ZHANG, DAMING AND TAO SANG.* Laboratory of Systematic and Evolutionary Botany, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China; Department of Botany and Plant Pathology, Michigan State University, East Lansing, MI 48824. - Evolution of nuclear ribosomal DNA in peonies (Paeonia): evidence from physical maps generated by fluorescent in situ hybridization.
Physical maps of ribosomal RNA genes (rDNA) have been generated by
fluorescent in situ hybridization for seven diploid (2n
= 10) Paeonia species: P. delavayi and P. rockii
of section Moutan, P. brownii and P. californica
of section Oneapia, and P. emodi, P. tenuifolia,
and P. veitchii of section Paeonia. For species in
sections Moutan and Paeonia, all of the rDNA loci are
located near telomeres on the short arms of the chromosomes. For the
two species in section Oneapia, rDNA sites were found on both
arms of certain chromosomes. This result supports previous
observations, based on meiotic configurations, that chromosomal
structural rearrangement occurred frequently in P. brownii and
P. californica. The number of rDNA loci varies within each
section. Paeonia rockii, P. californica, and P.
tenuifolia have three rDNA loci. Paeonia delavayi and
P. brownii have four loci. Strikingly, P. emodi and
P. veitchii have a rDNA site on each of the ten mitotic
chromosomes. The most parsimonious interpretation of evolution of the
number of rDNA loci in Paeonia is that the most recent common
ancestor of extant peony species had three rDNA loci located on
chromosomes 3, 4 and 5. The additional loci were gained independently
in each of the three sections. The increase of the number of rDNA
loci is likely to result from translocation of rDNA repeats from
chromosomes bearing rDNA loci to chromosomes without rDNA loci. The
translocation may have been facilitated by telomeric location of rDNA
loci. For allotetraploid peony species lacking polymorphism in
sequences of the internal transcribed spacers of rDNA, the
polymorphism that originated from hybridization between divergent
diploid parental species may have been homogenized through concerted
evolution among at least six rDNA loci.
Key words: concerted evolution, in situ hybridization, Paeonia, physical map, rDNA