Chromosomal organization of four classes of repetitive DNA sequences in killifish Orestias ascotanensis Parenti, 1984 (Cyprinodontiformes, Cyprinodontidae).

Cristian Araya-Jaime,Patricia Iturra,Natalia Lam,Irma Vila Pinto,Marco A Méndez

doi:10.3897/compcytogen.v11i3.11729

Abstract

Orestias Valenciennes, 1839 is a genus of freshwater fish endemic to the South American Altiplano. Cytogenetic studies of these species have focused on conventional karyotyping. The aim of this study was to use classical and molecular cytogenetic methods to identify the constitutive heterochromatin distribution and chromosome organization of four classes of repetitive DNA sequences (histone H3 DNA, U2 snRNA, 18S rDNA and 5S rDNA) in the chromosomes of O. ascotanensis Parenti, 1984, an endemic species restricted to the Salar de Ascotán in the Chilean Altiplano. All individuals analyzed had a diploid number of 48 chromosomes. C-banding identified constitutive heterochromatin mainly in the pericentromeric region of most chromosomes, especially a GC-rich heterochromatic block of the short arm of pair 3. FISH assay with an 18S probe confirmed the location of the NOR in pair 3 and revealed that the minor rDNA cluster occurs interstitially on the long arm of pair 2. Dual FISH identified a single block of U2 snDNA sequences in the pericentromeric regions of a subtelocentric chromosome pair, while histone H3 sites were observed as small signals scattered in throughout the all chromosomes. This work represents the first effort to document the physical organization of the repetitive fraction of the Orestias genome. These data will improve our understanding of the chromosomal evolution of a genus facing serious conservation problems.

Highlights

Cytogenetic analysis is a useful tool for describing evolutionary patterns and the histories of closely-related species or species complexes
Previous cytogenetic studies involving the seven species of O. agassii complex of the Chilean Altiplano were limited to characterizing the chromosome number and morphology of the species
Characterization of the repetitive fraction of the genome is a useful tool for identifying recent genomic changes during the evolutionary process as well as possible hotspots associated with chromosomal rearrangements (Valente et al 2011, Ozouf-Costaz et al 2004, Yano et al 2014)

Summary

Introduction

Cytogenetic analysis is a useful tool for describing evolutionary patterns and the histories of closely-related species or species complexes. Conventional karyotyping studies involving the seven species of O. agassii complex found in the Chilean Altiplano (17°and 22°S) have revealed variations in the chromosome number (2n=48-55) and the presence/absence of microchromosomes, suggesting that Robertsonian rearrangements may play a role in the karyotypic evolution of these species (Arratia 1982, Vila et al 2007, 2010, 2011, Habit et al 2006, Villwock and Sienknecht 1996). Studies involving chromosomal mapping of repetitive sequences in fish are scarce and typically focus exclusively on the location of ribosomal DNA sites. Studies involving physical mapping of histone genes and mobile elements are limited, and data is available for only a few species (Pendas et al 1994, Hashimoto et al 2011, Ferreira et al 2011)

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Conclusion