An X sub(1)X sub(1)X sub(2)X sub(2)/X sub(1)X sub(2)Y multiple sex chromosome system in a new species of the genus Gymnotus (Pisces, Gymnotiformes)

The authors characterize the karyotype of Gymnotus sp., an undescribed fish species from the Parana River. The diploid number is 2n = 40 chromosomes for females and 2n = 39 chromosomes for males, showing a multiple sex chromosome system of the X sub(1)X sub(1)X sub(2)X sub(2)/X sub(1)X sub(2)Y type, a first description in the genus Gymnotus. The nucleolar organizing regions sites are located in only one chromosome pair, a common feature among Gymnotiformes, and heterochromatin is present in the centromeric region of all chromosomes. Although the metacentric neo-Y chromosome appears to have arisen by centric fusion in three species from different Gymnotiformes families, the presence of the X sub(1)X sub(1)X sub(2)X sub(2)/X sub(1)X sub(2)Y sex chromosome system seems to be a homoplasic event in this group.