Abstract
Homoploid hybrid speciation (HHS), characterized by hybrid speciation without a change in chromosome number and facilitated by ecological divergence, is well known in angiosperms but rare in gymnosperms. Picea purpurea as one of two demonstrably conifer diploid hybrid species in gymnosperms has been found to occupy colder alpine habitats than its parents. However, studies on whether leaf frost tolerance and hydraulic safety exhibit transgressive segregation and thus play a role in conifer HHS are still lacking. In this study, we compared the frost tolerance of photosystem stability (the maximum efficiency of PSII, Fv/Fm), pressure-volume parameters, and xylem resistance to dysfunction of leaves (current-year twigs) and stems (annual shoots) between P. purpurea and its progenitors. The results indicated that P. purpurea had significantly lower osmotic potential at full turgor, water potential at turgor loss point, water potential at 12 % loss of conductance of stem, the maximum hydraulic conductance of stem and the temperature causing a 50 % reduction in initial Fv/Fm than its parental species. In contrast, the leaf and stem xylem pressure inducing 50 % loss of hydraulic conductivity (leaf Ψ50 and stem Ψ50, respectively) and hydraulic safety margin in leaf Ψ50, stem Ψ50 in P. purpurea showed no significant difference with those of P. wilsonii, but significantly larger than those of P. likiangensis. This suggests that the frost tolerance of photosystem stability and the cell dehydration tolerance in P. purpurea are superior to its parental species, facilitating its successful colonization and establishment in colder habitats.
Highlights
Homoploid hybrid speciation (HHS) entails the establishment of novel lineages through hybrid speciation with no change in chromosome number, and is facilitated by ecological divergence
Wang et al – Enhanced leaf frost tolerance in a homoploid hybrid species, Picea purpurea originated from this pattern of speciation, and both of them occupy colder alpine habitats than their parental species (Wang et al 2011; Sun et al 2014)
We found that the stem Kmax of P. purpurea was significantly lower than those of its parental species, implying that this would have effects on the twig growth of P. purpurea: the lower stem Kmax will limit the water transported from roots to terminal shoots, reducing the gas exchange and the capacity of carbon gain of leaves, which further limits the growth of seedlings
Summary
Homoploid hybrid speciation (HHS) entails the establishment of novel lineages through hybrid speciation with no change in chromosome number, and is facilitated by ecological divergence. Species distributions along altitude gradients are potentially determined by differential frost tolerance and resistance to cold-induced drought conferred by physiological or morphological traits such as increased cell survival at low water potentials and reduced vulnerability of xylem vessels to embolism (Baltzer et al 2005). This is important for homoploid hybrid species: the transgressive expression of such traits enables the initial hybrid generation to occupy novel habitats and establish niche separation from its progenitors, thereby avoiding the homogenizing effects of gene flow and competition from the parental species (Rieseberg et al 2003; Gompert et al 2006). Quantitative studies on transgressive segregation in the frost tolerance and drought tolerance of conifers in homoploid hybrid species, and its potential contribution to their establishment in cold habitats, are lacking
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