Comparative Chromosome Morphology and Physical Mapping of 18S-5.8S-26S and 5S Ribosomal DNA Loci in Diploid and Newly Synthesized Autotetraploids of Phlox drummondii Hook.

摘要

Till recently, polyploids have been considered to represent evolutionary dead-ends. However, the use of molecular biology and molecular cytogenetics tools revealed that polyploidy genomes are highly dynamic with enormous potential for generating novelgenetic and epigenetic changes. Previous results had shown that neoautotetraploid Phlox drummondii Hook. (2n=4x=28) exhibit genetic and epigenetic changes comparable to allopolyploidization of the plant taxa. In the present study, induced autotetraploids of P. drummondii were analysed for any possible chromosomal changes by conventional and molecular cytogenetic techniques. Except for two plants showing aneuploid numbers (2n+l=29, 2n-l=27), the chromosome complements of normal autotetraploids (2n=4x=28) could be arranged into seven homomorphic groups of 4 chromosomes each corresponding to seven pairs in the diploid (2n=14) including one pair/ group of nucleolar chromosomes. The distribution of the 18S-5.8S-26S and 5S ribosomal RNA gene families on thechromosomes of the diploid and the autotetraploid P. drummondii was analysed by fluorescence in situ hybridisation (FISH) with pTa71 (18S-5.8S-26S rDNA) and pTa794 (5S rDNA) clones. Two and four major 18S-5.8S-26S rDNA loci with intense signals were found in the nucleolus organiser regions (NOR) of the diploid and autotetraploid, respectively. There was no indication of low copy number and (or) silencing of rDNA loci in two out of four sites in the autotetraploids. FISH studies also indicated that at least two out of seven groups of chromosomes bearing 18S-5.8S-26S and 5S rDNA loci were not involved in any chromosome rearrangements.