Is a new invasive herb emerging? Molecular confirmation and preliminary evaluation of natural hybridization between the invasive Sphagneticola trilobata (Asteraceae) and its native congener S-calendulacea in South China

Sphagneticola trilobata is an invasive herb in South China. Here, we reported some morphological intermediates between S. trilobata and the indigenous congener S. calendulacea in the field for the first time, which were suspected to be hybrids. Morphological and molecular methods were used to test this hybrid origin hypothesis, and eco-physiological parameters were measured to evaluate the hybrid's invasion potential. The hybrid origin hypothesis was confirmed by analysis of microsatellite and nrITS sequences. Bayesian assignment analysis based on six microsatellite loci genotypes separated the studied individuals into three groups, which corresponded to the two parent species and the putative hybrids that were identified on the basis of leaf morphology. The hybrid individuals were categorized as F1 hybrids with membership coefficients (Q(i) values) close to 0.5. Phylogenetic analysis based on nrITS data showed that the putative hybrid individuals possessed two types of species-level divergent clones, and were inseparate with S. trilobata clade and S. calendulacea clade, respectively. The maternally-inherited chloroplast marker atpB-rbcL demonstrated that the invasive species functioned as the pollen donors. Ecophysiological studies on photosynthesis, specific leaf area (SLA), leaf nutrient concentration, and construction cost in the parental species and the hybrid demonstrated that the hybrid showed slightly lower or equivalent growth potential compared to the invasive species and significant higher energy-use efficiency than both parents. As such, it seems that a new invasive herb is emerging. As a consequence of this hybridization, the native species S. calendulacea may be disappearing rapidly, which presents a major concern for conservation efforts.