A New Variant B Chromosome in Auchenipteridae: The Role of (GATA)n and (TTAGGG)n Sequences in Understanding the Evolution of Supernumeraries in Trachelyopterus

Abstract

Basic and molecular cytogenetic techniques were carried out in 3 Neotropical region populations of catfishes, two of <i>Trachelyopterus galeatus</i> (one from the marshlands of Paraguay River basin and another from Lago Catalão, Amazon River basin) and one of <i>Trachelyopterus porosus</i>, a sympatric population to <i>T. galeatus</i> from the Amazon River basin. This study aimed to describe and understand the structure and evolution of <i>Trachelyopterus</i> B chromosomes, mainly through physical mapping of repetitive elements. A diploid number of 58 chromosomes was found for all individuals, as well as the presence of B chromosomes. For <i>T. porosus</i> this is the first report of a supernumerary. The sympatric species of <i>T. galeatus</i> and <i>T. porosus</i> from Amazon River had 1–3 B chromosomes and <i>T. galeatus</i> from Paraguay River had 1–2 B chromosomes, all of them showed intra- and interindividual numerical variation. Two females of <i>T. porosus</i> exhibited a new variant B chromosome (B2), previously not seen in Auchenipteridae, which might have originated from B1 chromosomes. All B chromosomes were entirely heterochromatic. In contrast to all complement A and B2 chromosomes, in which the telomeric sequences were found in the telomeric regions, B1 chromosomes of all populations were totally marked by (TTAGGG)_n probes. (GATA)_n sequence sites were found through all complement A chromosomes, but B1 and B2 chromosomes exhibited only a clustered block in one of the chromosome arms. The most frequent B chromosomes (B1) in all populations/species, including those previously studied in Auchenipteridae catfishes, share the following characteristics: totally heterochromatic, small, metacentric, with accumulation of repetitive (TTAGGG)_n sequences, and a low number of (GATA)_n copies, which might suggest a common ancient origin in <i>Trachelyopterus</i> species/populations.