Biosystematic studies on the genus Polygonatum (Asparagaceae) VII. Plastome phylogeny and karyotype evolution

Polygonatum exhibits an extremely large infrageneric variation in basic chromosome numbers (x = 8–15) and karyotypes, whose evolutionary history has never been estimated. We therefore aimed for this study to produce a detailed phylogeny of Polygonatum and to estimate the evolution of basic chromosome numbers and karyotypes based on the phylogenetic tree. Plastome sequences (160,134 bp) from 71 samples of 44 Polygonatum species, in addition to four Disporopsis and two Heteropolygonatum samples as an outgroup, were utilized. The evolution of basic chromosome numbers and karyotypes was estimated based on a phylogenetic tree, using ChromEvol and BayesTraits, respectively. The basic chromosome number of Polygonatum was estimated to be originally x = 15 or 16, then decreased over the course of evolution of Polygonatum by dysploidy from x = 15 or 16 to x = 8. A large metacentric chromosome pair was present in the ancestral karyotype of Polygonatum, then was lost nine times and regained twice during the evolution. It might be probable for the wide range of descending dysploidy from x = 15 or 16 to x = 8 to have yielded many species in Polygonatum. Further, the current results suggest that the loss or regain of a large metacentric chromosome pair might not simply result from Robertsonian rearrangements but from the more complex chromosomal rearrangements, which may have contributed to the present high species diversity in the genus in addition to the descending dysploidy.