Identification of early and late replicating heterochromatic regions on platyfish (Xiphophorus maculatus) chromosomes.

The platyfish (Xiphophorus maculates) is one of the best studied lower vertebrates, however cytogenetic data on this species is rather limited. This paper presents advances in platyfish cytogenetics, performed by classical techniques enabling the identification of heterochromatin as well as early and late replicating chromosomal regions. Analysis of chromatin resistant to restriction enzymes (Alu I, Dde I and Hinf I) showed pericentromeric, telomeric and interstitial blocks of heterochromatin. DAPI fluorochrome staining revealed that some of the pericentromeric bands are discrete AT rich clusters of chromatin. Analysis of platyfish DNA replication on the chromosomal level showed heterogeneity of fish heterochromatin regarding its replication time; moreover, the replication banding pattern distinguished pairs of homologous chromosomes.