molecular analysis of the b microchromosome in steindachnerina insculpta characiformes curimatidae by microdissection b chromosomes are additional elements to standard karyotypes observed in different species of fishes especially in curimatidae however despite studies demonstrating the occurrence of bs little is known about their origin and evolution to better understand the genomic composition and evolutionary processes involving b chromosomes microdissection of b microchromosomes in steindachnerina insculpta was conducted chromosome painting revealed the totally hybridized b and markings on a chromosomes both in s in sculpta and in cyphocharax spilotus demonstrating a strong homology between these different species in specimens of c modestus which do not have bs the signals were observed on a chromosomes cloning and sequencing of some b fragments revealed that the b microchromosome in s insculpta is composed of repetitive elements homologous to the dirs4 ltr retrotransposon of xenopus silurana tropicalis fish with clone psi48 with the dirs4 retroelement revealed signals on all a chromosomes in the 2 species and also on the b suggesting the insertion of repetitive elements in these species