Chromosomal Evolution Of The Tribe Polygonateae (Asparagaceae) Based On Molecular Phylogeny

The tribe Polygonateae belongs to Asparagaceae and includes ca. 100 species from four genera, i.e., Maianthemum F.H. Wigg., Disporopsis Hance, Heteropolygonatum M.N. Tamura & Ogisu, and Polygonatum Mill. Polygonateae is widely distributed in the Northern Hemisphere, with most found in eastern Asia, especially from the Qinghai-Tibetan Plateau and the Hengduan Mountains of SW China. Cytological evolution is complex in the tribe and chromosomal data are basic and very important for a better understanding of the origin and evolution of the tribe. Based on comprehensive cytological survey and molecular phylogenetic investigations, Polygonateae was used as a model to examine chromosomal evolution in plant speciation in view of their phylogenetic relationships, especially implications for taxonomy and systematics.1.CytologyIn this section, chromosome numbers of Polygonateae are completely surveyed. A total of 322 counts were collected from literatures representing 75 species from 4 genera of the tribe. Main results are described as follows:(1) Maianthemum comprises about 30 species with 20 have been counted. The chromosome number of the genus usually are found to be diploids with 2n=36 and x=18. A few are also observed as triploid(2n=54) and tetraploid(2n=72).(2) Four of the six species from Disporopsi have cytological records and all were reported as diploids with 2n=40 and x=20.(3) Four out of the six Heteropolygonatum species were counted with chromosome number of 2n=32 and x=16, except for the tetraploidy H. ogisui.(4) Polygonatum is comprised of approximately 60 species, 47 of which have chromosome reports. It is found that chromosome numbers are of great variation ranging from x=8 to x=16 in the genus, mainly reported as diploids with a few triploid to hexaploids.2.Molecular phylogenyOne hundred and fifteen taxa from the tribe, representing 18 species of Maianthemum,four species of Heteropolygonatum,four Disporopsis and 21 species of Polygonatum are sampled. Phylogeny relationships are reconstructed based on four plastid markers(rbc L,trn K,psb A–trn H and trn C–pet N) using Bayesian inference(BI)、maximum Likehood(ML) and maximum parsimony(MP) methods. BI, ML and MP analyses produced similar results. Two lineages have recognized in Polygonateae: one comprising Disporopsis, Heteropolygonatum and Polygonatum, and the other, Maianthemum sensu stricto and Smilacina. The phylogenetic analyses supported the monophyly of Maianthemum with Maianthemum sensu stricto nested within Smilacina. Almost all species from the eastern Himalayan region in SW China form a well supported clade. M. tatsienense and M. stenolobum from SW to central China form another clade. The other species from eastern Asia(central to NE China and Japan) and the New World fall into several clades. The phylogenetic analyses supported the current generic delimitation of Polygonatum, w Heteropolygonatum and Disporopsis as monophyletic genus. In this group, Polygonatum is sister to a clade including Heteropolygonatum and Disporopsis. Two main lineages are recognized in Polygonatum: one includes most Polygonatum species centered in southern to south-western China, with some species extending into Europe and Central Asia, and another belongs to the ‘northern’ group, including taxa mainly in the north-eastern part of the Sino-Japanese region, with some species endemic to central China. P. sibiricum is placed between these two main clades.3.The pattern of chromosome evolutionBase on the cytological survey and molecular phylogeny of the tribe, we analyzed chromosomal evolution within the tribe Polygonateae using likelihood model. The polyploidization and aneuploidization events, and the correlation with phylogenetic relationships and geographic distribution are also involved. Polygonatum shows a wide variation of chromosome base numbers mainly from x =8 to 16 with both polyploidization and aneuploidization simultaneously, quite different from the other three genera with stable basic chromosomal numbers, i.e., Maianthemum(x=18), Heteropolygonatum(x=16), and Disporopsis(x=20). Our results support the recognition of the 4 genera in the tribe, each showing a distinct chromosomal pattern. We also find that a decreasing trend of chromosomal basic number of taxa in the tribe. The chromosomal evolution of the tribe is dominantly on the diploidy level with less polyploidization and the aneuploidization events are only found in Polygonatum. Cytological data are highly consistent with the phylogenetic relationships and provide useful implications for the evolution of the tribe.