Study on karyotype and occurrence of B chromosomes in two endemic species of the genus Pimelodus (Siluriformes, Pimelodidae) from the river Igua ?u

摘要

The ichthyofauna from the river Igua ?u has been characterized by a small number of species and high endemism (Severi and Cordeiro 1994). Among the fish in this basin and in particular among the Siluriformes, the Pimelodidae family ranks second in size and one of the most diversified, with approximately 60 genera and 300 species (Mess 1974). This group is of great economic importance due to its varied sizes and to its consumption by humans (Britski et al. 1984).Cytogenetic studies on the Pimelodidae family indicate a constant diploid number of 56 chromosomes for most species. Variations in the diploid number in some species of the group have been attributed to the presence of B chromosomes (Dias and Foresti 1990, 1993; Fenocchio and Bertollo 1990; Almeida-Toledo et al. 1992; Silva et al. 1996; Vissoto et al. 1999a; Abucarma and Martins-Santos 2001).Supranumerary, or B chromosomes, represent one of the many factors that cause numerical chromosome variation. Although most do not affect the organisms they belong to, not all are totally inert and may present transcriptional activity (Green 1988, 1990; Beukeboom 1994). Although there are many explanations for the origin of these chromosomes, two general hypotheses have been raised. One insists on an intra-species origin, descending from chromosomes of the normal complement of the species (Jones and Rees 1982); the other hypothesis foregrounds that B chromosomes have an inter-species origin because of crossings between closely related species (Bataglia 1964; McAllister and Werren 1997; Perfectti and Werren 2000).This work describes the cytogenetic differences between Pimelodus sp. and P. ortmanni, both endemic in the Igua ?u river basin, and compares them with other species of the genus, especially with those from the Paraná river basin. It also reports, for the first time, the occurrence of B chromosomes in genus Pimelodus.