Laying the Groundwork for Association Mapping in Black Walnut (Juglans nigra L.)

Pests, disease and drought now threaten many angiosperm forest trees of critical economic and ecological importance in the northern hemisphere.  Association mapping in natural populations of these species could identify gene networks that influence tolerance to biotic and abiotic stress, facilitating conservation and sustainable harvest of these species.  We have focused our genomics effort on black walnut (Juglans nigra L.), the most valuable fine hardwood tree species in North America, which also produces nutritious nuts and walnut shell products used in numerous industrial applications.  Black walnut now faces an imminent threat posed by Thousand Cankers Disease (TCD), a lethal fungal disease caused by Geosmithia morbida and vectored by the walnut twig beetle (Pityophthorus juglandis).  An intensive effort to develop genomic resources for existing breeding programs and for natural populations will enable us to save this species.

Using gSSRs, EST SSRs, and a BAC library from an orchard of mature black walnut cultivars we report multiple levels of genomic and genetic information: a multi-year parentage analysis and pollination neighborhood study including adjacent wild trees, a detailed phenology study, the development of a full sib mapping population and our progress on a densely populated genetic map.  This information will permit us to recommend an isolation distance for improved seed orchards, design breeding programs for tolerance to TCD, conduct association mapping studies on natural stands and finally, to detect the genetic mechanisms that permit local adaptation in long-lived forest trees in the presence of intense competition, human–mediated disturbance and rapid climate change.