Initial Steps of XY Sex Chromosome Differentiation in the Armored Catfish (Siluriformes: Loricariidae) Revealed by Heterochromatin Accumulation.

In fish species, heterochromatinization is one process that could trigger sex chromosome differentiation. The present article describes a nascent XX/XY sex chromosome system evidenced by heterochromatin accumulation and microsatellite (GATA) in from two populations of the Paraná River basin. The specimens of from the Campo and Bossi Rivers share the same karyotype. The species exhibits 74 chromosomes (8m+14sm +16st +36a, fundamental number = 112). The C-banding technique suggests male heterogamety in , where the Y-chromosome is morphologically like the X-chromosome but differs from it for having long arms that are entirely heterochromatic. Double fluorescence hybridization (FISH) with 18S and 5S rDNA probes confirmed the Ag-nucleolus organizer region sites in a single pair for both populations, and minor rDNA clusters showed interpopulational variation. FISH with the microsatellite (GATA) probe showed a dispersed pattern in the karyotype, accumulating these sequences of sex chromosomes of both populations. FISH with microsatellite (CGC) probe showed interpopulational variation. The absence of differentiated sex chromosomes in is described previously, and a new variant is documented herein where XY chromosomes can be seen in an early stage of differentiation.