Abstract:

The Qinghai–Tibet Plateau (QTP) comprises a platform (sometimes called the Qinghai–Tibet Plateau sensu stricto), the Himalayas, and Hengduan Mountains (Liang et al., 2018; Mao et al., 2021). The latter two parts and adjacent highlands are also called the Pan-Himalaya. Numerous plants are distributed there with many endemic species, probably because of the high diverse landscapes created by continuous geological and climatic activities (Favre et al., 2015; Mao et al., 2021). As the well known biodiversity hotspot of the alpine plants in the world (Sun et al., 2017), many studies have been conducted on evolutionary origin and ecological adaptation of those species occurring in the QTP (e.g., Wen et al., 2014, 2019; Zhang et al., 2019). In the present special issue, we collected 15 related papers on this topic. Among them, two are invited reviews. Mao et al. (2021) provide a comprehensive review of evolutionary origin of species diversity on the QTP. Especially, they outlined major disputes and likely causes in this research topic, including circumscribing and naming the QTP, the QTP uplifts, dating of molecular phylogenetic trees, non-causal correlations between QTP uplifts and species diversification and the unified ice sheet. The authors also summarized genomic advancements related to high-altitude adaptation of both plants and animals. Tong et al. (2021) reviewed the reproductive strategies of animal-pollinated alpine plants on the QTP, involving pollination system, pollen limitation, self-pollination, and sexual system. In this region, 95.4% of animal-pollinated plants are pollinated by insects (i.e., bees, moths, butterflies, and flies) with only 4% by vertebrates (i.e., bats and birds). Self-pollination through self-compatibility shift from outcrossing has become an effective reproductive strategy to overcome pollen limitation in alpine plants. The other 13 research papers aimed to address origin and adaptation of alpine flora involving three major lines of evidence: genomics, ecology, and paleobotany. We hope that the collection of these papers will increase our understanding of the origin, speciation, and adaptation of alpine species on the QTP.

1 Genomics: Phylogeny, Speciation, and High-Altitude Adaptation

Because of the decreasing cost, it is becoming easy to obtain numerous homologous sequences to construct the high-solved and well supported phylogeny for diverse genera. Here, we collect three related case studies. A phylogenomic analysis using nuclear and plastome genes by Zhou et al. (2021) was conducted for the peonies (Paeoniaceae, Paeonia L.). Their results suggest that the Paeoniaceae is a relict and ancient lineage with a divergence from the close relatives in the late Cretaceous. The common ancestor of the extant peonies may have survived in the Pan-Himalaya during the ice ages. The further diversification evolved into two subgenera and seven sections that are widespread in the Northern Hemisphere. In addition, climatic oscillations since the late Pliocene promoted polyploidy speciation including both allotetraploids and autotetraploids. Ye et al. (2021) constructed phylogeny of two species-rich bamboo genera (Poaceae) in the Himalayas and Hengduan Mountains based on simplified genome sequences. They revealed the high discordance of gene topologies and extensive hybridization between identified lineages. They conclude that reticulate evolution seems to be common during species diversification of these two genera. Chen et al. (2021) constructed phylogeny and diversification of the subtribe Gentianinae (Gentianaceae) in the QTP and adjacent regions based on transcriptomes. Similarly, they found the inconsistent phylogenetic relationships of a few identified clades based on nuclear and plastome genes. Both hybridization and incomplete lineage sorting may account for these discordant relationships. Especially, they found many gene duplication events in several phylogenetic nodes. Therefore, hybridization and gene duplication might have together facilitated species diversification of this subtribe in the alpine regions of the QTP in addition to the previously assumed geographic isolation.

The diverse habitats of the QTP provide chances for fast species divergence and adaptive evolution. Similarly, in this issue, we publish three related works. Based on genomic data, Li et al. (2021b) examined the cryptic divergence in an alpine ginger Roscoea tibetica Batalin (Zingiberaceae) in Hengduan Mountains of the QTP. They identified two deeply diverged lineages. However, gene flow occurred because of the second contact of these two lineages after the initial divergence. For two aspens occurring in the high- and low-altitude regions, however, Li et al. (2021a) revealed continuous gene flow since the initial divergence. A clear hybrid zone was recovered in the contacting region of the two species. In addition, for the high-altitude species, many genes related to the high-altitude adaptation were found to experience positive evolution. Therefore, natural selection may also play an important role during the divergence history of these two species. Such a strong selection posted by the arid habitat of the QTP may also exist for the lower plants. Zhang et al. (2021) used transcriptome data to examine the adaptive evolution of one high-altitude algae. Compared with the closely related low-altitude algae, a few genes involved in the antioxidative response, DNA repair, and translational/post-translational modifications were found to show positive evolution in the high-altitude species, possibly because of strong abiotic stresses, for example, extensive UV-B radiation on the QTP. All of these case studies suggest that geographic isolation, natural selection (especially high-altitude stresses), and hybridization may have together promoted the speciation of plants occurring there.

