Homeology of sex chromosomes in Amazonian Harttia armored catfishes supports the X-fission hypothesis for the XXY sex chromosome system origin.

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 XXY 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 X and X chromosomes of XXY system from H. punctata, we confirm previous assumptions that XXY 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 XXY sex chromosomes suggests they might have originated once in the common ancestor of these closely related species. A joint arrangement of mapped H. punctata X and X sex chromosomes in early diverging species of different Harttia clades suggests that the XXY sex chromosome system may have formed through an X chromosome fission rather than previously proposed Y-autosome fusion.