Sex chromosome differentiation revealed by genomic in-situ hybridization

In this work, genomic in-situ hybridization (GISH) was used to study the se x chromosome molecular differentiation on chromosomes of male and female in dividuals of the isopod crustacean Asellus aquaticus. As a composite hybrid ization probe, we contemporaneously used male and female whole genomic DNA differently labelled in the presence of an excess of unlabelled DNA of the female homogametic sex. The karyotype of A. aquaticus normally displays eight homomorphic chromosom e pairs, but a heteromorphic sex chromosome pair is present in about a quar ter of the males of a natural population previously identified by us. GISH did not reveal any sex chromosome molecular differentiation on the mal e and female homomorphic sex chromosome pair, and the karyotypes of these i ndividuals were equally labelled by the male- and female-derived probe, whi le the heteromorphic Y chromosome showed a differentially labelled region o nly with the male-derived probe. This region evidently contains male-specif ic sequences but, because no similar hybridized region is observed on the m ale homomorphic chromosome pair, they are probably not important for sex de termination but represent a molecular differentiation acquired from the Y c hromosome.