2 Ecological Adaptation

It remains an interesting topic to explore how plants adapt to diverse habitats of the QTP. Here, we collected three research papers related to ecological adaptation in pollination, climate, and functional traits. Harsh environments in the high-altitude region of the QTP, such as low temperature, strong wind, and extensive solar radiation, may change the pollinator spectrum of the outcrossing plants. Pi et al. (2021) compared the animal composition of the effective pollinators of one spring-flowering shrub Elaeagnus umbellata Thunb. (Elaeagnaceae) along three altitudinal gradients in the Hengduan Mountains. They found that the flowers of the high-altitude individuals became smaller with longer floral tubes and more nectar sucrose. Correspondingly, the number of bee pollinators decreased while sunbird pollinators increased. They revealed an obvious association between altitude, change of floral traits, and animal composition of the effective pollinators. In the second paper, He et al. (2021) examined the drought distribution limit of one evergreen oak species (Quercus pannosa Hand.-Mazz. s.l.) in the dry valleys in the SE Himalaya. They measured leaf functional traits of this species in the non-monsoon and monsoon seasons. They found that this oak species decreased growth at the drought stage but increased growth in the monsoon season. Therefore, monsoon duration seems to be a major factor in determining the distribution limit of this species. In the third paper, Zou et al. (2021) explored the relationship between functional traits and phylogenetic relatedness along altitudinal distributions of the Rhododendron species. They found that closely related species had high trait similarity. In addition, trait convergent selection occurred among closely related species with the similar altitudinal distributions. Therefore, both evolutionary history and trait selection may together shape species coexistence along altitudinal gradients.

3 Paleobotany for Ancient Flora of the QTP and Related Paleo-Altitude

Paleontology provides direct evidence for ancient flora of the QTP and further evolution in both floral composition and altitude change with time. In the recent past, many fossils of tropical or subtropical plants have been discovered on the central QTP, which suggested that the ancient climate in some subregions was very warm and humid and the altitude should not have been very high at the age when these fossils formed (Su et al., 2020). In the present issue, we collect four related papers. The extant species of the genus Illigera Blume (Hernandiaceae) are distributed in tropical Africa and Asia. The first fossil record of this genus was reported from western North America. Wang et al. (2021) reported 10 fossil fruits of this genus in the central QTP during the Eocene. These fossils could be placed under Illigera eocenica Manchester & O'Leary, which was first found in western North America in the Eocene. This finding indicates a warm, humid climate and low altitude in the central QTP during this stage and a close floristic link between the QTP and North America. Del Rio et al. (2021) reported Cissampelos L. and Menispermites Lesq. for the family Menispermaceae from the Middle Eocene of the central QTP. This family is now characterized with a pantropical and temperate distribution. Previous fossils for the family have been found in Europe and North America. Therefore, this study collected the ancient floristic connection between the QTP and Europe. In addition, Li et al. (2021c) reported a fossil for the genus Cercis L. (Fabaceae) from the Miocene sediments of the northeastern QTP. This genus currently shows the well known intercontinental disjunct distribution in subtropical and warm temperate regions of the Northern Hemisphere (Li et al., 2019; Wen et al., 2019). The paleo-altitude of this region was further inferred to be less than 2400 m at this stage. In addition, we also include one fossil report from the low-altitude foot of the southern Himalaya (Hazra et al., 2021).

Three of the four fossil reports from the high-altitude QTP suggest that the ancient flora there might comprise many tropical and subtropical genera from the Eocene to Miocene. These genera showed the widespread floristic connections with Northern America and Europe in their historical distributions. These findings and those reported before (Jia et al., 2019; Xu et al., 2019; Tang et al., 2019; Su et al., 2021) suggest that some subregions of the QTP were still at relatively low altitudes at these stages compared with the present high altitudes. Therefore, the total QTP might have never been uplifted together to a high elevation at a certain stage and each subregion was subjected to different uplifts at different stages (Mao et al., 2021).

4 Recommendations

In order to fully understand evolutionary histories and ecological adaptations of alpine plants on the QTP, all lines of evidence exemplified here by 13 research papers and those listed by two reviews (Mao et al., 2021; Tong et al., 2021) are encouraged in the future (also see Anderson & Song, 2020). However, the following related research is especially enforced: (1) species delimitation based on statistical analyses of morphological traits and population genetic data; (2) speciation pattern and species radiation; (3) molecular mechanism and allelic variation for special adaptation; (4) ecological adaptation linking species and community; and (5) continuous evolution from tropical to alpine floras since the Eocene.