First Report of Sex Chromosomes in Achiridae (Teleostei: Pleuronectiformes) with Inferences About the Origin of the Multiple X 1 X 1 X 2 X 2 /X 1 X 2 Y System and Dispersal of Ribosomal Genes in Achirus achirus

American soles (family Achiridae) have been characterized by remarkable chromosomal variation even though several species lack basic cytogenetic information. This trend indicates that chromosomal traits can be useful to taxonomy once the morphological identification of some taxa in this family (e.g., Achirus species) is controversial. In this work, we expand the cytogenetic data in Achiridae by providing the first karyotypic analysis of Achirus achirus. An unusual multiple sex chromosome system (X X X X /X X Y) was detected, once males presented 2n = 33 with three unpaired chromosomes (X X Y) while females presented 2n = 34 with two acrocentric pairs (X X and X X ) found in hemizygosis in males. The 18S rDNA clusters were observed interspersed with GC-rich sites in a single pair. However, the 5S rRNA genes were dispersed through the genome of A. achirus in a sex-specific manner (10 clusters in males and 12 in females), as a result of the presence of ribosomal cistrons in X and X chromosomes. This pattern allowed us to infer that Y chromosome has evolved by partial deletion followed by fusion of proto-X and proto-X homologous chromosomes. The high rate of genomic evolution in Achiridae could have favored their reproductive isolation and speciation even in sympatric conditions.