A novel ZZ/ZW chromosome morphology type in Eigenmannia virescens (Gymnotiformes: Sternopygidae) from upper Paraná River basin

Abstract

Gymnotiformes fishes present a great variation in chromosome numbers. Sternopygidae species are either sex-homomorphic or sex-heteromorphic. This study reports a novel ZZ/ZW chromosome morphology type in Eigenmannia virescens collected from the upper Parana River basin, which presented karyotype composed of 2n = 38 chromosomes, 6 metacentric (m), 10 submetacentric (sm), 4 subtelocentric (st) and 18 acrocentric (a) for males and 5 m, 11 sm, 4 st and 18 a for females, where the Z is a metacentric chromosome and the W is submetacentric chromosome. A single pair of chromosomes carrying the nucleolar organizing regions (NORs) was detected in the acrocentric chromosome pair No. 15 by silver-staining and confirmed by the 18S rDNA-FISH. Multiple 5S rDNA sites were detected in the karyotype, all of them in pericentromeric regions. The heterochromatin was mainly located in the pericentromeric regions of all chromosomes, and a conspicuous accumulation of heterochromatin colocated in NORs sites. Thus, the variation of Z and W chromosome types, associated with different chromosome sex systems (undifferentiated, XX/XY and ZZ/ZW) and different karyotypic formulae, reinforce that E. virescens should be considered a species complex.