Divergent location of ribosomal genes in chromosomes of fish thorny-headed worms, Pomphorhynchus laevis and Pomphorhynchus tereticollis (Acanthocephala)
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We studied distribution of ribosomal DNA (rDNA) sequences along with chromosomal location of the nucleolar organizer regions (NORs) in males of two fish parasites, Pomphorhynchus laevis and Pomphorhynchus tereticollis (Acanthocephala). Fluorescence in situ hybridization with 18S rDNA probe identified two clusters of rDNA in each species, but revealed a remarkable difference in their location on chromosomes. In P. laevis, the rDNA-FISH signals were found in long arms of the first chromosome pair and in short arms of the second pair. Whereas in P. tereticollis, rDNA clusters were located in long arms of both the first and second chromosome pairs. The divergent location of rDNA clusters in the chromosome No. 2 supports current classification of P. tereticollis, previously considered a synonym of P. laevis, as a separate species. A possible scenario of the second chromosome rearrangement during karyotype evolution of the two species involves two successive pericentric inversions. In both species, one or two prominent nucleoli were apparent within interphase nuclei stained with either silver nitrate or a fluorescent dye YOYO-1. However, a single large nucleolus was observed in early stages of mitosis and meiosis I regardless the number of rDNA clusters. Nevertheless, two bivalents with silver-stained NORs in diakinesis and two silver-stained sites in early prophase II nuclei indicated that all NORs are active. This means that each Pomphorhynchus NOR generates a nucleolus, but the resulting nucleoli have a strong tendency to associate in a large body.
KeywordsAcanthocephala FISH Karyotype evolution Nucleolar organizer region Pomphorhynchus rDNA Syndermata
M.B. and M.S. are highly indebted to Tomas Scholz, Biology Centre, ASCR, Institute of Parasitology, České Budějovice, Czech Republic, for a kind providing fish hosts of P. laevis and the overall support of parasitological part of the work. This study was supported by grant No. 2/6191/6 of the Slovak Grant Agency VEGA, Bratislava, and from the MVTS programme Stefanik No. 10 of the Ministry of Education of the Slovak Republic. M.B. acknowledges a research fellowship from the Academy of Sciences of the Czech Republic—Slovak Academy of Sciences Exchange Programme. F.M. and P.N. were supported by grants A6007307 of the Grant Agency of the Academy of Sciences of the Czech Republic (until 2005) and 206/06/1860 of the Grant Agency of the Czech Republic (since 2006), both Prague, and from the Entomology Institute project Z50070508.
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