Molecular structure of b chromosomes in the genus Characidium ( Characiformes, CRENUCHIDAE )

Abstract

Supernumerary or B chromosomes are extra genomic structures to standard chromosome complemente and occur in several groups of eukaryotic organisms such as plants, fungi and animals. They may have different origins, including derivation from the autosomal set or sex chromosomes and even as a result of interspecific crosses. The molecular mechanisms that drive the evolution of these segments resemble those of univalent sex chromosomes and may involve gene silencing, heterochromatinization processes, accumulation of repetitive DNA and transposons. The recent advent of resolutive cytogenetic techniques such as microdissection, method that allows the direct isolation of DNA from any chromosomal region associated with the technique of in situ hybridization with fluorescent dyes (FISH) has provided advancements in the understanding about the structure and composition of genomic elements in a significant number of organisms carrying B chromosomes. In this sense, this project aims to know the molecular content of B chromosomes present in representatives of three fish species of the genus Characidium, C. gomesi, C. pterostictum and C. oiticicai using amplification and cloning of several repetitive sequences (45S and 5S rDNA, telomeric DNA and genes for histones H1, H3 and H4) from the total DNA (g-DNA) of individuals do not carry B chromosomes of each specie, and from their microdissected B chromosomes (µB-DNA). The results obtained from the comparison of amplified sequences should provide an overview of the relationship between the molecular components of B chromosomes with the A chromosomes. This information will certainly allow learn more about the molecular structure of this type of DNA segment in the genome of these species as well as clarify aspects of their origin and possible functions. (AU)