The Neotropical monophyletic catfish genus Harttia represents an excellent model to study karyotype and sex chromosome evolution in teleosts. Its species split into three phylogenetic clades distributed along the Brazilian territory and they differ widely in karyotype traits, including the presence of standard or multiple sex chromosome systems in some members. Here, we investigate the chromosomal rearrangements and associated synteny blocks involved in the origin of a multiple X1X2Y sex chromosome system present in three out of six sampled Amazonian-clade species. Using 5S and 18S ribosomal DNA fluorescence in situ hybridization and whole chromosome painting with probes corresponding to X1 and X2 chromosomes of X1X2Y system from H. punctata, we confirm previous assumptions that X1X2Y sex chromosome systems of H. punctata, H. duriventris and H. villasboas represent the same linkage groups which also form the putative XY sex chromosomes of H. rondoni. The shared homeology between X1X2Y sex chromosomes suggests they might have originated once in the common ancestor of these closely related species. A joint arrangement of mapped H. punctata X1 and X2 sex chromosomes in early diverging species of different Harttia clades suggests that the X1X2Y sex chromosome system may have formed through an X chromosome fission rather than previously proposed Y-autosome fusion.

Departamento de Biologia Estrutural Molecular e Genética Universidade Estadual de Ponta Grossa Ponta Grossa PR Brazil

Institut für Humangenetik Universitätsklinikum Jena 07747 Jena Germany

Laboratório de Citogenética de Peixes Departamento de Genética e Evolução Universidade Federal de São Carlos São Carlos SP 13565 905 Brazil

Laboratory of Fish Genetics Institute of Animal Physiology and Genetics Czech Academy of Sciences Rumburská 89 Liběchov Czech Republic

Museu de Zoologia Universidade de São Paulo São Paulo SP 04263 000 Brazil

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