Homoploid hybrid speciation – hybrid speciation without a change in chromosome number and facilitated by ecological divergence – is well known in angiosperms but rare in gymnosperms. To date, only two conifer species are known to have originated in such a manner, Pinus densata and Picea purpurea. There is molecular evidence that P. purpurea originated from the hybridization of P. wilsonii and P. likiangensis. It is also clear that P. purpurea prefers habitats with cold and humid climates, while its parents prefer habitats with warm or mild climates. This makes them a unique model system to study the mechanisms that enable homoploid hybrid species to colonise cold habitats.
A recent study by Wang et al. and published in AoBP compared the frost tolerance of photosystem stability, pressure-volume parameters, and xylem resistance to dysfunction of leaves and stems between P. purpurea and its progenitors. Their results revealed that P. purpurea exhibited greater cell dehydration tolerance and frost tolerance of photosystem stability than its parental species. These traits may have contributed to its adaptation to higher altitude and latitude regions where frost stress is frequent. It was also noted that the xylem resistances of leaves and stems of P. purpurea were similar to those of one of its parental species P. wilsonii yet much larger than those of P. likiangensis. This phenomenon may relate to their specific habitat: although P. wilsonii prefers warmer habitats and P. purpurea colonizes colder ones, both of them would experience frequent water stress (induced by drought and cold, respectively).