## 3 Discussion
### Species Divergence of Sect. Tubulosae  
This study sampled all the recognized taxa of the sect. _Tubulosae_ and well-resolved the species relationship of this section. There are two species and one variety of sect. _Tubulosae_ distributed in mainland China. These three continental taxa have often been treated as a single species, i.e., _C. heracleifolia_[65; 66; 72]. However, [PERSON] and [PERSON] [44] considered that the continental plants of sect. _Tubulosae_ could be classified into three taxa, _C. heracleifolia_, _C. tubulosa_, and _C. tubulosa_ var. _ichangensis_ (Figure 1). Although very similar and often occurring together, _C. heracleifolia_ and _C. tubulosa_ can be distinguished by several subtle characteristics that are easily overlooked. For example, flowers of _C. heracleifolia_ are bisexual with slender pedicels and slightly dilated sepal apex, whereas flowers of _C. tubulosa_ are polygamous with robust pedicels and strongly dilated sepals. The pollen grains of _C. heracleifolia_ are tricolpate, and those of _C. tubulosa_ are pantoporate [44]. [PERSON] and [PERSON] [44] put var. _ichangensis_ under _C. tubulosa_ mainly due to its robust and velutinous pedicels and pantoporate pollens. Nonetheless, flowers of _C. tubulosa_ var. _ichangensis_ are bisexual with slightly dilated sepal apex, which are more similar to _C. heracleifolia_ (Figure 4). Our phylogenomic analysis using SNPs data (Figure 2C) confirmed that plants of sect. _Tubulosae_ from mainland China formed three clades which well represented the three taxa of [PERSON] and [PERSON] [44]. However, the results revealed that _C. tubulosa_ var. _ichangensis_ is sister to _C. heracleifolia_ rather than to _C. tubulosa_. On the other hand, _C. tubulosa_ was tested to be more closely related to all the other island species of the section (Figures 2C, 3 and 4). This can be supported by the evidence that the flowers of _C. tubulosa_ and all the other island species are polygamous or unisexual. Our phylogenomic analysis revealed that the bisexual flower vs. polygamous (or unisexual) flower is the most important character separating sect. _Tubulosae_ into two lineages (Figure 4). Nonetheless, pollen morphology did not reflect well the species relationship of the sect. _Tubulosae_. Only one species, _C. heracleifolia_, has tricolpate pollen grains, whereas the others have pantoporate pollens [44; 46]. According to [PERSON] and [PERSON] [73], tricolpate pollens are widespread and exist in all the major lineages of _Clematis_. This pollen type can be considered the primitive type in the genus. All the species from the sister clade of the sect. _Tubulosae_ also have tricolpate pollens [73]. In sect. _Tubulosae_, only the continental species _C. heracleifolia_, keeps the primitive pollen type, indicating that other taxa, including island species, may be derived species in the section (Figure 4). Our results showed that all the island species of sect. _Tubulosae_ are derived species and are closely related to the continental species _C. tubulosa_. Within the island species, _C. tsugetorum_ and _C. psilandra_ from Taiwan island are grouped together but not clearly separated by the SNPs data (Figure 2C). _Clematis tsugetorum_ is a unique species with special morphological characteristics. The plants are small with solitary terminal flowers (Figure 4). [PERSON] and [PERSON] [44] established a series of _Uniforae_ for this species. However, this series was unnecessary because the other series _Tubulosae_ (including all the other species of sect. _Tubulosae_), is paraphyletic to ser. _Uniforae_. Three Japanese taxa also formed a monophyletic group. Among these three taxa, _C. stans_ is distributed in Hokkaido and Honshu, while the other two taxa occur in Kyushu and Shikoku. The leaf morphology of _C. speciosa_ is very special in the section with very reduced teeth on the leaf margins. Although multiple samples of _C. stans_ were grouped together, its variety, _C. stans_ var. _austrojaponensis_, did not cluster with _C. stans_ (Figure 2C).