Seeing the Forest for the Trees: Conserving the Carya Genus for Utilization.

Pecan [Carya illinoinensis (Wangenh.) K. Koch] is a tree species native to North America that is the most economically valuable member of its genus.  Worldwide pecan production has averaged over 211,000 tonnes over the past 10 years with ~60% from the US. While the development of regionally adapted pecan cultivars is the goal of national and state breeding programs, the crop is still in the early stages of domestication.  The first successful propagation by grafting occurred less than 180 years ago, within the life-span of these long-lived perennial trees.  The successes of improved cultivar selection that have fueled the growth of the US pecan industry have been accompanied by a decline in native pecan acreage, eroding the genetic base upon which crop development depends and potentially obscuring the record of adaptation that survives in native populations.  Ten other species of Carya occur in the US, where they help comprise the oak-hickory forest, the nation's largest forest cover type.  These diverse species include diploid (2n=2x=32) members (pecan, C. aquatica, C. cordiformis, C. myristiciformis, C. ovata and C. laciniosa), as well as tetraploid (2n=4x=64) species (C. glabra, C. tomentosa, C. texana, C. pallida, C. floridana).  Sympatric species with the same ploidy level form fertile natural hybrids.  Molecular genetic evidence indicates that some pecan cultivars that have contributed to early nut maturation have mixed ancestry. Strategies such as mapping by admixture linkage disequilibrium may prove useful in identifying sources of disease resistance from crop wild relatives.  This is an appropriate time to critically assess what we know about this important North American tree crop in order to strategically prepare for its future development globally, to conserve the valuable genetic foundations of pecan and its congeners, and to refine the role of the USDA ARS National Collection of Genetic Resources-Carya in those processes.