A ZZ/ZW sex chromosome system in Imparfinis mirini (Pisces, Siluriformes)

Abstract

The cytogenetic analysis of 21 specimens of Imparfinis mirini showed the presence of 2n = 58 chromosomes in animals of both sexes. The basic karyotype consists of 22 metacentric and 34 submetacentric chromosomes plus one homomorphic pair (M/M) in males and one heteromorphic pair (M/SM) in females. Ag-NORs were located interstitially on the long arms of the largest metacentric pair and differences were frequently found concerning NOR size in the homologues. I. mirini presents a relatively large quantity of constitutive heterochromatin located in pericentromeric, interstitial and telomeric positions in several chromosome pairs. The occurrence of a heteromorphic chromosome pair in females was demonstrable by the presence of one extra C-banded segment added to the long arm of the W chromosome. The occurrence of detectable heteromorphism involving the sex chromosomes among fishes is discussed.