and from the Paramaribo region (Surinam) were analyzed. The specimens from Corrientes, Miranda and Porto Velho presented a diploid number equal to 48, all M-SM chromosomes, with a fundamental number (NF) equal to 96. The specimen from Surinam had 2n=48 (46 M-SM + 2 A) and NF =94. The specimens from Parque Estadual do Rio Doce had 2n =52 (46 M-SM+6 A) and NF=98. These structural and numerical chromosome variations among H. unitaeniatus populations suggest that Hoplerythrinus may not be a monotypic genus as presently thought.

-Specimens of Apareiodon affinis collected from four sites of the Paraná River (called Lower Paraná, Argentina) were analyzed. This species presents a diploid number of 54 chromosomes, without any apparent sex chromosomal heteromorphism. However, it was possible to verify the occurrence of 10 cytotypes, some of them common to all collected sites, characterizing an structural chromosomal polymorphism. Data obtained from chromosomal banding methods, results further suggest an inter-individual NOR polymorphism. Such variation is possibly caused by pericentric inversions and other chromosomal rearrangements. The 5S rDNA was separated from 18S rDNA clusters, these cistrons seems to present variation, suggesting that other chromosomes could also be involved in the polymorphism. The results obtained confirmed the structural polymorphism in A. affinis.

Summary An X 1 X 2 Y sex chromosome system is reported for the first time in Gymnotus sp. The chromosome number observed was 2nϭ40 (14 M-SMϩ26 ST-A) in females and 2nϭ39 (15 M-SMϩ24 ST-A) in males, with the same fundamental number in both sexes (FNϭ54). The multiple sex chromosome system might have been originated by a Robertsonian translocation of an ancestral acrocentric Y-chromosome with an acrocentric autosome, resulting in a metacentric neo-Y chromosome observed in males. Single NORs were detected on the short arm of a middle-sized acrocentric chromosome pair. Constitutive heterochromatin was observed in the pericentromeric regions of several chromosome pairs, including the neo-Y chromosome and the NOR carrier chromosomes. The DAPI/CMA 3 stain revealed that all the pericentromeric heterochromatin are AϩT rich whereas the NORs were associated with GϩC rich base composition. The possible ancestral condition characterized by an undifferentiated Y-chromosome from all the Gymnotiformes fishes is discussed.

Summary In the present study six specimens of Triportheus paranaensis from the Parand river, in Corrientes (Argentina) were analyzed. All the specimens showed 2n=52 chromosomes with a karyotype formula composed of 24 M +22 SM +4 ST +ZZ (M/M) in males or ZW (M/SM) in females. The constitutive heterochromatin pattern was similar to those of other Triportheus species previously analyzed. Two chromosome pairs with NORs were detected. One of these is located in the telomeric region of a metacentric chromosome pair. This position was unreported for any other Triportheus species, constituting a chromosome marker for the species. The comparative analysis between our results and the available in the literature suggests that the karyotype diversification of the group would have mainly produced by chromosome pericentric inversions and translocations. These structural changes in addition to duplications and translocations of the nucleolar organizer regions, could have modified the karyotype formula preserving the diploid chromosome number.

Resumen Caramello, C.S.; Cowper, C.F.; Jorge, M.J.; Pérez, J.E.; Jorge, L.C.: Anormalidades morfológicas nucleares en hematíes del pez Prochilodus linneatus expuesto al clorpirifos. Rev. Vet. 30: 2, XX-XX, 2019. Recientemente adquirió importancia el monitoreo de xenobióticos (como pesticidas) en el medio ambiente, debido a la persistencia de la actividad biológica de muchos de ellos. Clorpirifos es un insecticida organofosforado empleado a nivel agrícola y doméstico. Este plaguicida puede alcanzar accidentalmente el ecosistema acuático y la fauna íctica. Al ingerir sustancias contaminadas, los peces desarrollan alteraciones debidas a la bioacumulación, constituyéndose en eficaces indicadores de polución ambiental. El propósito del estudio fue evaluar la genotoxicidad del clorpirifos en P. linneatus (Pisces, Prochilodontidae) a través de la frecuencia de micronúcleos (MN) y de alteraciones de la morfología nuclear (AMN) en eritrocitos de sangre periférica. Los ensayos se realizaron con clorpirifos en su forma pura (Sigma Aldrich). Los peces se dividieron en grupos control (C) y tratados (T1 = 10 μg/l; T2 = 30 μg/l y T3 = 90 μg/l). Se realizaron tres replicas por grupo, con un total de 3 animales por pecera, los cuales permanecieron allí durante 7 días, tras lo cual fueron sacrificados con sobredosis de anestésico. Se extrajo sangre de la vena caudal, realizándose frotis que se colorearon con Giemsa, determinándose el número de MN y AMN luego de la observación de 2000 células por animal. Para el estudio se consideraron las células con sus membranas citoplasmáticas intactas. En ambos grupos, los frotis sanguíneos revelaron la presencia de micronúcleos, así como algunas de las siguientes alteraciones de la morfología nuclear: muesca, lobulación, evaginación, forma de ocho, segmentación y vacuolización nuclear. Los individuos del grupo T3 mostraron variaciones significativas en la frecuencia de MN y AMN respecto a los controles (p<0,05), no así los ejemplares de los tratamientos T1 y T2. Por lo expuesto, surge que el clorpirifos a la dosis de 90 μg/l podría ser nocivo para el medio ambiente. Abstract Caramello, C.S.; Cowper, C.F.; Jorge, M.J.; Pérez, J.E.; Jorge, L.C.: Morphological abnormalities in nucleous of hematies from the fish Prochilodus linneatus exposed to clorpirifos. Rev.