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Physical mapping of rDNA and genome size in three European Fraxinus species

The genus Fraxinus L. (Oleaceae) comprises around 52 taxa at the specific or subspecific level. It is widely distributed in the Northern hemisphere and absent from the Southern hemisphere. In Europe the genus is represents by only three species (Fraxinus angustifolia, F. excelsior, F. ornus). Two species (Fraxinus angustifolia and F. excelsior) belong to the section Fraxinus and one (F. ornus) to the section Ornus. These two sections are clearly different in the light of molecular phylogeny. The physical mapping of two rRNA gene families 5S and 18S-5.8S-26S and the distributional pattern of GC-rich regions in the chromosomes of 3 European Fraxinus species have been established for the first time by means of fluorescence in situ hybridization (FISH) and fluorochrome banding with chromomycin A3. Heterochromatin and rDNA organization was conserved and almost identical for two species from Fraxinus section (F. angustifolia and F. excelsior). The number and position of two rDNA families in Fraxinus ornus were almost the same, but the gene organisation was quite different. In this species the 5S and 18-26S rRNA genes were colacalized and dispersed with each other at the level of satellites and secondary constrictions. In F. angustifolia and F. excelsior only 18-26S were situated at the level of satellites and secondary constrictions, the 5S was located just below. The number and position of GC-rich DNA correspond to those of 18S-26S loci. The genome size was assessed by flow cytometry. It ranges from 1, 54 to 1, 68 pg for F. angustifolia and F. excelsior respectively. Fraxinus ornus possessed the bigger 2C DNA value (1, 98 pg). In the light of these cytogenetic features the clear differentiation between two sections (Fraxinus and Ornus) was observed both at the rDNA organization and genome size level.

rDNA; physical mapping; genome size; FISH; Fraxinus

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