Molecular cytogenetic studies in western Mediterranean Juniperus (Cupressaceae): a constant model of GC-rich chromosomal regions and rDNA loci with evidences for paleopolyploidy
2015
Vallès, Joan | Garnatje, Teresa | Robin, Odile | Siljak-Yakovlev, Sonja
We carried out the first cytogenetic characterisation of Mediterranean species of Juniperus (Cupressaceae); to date, nuclear DNA amount and chromosome numbers were known, but a fine karyotype analysis had been only done in three eastern Asian taxa. We performed chromomycin A₃ banding, for the first time in the family, and fluorescent in situ hybridisation (FISH) in order to detect 18S-5.8S-26S (also termed 35S and 45S) rRNA genes in five species of the genus, four diploid (Juniperus communis, Juniperus oxycedrus, Juniperus phoenicea and Juniperus sabina) and the only Mediterranean one that is exclusively tetraploid (Juniperus thurifera). The results show a very homogeneous pattern, with one chromosome pair with chromomycin-positive bands at the secondary constrictions and colocalised 35S rDNA signals. In diploid species, this model agrees with that of the two Asian taxa previously investigated with FISH. In the tetraploid species, conversely, the results are different: in the Asian taxa, signal number and location are exactly double in tetraploid than in diploid taxa, whereas in our case, the tetraploid species bears the same number of signals (not twice) as diploid ones. This fact can be interpreted as accounting for the age of polyploidy in J. thurifera: after the polyploidisation event and the stabilisation of the doubled genome, there has been time left for genome rearrangements implying a loss of GC-rich chromatin and one of the rDNA loci; this argument is a support for the ancient and relict character attributed to this taxon.
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