Cytogenetic analysis of platyfish (Xiphophorus maculatus) shows location of major and minor rDNA on chromosomes

摘要

The platyfish Xiphophorus maculatus is an interesting species from genetical point of view because of its three chromosome (X, Y, W) sex determination system and sex-linked pigment loci (Orzack et al. 1980; Kallman 1984; Nanda et al. 1992). Because of this it is considered as a fish model for molecular mechanism of tumor formation (tumorigenesis) (Schartl 1995; Nanda et al. 1996, 2000Gutbrod and Schartl 1999), mutagenesis (Setlow and Woodhead 2001) and for comparative gene mapping in fish (Morizot et al. 2001). These issues have resulted in establishing the programme of platyfish gene mapping performed at the Xiphophorus Genetic Stock Center (San Marcos, USA), among others. Scientists in San Marcos Center and their collaborators have published data on gene linkage groups of platyfish (Morizot et al. 2001). As tumor formation and mutagenesis tests performed on platyfish could be followed by chromosome aberrations (rearrangements), knowledge about platyfish chromosomes may be important. Moreover, cytogenetic analysis can help in making progress in physical gene mapping.Karyotype of platyfish consists of 24 pairs of small acrocentric and subtelocentric chromosomes. Chromosome number and morphology were previously analyzed only by simple banding techniques (Nanda et al. 1993) which did not reveal chromosomes with ribosomal RNA genes (rDNA). Nucleolus organizer regions (NORs) formed by major rDNA (18S, 5,8S, 28S) can be visualized by using chromomycin A3 (CMA3) or AgNO3 staining techniques. These methods do not detect regions containing 5S (minor) rDNA, another ribosomal gene family not involved in the formation of the nucleolus. The aim of present work was to describe the distribution of major rDNA and its relation to minor rDNA in platyfish chromosomes through a combination of CMA3 and AgNO3 staining techniques and 5S PRINS (Primed in situ labeling).