Análise citogenética de Tetranematichthys wallacei: contribuições à compreensão da evolução cromossômica em Auchenipteridae

Abstract

Considering the evidence of chromosomal fusion found in Ageneiosus inermis as basal for Ageneiosus, it becomes possible to suppose that this event may have occurred in an ancestor prior to the diversification that defined the groups representing the tribe Agneiosini ((Ageneiosus + Tympanopleura) Tetranematichthys), which can be clarified through the analysis of Ageneiosini species. In addition, in 25 genera currently valid in Auchenipteridae, there are only eight with cytogenetic studies, and of the 21 species of Ageneiosini, only two had been previously studied through cytogenetics analysis. Given this reality, the objective of this research has been to analyze through cytogenetic methods a first sample of the genus Tetranematichthys. For this, 11 individuals (3 females and 8 males) of Tetranematichthys wallacei from the Guamá river basin have been collected and examined. The diploid number found has been 52 chromosomes (32m+18sm+2st), with no differences between the sexes, and with a fundamental number of 104. Blocks of heterochromatin predominantly terminal and some interstitial has been observed. Hybridization with telomeric sequence [TTAGGG]n showed markings only in the terminal regions, without ITS and hybridizations with 18S rDNA showed its presence in pairs 7 metacentric and 17 submetacentric, and pairs 8 and 10 metacentric with 5S rDNA. Only one chromosome of the pair 7 presented 18S rDNA site in only 3 specimens, which constituted a polymorphism. Tetranmatichthys wallacei has some characteristics that call attention to its current phylogenetic position, as it presents a probable reduction in the diploid number in Auchenipteridae. Furthermore, it shares with Tympanopleura atronasus the 18S rDNA allocated on the long arm of a large submetacentric chromosome (first pair), and does not seem to share with Ageneiosus the first metacentric pair with evidence of fusion in A. inermis. The data generated for T. wallacei added to the other two taxa of the tribe previously studied suggest Ageneiosini as the most different clade in the family by now.