Sex chromosome evolution in fish. II. Second occurrence of an X1X2Y sex chromosome system in Gymnotiformes

A multiple sex chromosome system of the X1X1X2X2:X1X2Y type is reported to occur in the fish species Brachyhypopomus pinnicaudatus (Gymnotiformes, Hypopomidae), being the second occurrence of this sex chromosome system in Gymnotiformes and the fifth among Neotropical freshwater fish. The possible origin of this system was hypothesized to be a centric fusion, which occurred in an ancestral form, of two medium-sized acrocentrics, giving origin to the metacentric neo-Y. Heterochromatic DAPI-positive regions were visualized in the pericentromeric region of all the chromosomes, including the Y-chromosome. In-situ hybridization with (TTAGGG)n (all-human-telomeres probe) did not detect any telomeric interstitial regions (ITS), indicating a possible loss of terminal segments of the chromosomes involved in the neo-Y formation.