Initial steps in XY chromosome differentiation in Hoplias malabaricus and the origin of an X1 X2 y sex chromosome system in this fish group

Abstract

The neotropical fish, Hoplias malabaricus, is well known for its population-specific karyotypic diversity and the variation of its sex chromosomes. Seven karyomorphs (A to G) have been previously described with an XY, X1 X2 Y and XY1 Y2 sex chromosome system found in karyomorphs B, D and G, respectively. We compared the chromosomal characteristics of karyomorphs C and D using C-banding, staining with CMA 3 and DAPI, and by mapping the location of 18S rDNA, 5SHindIII-DNA and (TTAGGG) n repeat sequences. Our results show conserved karyotypes in both karyomorphs, a nascent XX/XY sex chromosome system in karyomorph C and the origin of neo-Y chromosome in karyomorph D. The X and Y chromosomes of karyomorph C differ only slightly because of the amplification of repetitive sequences on the X chromosome, resulting in a homomorphic condition in all females and a heteromorphic condition in all males examined. Our study showed that chromosomes X and 20 of karyomorph C have similar patterns to the X 1 and X2 chromosomes of karyomorph D, and are probably homologous. We showed that the neo-Y chromosome of karyomorph D shares similar patterns to the chromosomes Y and 20 of karyomorph C, and probably evolved through tandem fusion between Ypter/20pter. An interstitial site of the satellite 5SHindIII-DNA on the neo-Y reinforces the hypothesized dicentric nature of this chromosome. Our study shows the initial steps in XY chromosome differentiation in H. malabaricus and, in a broader context, contributes to the understanding of the evolutionary pathway leading to a multiple X1 X2 Y sex chromosome system in fishes. © 2010 Macmillan Publishers Limited All rights reserved.