Karyotypic similarities between two species of Rhamphichthys (Rhamphichthyidae, Gymnotiformes) from the Amazon basin

Julio Pieczarka,Patrícia Silva,Danillo Silva,Cleusa Nagamachi,Jonas Oliveira,Adauto Cardoso,Susana Milhomem

doi:10.3897/compcytogen.v7i4.4366

Abstract

The family Rhamphichthyidae includes three genera: Rhamphichthys Müller et Troschel, 1846, Gymnorhamphichthys M. M. Ellis, 1912 and Iracema Triques, 1996. From this family, only the species Rhamphichthys hanni Meinken, 1937 has had its karyotype described. Here, we describe the karyotypes of two additional Rhamphichthys species: Rhamphichthys marmoratus Castelnau, 1855 from the Reserva de Desenvolvimento Sustentável Mamirauá, Amazonas state and Rhamphichthys prope rostratus Linnaeus, 1766 from Pará state, both in Brazil. Our karyotypic analyses demonstrated that the diploid number is conserved for the genus (2n = 50), but the karyotypic formulas (KFs) differed between Rhamphichthys marmoratus (44m/sm+6a) and Rhamphichthys prope rostratus (42m/sm+8a). In both species, the constitutive heterochromatin (CH) was located in the centromeric region of most chromosomes. Large heterochromatic blocks were found on the long arms of pairs 4 and 14 in Rhamphichthys marmoratus and on chromosomes 3, 4 and 19 in Rhamphichthys prope rostratus, which also has a heteromorphism in chromosome pair 1. The CH was DAPI positive, indicating that it is rich in AT base pairs. The Nucleolus Organizer Region (NOR) showed staining at a single location in both species: the long arm of pair 1 in Rhamphichthys marmoratus and the long arm of pair 12 in Rhamphichthys prope rostratus, where it showed a size heteromorphism. CMA3 staining coincided with that of Ag-NOR, indicating that the ribosomal genes contain interspaced GC-rich sequences. FISH with an 18S rDNA probe confirmed that there is only one NOR site in each species. These results can be used as potential cytogenetic markers for fish populations, and comparative analysis of the karyotypes of Hypopygus Hoedman, 1962, Rhamphichthys and Steatogenys Boulenger, 1898 suggests that the first two genera diverged later that the third.

Highlights

The family Rhamphichthyidae comprises three genera: Rhamphichthys Müller et Troschel, 1846, with eight described species, Gymnorhamphichthys Ellis, 1912, with six species, and Iracema Triques, 1996, with only one species (Ferraris 2003, Lundberg 2005, Triques 2005, Carvalho et al 2011) (Table 1)
Our karyotypic analyses demonstrated that the diploid number is conserved for the genus (2n = 50), but the karyotypic formulas (KFs) differed between R. marmoratus (44m/sm+6a) and R. prope rostratus (42m/sm+8a)
Large heterochromatic blocks were found on the long arms of pairs 4 and 14 in R. marmoratus and on chromosomes 3, 4 and 19 in R. prope rostratus, which has a heteromorphism in chromosome pair 1

Summary

Introduction

The family Rhamphichthyidae comprises three genera: Rhamphichthys Müller et Troschel, 1846, with eight described species, Gymnorhamphichthys Ellis, 1912, with six species, and Iracema Triques, 1996, with only one species (Ferraris 2003, Lundberg 2005, Triques 2005, Carvalho et al 2011) (Table 1). These numbers are likely to be an underestimate, since the number of species described in Gymnotiformes has increased over the last 15 years (Albert and Crampton 2005). This trait allows them to be nocturnal and live in rivers with dark waters (Kawasaki et al 1996, Crampton 1998, Nanjappa et al 2000, Gouvêa et al 2002)

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Conclusion