Abstract
The 18S-5.8S-25S (pTa71) and 5S (pTa794) rDNA were used as probes for in situ hybridization to reveal the physical localisation of ribosomal genes in some species of orchids. Earlia robertiana (2n = 36) revealed one chromo- some pair with both pTa71 and pTa794 signals; Anacamptis papilionacea (2n = 32) showed two pairs of 5S rDNA and one pair of 18S-25S sites; A. morio (2n = 36) had two pairs of 18S-25S rDNA and one pair of 5S sites; the natural hybrid A. x gennarii (A. morio x A. papilionacea, 34 chromosomes) showed three 18S-25S and three 5S rDNA sites; finally, A. collina (2n = 36) showed only one pair of 18S-25S rDNA and one pair of 5S rDNA sites. This preliminary contribution indicates that repetitive DNA sequences in orchids may prove very useful for the understanding of evolutionary trends.
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