Submitted to: Tree Genetics and Genomes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 1, 2011
Publication Date: September 15, 2011
Citation: Grauke, L.J., Mendoza-Herrera, M., Miller, A., Wood, B.W. 2011. Geographic patterns of genetic variation in native pecans. Tree Genetics and Genomes. 7(5):917-932. Interpretive Summary: Some of the DNA in pecans comes only from their seed parent and is found in structures in the cells called plastids. Other DNA, found in the cell nucleus, is inherited from both the seed parent and the pollen parent. When seeds are heavy, as in nut trees like pecans, they are not very mobile. Pollen of pecans is wind-blown and moves great distances. We tested for genetic differences in geographic populations of native pecan trees, using the maternally inherited seed markers as well as the pollen influenced nuclear markers. We found that the different sources of DNA told us different stories. More plastid patterns were found in Mexico, with fewer in Texas and even fewer in more northern states. This may be related to the age of the populations, since the northern regions did not have pecans during the last ice age, 20,000 years ago. The pattern of variation in nuclear markers was not so clearly related to geography. By continuing to study the patterns found in pecan DNA, we hope to do a better job of conserving and improving this important North American tree species.
Technical Abstract: A structured collection of eighty seedling pecan trees [Carya illinoinensis (Wangenh.) K. Koch] representing nineteen putatively native pecan populations across the species range were evaluated at three plastid and 14 nuclear microsatellite (simple sequence repeat, SSR) loci. Data were analyzed using a priori population designations, and also within a Bayesian framework in which individuals were assigned to clusters regardless of population of origin. Population genetic analyses using a priori populations, clusters based on chloroplast microsatellite data (cpSSR), and clusters based on nuclear microsatellite data (nSSR) yielded consistent results. For all groupings, cpSSR variation exhibited more geographic structure than the nSSR data. Further, cpSSR microsatellite diversity decreased with increasing latitude, but this pattern was not observed with the nuclear data. Contrasting patterns in plastid and nuclear genetic diversity demonstrate unique aspects of postglacial recolonization reflected in the movement of seeds vs. pollen. These data suggest that plastid SSRs are useful tools for identifying population structure in pecan and hold promise for ongoing efforts to identify and conserve representative germplasm in ex situ collections.