Characterization of chromosomes and genome organization of ¤Thapsia garganica¤ L. by localizations of rRNA genes using fluorescent in situ hybridization

The karyotype (2n = 22) of Thapsia garganica L. (Apiaceae, Apioideae, Laserpitieae) was constructed using molecular cytogenetics. The size of the 22 chromosomes ranged between 5 and 10 microns. Two chromosomes had satellites on the short arms. The 18S-5.8S-26S rDNA genes were located to the two satellites by fluorescent in situ hybridization (FISH).

The 5S rDNA repeat unit was amplified and located by FISH to one pair of the non-satellited chromosomes. Sequencing 5S rDNA of T. garganica revealed two classes of genes based on distinct intergenic spacer regions of 191/193-bp and 181-bp. respectively. The ITS1 and ITS2 of the 18S-5.8S-26S repeats were also amplified and phylogenetic analysis placed T. garganica in a distinct clade from the Daucus clade containing Laserpitium sp.

The organization of T. garganica L. genome is tentatively divided in group A of 14 chromosomes with median centromere and of which one pair (q/p 1.22) contains single 5S rDNA locus on the long arm. Group B consists of six chromosomes with subterminal centromeres (q/p between 4.23 and 7.92) and finally group C with the satellited chromosome pair containing the 18S-5.8S-26S rDNA locus at the secondary constriction.