First Observation of Wild Double-Flowered Rhododendron nivale With Stamen Petaloidy and Possible Parallel Evolution

Parallel evolution of similar phenotypes under similar selective pressures is often viewed as evidence for natural selection. However, how often such parallel variation occurs in floral traits, and its underlying mechanisms, remain poorly understood-especially in alpine plants. Here we report the first discovery of wild double-flowered plants in Rhododendron nivale, a dwarf shrub of the eastern Qinghai-Tibet Plateau, and the first record of double flowers in subsection Lapponica (small-leaved, high-altitude sky-island shrubs). Naturally occurring stamen petaloidy was found in two geographically isolated populations at high passes in Chagyab County, Tibet (4825 m) and Yajiang County, Sichuan (4650 m), China. Individuals from both populations show nearly identical floral modifications: complete stamen petalody, thickened corolla lobes, abnormal style morphology, and denser capsule scales. The two populations are separated by > 800 km with no possible gene flow, yet they occupy remarkably similar habitats (strong winds, low temperatures, short growing seasons). This parallel variation may suggest independent origins under similar selective pressures. Based on the ABC model of floral development, we hypothesize that the double-flower phenotype results from altered expression of C-class genes such as AGAMOUS. A plastid (rbcL-matK) phylogeny further shows that the two double-flowered populations are not each other's closest relatives but instead nest within their respective geographic clades, consistent with independent (parallel) origins. Our findings provide unique material for understanding the molecular mechanisms and environmental drivers of floral homeotic variation. A more systematic investigation of floral development genes across Rhododendron is warranted, as such rare mutations may reveal the genetic basis of flower evolution and the role of parallel evolution in generating phenotypic diversity in alpine sky-island ecosystems.