Biogeographical Studies Of Qinghai-Tibetan Endemic Genus Parapteropyrum (Polygonaceae)

Parapteropyrum, a monotypic shrubby genus with the only endangered species endemic to the arid valleys of the central Qinghai-Tibetan Plateau (QTP), was universally placed in the Atraphaxideae because of their shared morphological traits although the other woody species of this tribe occurred mainly in the central Asia. It is suggested that Parapteropyrum is closely related to the small genus Pteropyrum based on morphological traits. However, recent molecular phylogenetic studies suggested that Parapteropyrum is closely related to Fagopyrum. The accurate position of Parapteropyrum is still uncertain and further research works are still needed to clarify the origination and evolution of this monotypic genus. In our studies, the karyological data show that the chromosome number and karyotype of P. tibeticum are formulated as 2n=48=16m+16sm+16st, belonging to 2B type. Both Atraphaxis manshurica and A. bracteata have the same chromosome number of 2n=22 and similar karyotypes (22m, belonging to 1A). The basic chromosome number of another genus (Calligonum) of the Atraphaxideae is x=9 (2n=18 or 36). Across the published karyological data of Polygonaceae that occurred in the QTP or adjacent areas, only Fagopyrum (2n=16 or 32) shares the same basic chromosomes with Parapteropyrum. We studied sequence variations of the nuclear internal transcribed spacer (ITS) and chloroplast DNA (cpDNA; rbcL and accD) for this genus and the other ten related genera of the Polygonaceae occurred in the QTP and adjacent areas. The constructed phylogenies based on ITS, cpDNA or a combination of both datasets suggested that currently recognized tribe Atraphaxideae is polyphyletic. The woody Parapteropyrum nested deeply within the herbal Fagopyrum and it showed no any relationships with the other genera of the Atraphaxideae. Morphologically, many common characters were also shared by Fagopyrum and Parapteropyrum:racemous inflorescence,5 deeply lobed perianth, stamens 8 (5 outer,3 inner), styles 3 with head-like stigmas, triangular achenes. Totally, the karyological, genetic and morphological evidences all suggested that shrubby P. tibeticum has derived from herbal Fagopyrum. Our results suggested that Parapteropyrum should be reduced and its only species should be incorporated under the genus Fagopyrum. Molecular datings revealed that this woody species probably originated between 6.35 and 14.8 million years ago (Mya). Meanwhile, during this stage, geological evidence suggested that the Qinghai-Tibetan Plateau had extensively uplifted, especially between 15-13 Mya and 8-7 Mya. The high species diversity in the QTP was suggested to result mainly from radiative diversification and convergent evolution of plants because of the rapid plateau uplifts and the dramatic climate change. These results together suggested that the large-scale QTP uplifts since the Miocene promoted not only extensive speciation in this region, but also the secondary acquirements of woodiness in a few herbal lineages under the strong counter-selection of inbreeding depression because of the geographical isolation. It is confirmed that insular woodiness has also happened in the QTP as occurred in most islands and its evolutionary significance is also discussed in detail. In addition, the confirmation of this woody species within Fagopyrum further implies its possible usage as an important buckwheat resource because of the rising food crisis.