GENETIC MAPPING REVEALS A NUMBER OF EMBRYONIC LETHAL LOCI IN A SELFED FAMILY OF LOBLOLLY PINE

Forest Biotechnology Group, Department of Forestry, North Carolina State University, Raleigh, NC 27695-8008 USA

Several questions about the genetic basis of inbreeding depression have important implications for understanding the evolution of forest trees, and for the design of conservation and breeding programs. These questions include dominance vs. overdominance as the basis of inbreeding depression, the number and degree of lethality of loci contributing to inbreeding depression, and the importance of epistatic effects. We are using genetic mapping in a large family of loblolly pine (Pinus taeda L.), derived from self-pollination of a single clone (7-56), to investigate the basis for embryonic lethality associated with inbreeding. We have partially scored 270 selfed seedlings and 57 non-germinating embryos for framework markers from an AFLP linkage map that provides almost complete genome coverage at a 20 cM scale. Preliminary results suggest a minimum of 12 separate loci with complete or almost-complete embryonic lethal effects, as evidenced by highly skewed segregation ratios. Only three of the twelve linkage groups show no evidence of embryonic lethals, and three linkage groups appear to have multiple lethals. These results suggest that reduced seed production associated with inbreeding is largely the result of recessive mutant alleles with large lethal effects, at least in this family. We also plan to identify loci affecting seed germination and early seedling growth, and study the effects of known mutations on seedling survival and growth. Our methodology can be readily applied to other clones as well, providing a powerful tool for investigating inbreeding depression.