussion on the evolutionary and cytotaxonomic aspects of the genus. The two species presented 2n = 50 chromosomes but different karyotype formulas and occurrence of 1-2 B chromosomes. These elements are small metacentrics in M. bonita and small acrocentrics in M. forestii. In both species, B chromosomes were euchromatic. Ag-NOR sites were found in pair 3 (metacentric), coinciding with fluorescent in situ hybridization (FISH) by the 18S rDNA probe in both species. However, the species differed in terms of the number and position of 5S rDNA sites. Heterochromatic blocks, mapped in M. bonita showed the least amount of heterochromatin in the terminal and pericentromeric regions, while the M. forestii karyotype revealed a greater amount of interstitial heterochromatic blocks. The karyotype distinctions between the two species, including the morphology of B chromosomes, may contribute as a reference in the taxonomic studies in this group.(AU)

Moenkhausia é um gênero altamente especioso dentre os Characidae, composto por 96 espécies válidas, mas apenas doze espécies têm seus cariótipos conhecidos. Portanto, são apresentados aqui os primeiros dados citogenéticos de duas populações alopátricas de Moenkhausia bonita e uma de M. forestii, ambas pertencentes à bacia do alto rio Paraná (PR), com uma ampla discussão sobre os aspectos evolutivos e citotaxonômicos do gênero. As duas espécies apresentaram 2n = 50 cromossomos, mas diferentes fórmulas cariotípicas e ocorrência de 1-2 cromossomos B. Esses elementos são pequenos metacêntricos em M. bonita e acrocêntricos pequenos em M. forestii. Em ambas as espécies, os cromossomos B apresentaram-se eucromáticos. Sítios Ag-NOR foram encontrados no par 3 (metacêntrico), coincidindo com a hibridização fluorescente in situ (FISH) pela sonda 18S rDNA em ambas as espécies. No entanto, as espécies diferiram em termos de número e posição dos sítios de 5S rDNA. Blocos heterocromáticos mapeados em M. bonita revelaram pequena quantidade de heterocromatina nas regiões terminal e pericentromérica, enquanto o cariótipo de M. forestii revelou uma maior quantidade de blocos heterocromáticos intersticiais. As distinções cariotípicas entre as duas espécies, incluindo a morfologia dos cromossomos B, podem contribuir como uma referência em estudos taxonômicos neste grupo.(AU)

Diversity of Sex Chromosome Systems in Ancistrini (Loricariidae, Hypostominae): ZZ/ZW in Ancistrus taunayi Miranda Ribeiro, 1918.

The karyotype of the Ancistrini catfish Ancistrus taunayi was analyzed by conventional (Giemsa staining, AgNOR staining and C-banding) and molecular cytogenetic (5S and 18S rDNA-FISH) methods. The diploid chromosome number was 2n = 50 (22 metacentrics + 10 submetacentrics + 10 subtelocentrics + 8 acrocentrics) for both sexes. A single NOR-bearing acrocentric chromosome pair (No. 24) was detected after Ag-staining and 18S rDNA-FISH, while 5S rDNA was found only in the subtelocentric pair No. 21. Conspicuous GC-rich heterochromatin blocks corresponded to the NOR sites and were also observed in the distal regions of the acrocentric chromosome pairs Nos. 22 and 25. Chromosome pair No. 22 differed between males and females; in males, only a small interstitial block of GC-rich heterochromatin was present in both chromosomes, whereas in females, 2 blocks of GC-rich heterochromatin flanked a euchromatic region in one of the homologues, suggesting the occurrence of a ZZ/ZW sex chromosome system. Two mechanisms for the origin and evolution of this simple ZZ/ZW sex chromosome system in A. taunayi are proposed: (1) a paracentric inversion followed by amplification of the proximal heterochromatin and (2) amplification of the interstitial heterochromatin followed by a paracentric inversion. Although ZZ/ZW systems have already been described for other Ancistrus species, our results do not show the same pattern, suggesting an independent origin.

Coexistence of cytotypes and chromosomal mosaicism in Hoplias malabaricus (Characiformes, Erythrinidae).

Karyotypes of seventeen Hoplias malabaricus specimens, collected in the fish culture station of UNOPAR (University of Northern Paraná), were analyzed. The station is in the Claro River system in the Tibagi River basin. Two distinct and coexistent karyotype forms (cytotypes) were identified, comprising either 42 chromosomes (cytotype A) or 40 chromosomes (cytotype C), both presenting metacentric and submetacentric chromosomes. In two specimens, one male and one female, it was not possible to characterize a modal diploid number because different cell lines were observed, with a predominance of 2n=41 and 2n=42 chromosomes at a frequency of 38.24% and 41.12%, respectively. The karyotype with 2n=41 showed some putative monosomic and trisomic chromosomes, while the karyotype with 2n=42 showed 21 chromosomal pairs, similar to cytotype A. RAPD analysis showed that these two specimens have the same band pro file of cytotype A (Nei's genetic identity=92%), discarding a possible hybridization between both cytotypes and supporting the mosaicism hypothesis. These findings corroborate the isolation between cytotypes A and C.

Coexistence of cytotypes and chromosomal mosaicism in Hoplias malabaricus (Characiformes, Erythrinidae)

Karyotypes of seventeen Hoplias malabaricus specimens, collected in the fish culture station of UNOPAR (University of Northern Paraná), were analyzed. The station is in the Claro River system in the Tibagi River basin. Two distinct and coexistent karyotype forms (cytotypes) were identified, comprising either 42 chromosomes (cytotype A) or 40 chromosomes (cytotype C), both presenting metacentric and submetacentric chromosomes. In two specimens, one male and one female, it was not possible to characterize a modal diploid number because different cell lines were observed, with a predominance of 2n=41 and 2n=42 chromosomes at a frequency of 38.24 percent and 41.12 percent, respectively. The karyotype with 2n=41 showed some putative monosomic and trisomic chromosomes, while the karyotype with 2n=42 showed 21 chromosomal pairs, similar to cytotype A. RAPD analysis showed that these two specimens have the same band pro file of cytotype A (Nei's genetic identity=92 percent), discarding a possible hybridization between both cytotypes and supporting the mosaicism hypothesis. These findings corroborate the isolation between cytotypes A and C.