Copia retrotransposons of two disjunctive Panax species: P. ginseng and P. quinquefolius

Abstract

Sixty highly heterogeneous reverse transcriptase (RT) gene domains, each representing a different copia retrotransposon, were isolated from Panax ginseng and P. quinquefolius, two highly valued medicinal plant species representing classical eastern Asian and eastern North American disjunctive speciation. These RT domains were classifiable into 10 distinct families. While some families contained highly degenerate elements, others were largely composed of intact ones that had been subjected to purifying selection. DNA gel-blot analysis showed that all 10 families existed in both ginseng species, although the copy number of Family 1 showed marked difference between them. All element families appeared heavily methylated in both species, but a difference in cytosine DNA-methylation patterns between the two species was also evident. Thus, the copia retrotransposons in the two ginseng species are diverse and polyphyletic in origin, yet, they all appeared antique and presumably occurred before separation of P. ginseng and P. quinquefolius, followed by genetic and epigenetic differentiation in their respective host genomes.