Allopatric divergence of Stuckenia filiformis (Potamogetonaceae) on the Qinghai-Tibet Plateau and its comparative phylogeography with S. pectinata in China

Abstract

In the aquatic genus Stuckenia, the wide geographic range of S. pectinata and S. filiformis make them suited for examination of topographic and climatic effects on plant evolution. Using nuclear ITS sequence and ten chloroplast sequences, we conducted comparative phylogeographical analyses to investigate their distribution regions and hybrid zones in China, and compare their phylogeographical patterns and demographical histories. These two species were allopatric in China. S. filiformis occurred only on the Qinghai-Tibet Plateau (QTP), whereas S. pectinata occupied a wide range of habitats. These two species formed hybrid zones on the northeastern edge of QTP. Most of the genetic variance of S. filiformis was between the southern and eastern groups on the QTP, showing a significant phylogeographic structure. The geographical isolations caused by the Nyenchen Tanglha Mountains and the Tanggula Mountains promoted intraspecific diversification of alpine plants on the QTP. This study revealed the lack of phylogeographic structure in S. pectinata, due to the continued gene flow among its distribution regions. The ecological niche modeling showed that the distribution ranges of these two herbaceous species did not contract too much during the glacial period.

Figure 1. The geographical distribution of S. filiformis populations.

The map is created by DIVA-GIS. Right top is the map of China, indicating the Qinghai-Tibetan Plateau. Sampling locations are marked with population codes. Populations indicated by red and blue arrows are hybrids between S. filiformis and S. pectinata. Pie charts show the proportions of haplotypes within each population. Solid lines encircle SAMOVA-identified groups: green, ‘South group’; and red, ‘East group’. Left top is the network of 13 haplotypes of S. filiformis. The pie size is proportional to the haplotype frequency. Each dot between haplotypes indicates a mutational step.

Figure 2. The geographical distribution of S. pectinata populations.

The map is created by DIVA-GIS. Sampling locations are marked with population codes. Pie charts show the proportions of haplotypes within each population. The nuclear genotypes of S. pectinata are indicated by short arrows of different colors. Long arrows indicate the migration routes of widespread haplotypes. Right bottom is the network of 12 haplotypes of S. pectinata. The pie size is proportional to the haplotype frequency. Each dot between haplotypes indicates a mutational step.

Figure 3. Ecological niche modeling for S. filiformis and S. pectinata in China.

The maps are created by MAXENT and DIVA-GIS. The strength of prediction is indicated according to the key shown. Red and orange areas show strong predictions. (a) Regions in China considered suitable for S. filiformis under current climate conditions. (b) The paleodistribution model for S. filiformis in the LGM (22,000 years ago). (c) Regions in China considered suitable for S. pectinata under current climate conditions. (d) The paleodistribution model for S. pectinata in the LGM (22,000 years ago).

Figure 4. Phylogenetic relationships of haplotypes.

Support values (Bayesian posterior probability /maximum likelihood bootstrap) are next to the nodes. Arrow denotes node where fossil calibration was applied. The divergence times and ancestral areas are shown at nodes. The time scale is in Ma. The distributions were categorized into the following areas: northwest China (NW), north China (N), southwest China (SW), south QTP (SQTP) and east QTP (EQTP).

Figure 5

Mismatch distributions of the number of pairwise nucleotide differences of S. filiformis (a), S. pectinata (b) and the south and east groups of S. filiformis (c,d). The dashed line represents observed values (Obs) whereas the solid line shows expected values (Exp) under the population growth-decline model. The Y axis is frequency.