Hybridization can be a potent stimulus for the evolution of invasiveness in some introduced species, but it requires that hybrids can reproduce and establish to form wild populations. This was examined in Pyrus calleryana, an emerging invasive tree that was originally introduced as an ornamental from China, but is now found across much of the United States. Invasive populations are now known to consist of hybrid progeny of genetically distinct cultivars of the Callery pear, with high genetic admixture in invasive individuals. Thus intraspecific hybridization in this species serves to increase genetic diversity and create novel genetic combinations, but whether this translates into a fitness advantage for F1 or later generation hybrids remains unknown. The purpose of this study was to compare different hybrid types in terms of fitness components that impact the formation and establishment of invasive populations. The first goal was to compare maternal reproductive investment (fruit mass, seed mass and seed number) of cultivars and F1 hybrids. The second goal was to examine establishment ability (percent seed germination, percent mortality, photosynthetic rate, and biomass) of wild F1 hybrids and later generation hybrids (F2, BC1, etc.) within a common garden.
Results/Conclusions
Cultivars and F1 hybrids of Pyrus calleryana differed significantly for all maternal reproductive measures, with cultivated individuals setting larger seeds, likely the result of abundant resources in the cultivated setting. Additionally, F1 progeny appeared to have an initial establishment advantage by accumulating greater belowground biomass compared to later generation hybrids, although the groups did not differ in percent survival, photosynthetic rate or aboveground biomass. These results provide evidence for possible early-acting hybrid vigor in some traits followed by a breakdown of hybrid advantage in subsequent generations. No single cultivar or hybrid genotype was consistently more successful than the others, supporting our previous genetic study demonstrating the contribution of many cultivar genotypes within invasive populations.