CHARACTERIZING AND CONSERVING THE GENETIC DIVERSITY OF CARYA POPULATIONS
Location: Crop Germplasm Research
Title: Native walnuts of Texas
Submitted to: Acta Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 30, 2011
Publication Date: May 1, 2012
Citation: Grauke, L.J., Thompson, T.E., Mendoza-Hererra, M. 2012. Native walnuts of Texas. Acta Horticulturae. (ISHS) 948:199-210.
Interpretive Summary: Wild relatives of important crop species, when added to our national germplasm collections, have the potential to be used in breeding programs and make lasting contributions to the rootstock and scion characteristics of their domesticated kin. The geographic origins and interspecific associations of those materials are critical for their informed utilization. The valuable resources available in our local and national herbaria deserve careful study in developing targets of germplasm collection as well as in the interpretation of what has been collected. We studied herbarium collections of Texas walnut (Juglans) species at Texas A&M University and The University of Texas and then collected samples of leaves and nuts from trees of those species across Texas. Nut and leaf samples were measured and analyzed. DNA was extracted and studied using markers developed for the sister genus, Carya (pecans and hickories). The markers developed for Carya contributed to the interpretation of relationships among the Texas walnuts. Some of the samples we collected are mixtures between two different walnut species. Since one of the parents (J. nigra) is very susceptible to the deadly Thousand Cankers Disease, while the other (J. major) is very resistant, these hybrids may be useful in breeding for resistance. Nuts were sent to the National Clonal Germplasm Repository for Juglans in Davis, CA, and to the USDA Forest Service Hardwood Tree Improvement and Regeneration Center in West Lafayette, IN. Based on this study, the distribution map of Texas walnuts was revised and herbarium materials were collected and stored. This will help in planning the next plant collection.
Three species of Juglans occur naturally in Texas. Eastern Black walnut, J. nigra is found in eastern to central Texas. J. major, the Arizona walnut, is reported in scattered, disjunct populations from central to west Texas. J. microcarpa, the Little walnut, occurs from central to west Texas. Hybrids between all species have been suspected. Collections were made from several autochthonous populations of each species, some previously undocumented, for the purpose of providing germplasm for use in the NCGR-Davis. Samples were characterized for morphological descriptors of nuts and leaves, revealing considerable overlap between species. DNA was extracted from each individual and evaluated using plastid microsatellite markers developed for use in the sister genus Carya. Of three loci (ccmp2, ntcp40, ntcp9), all of which show polymorphism in Carya, only one (ntcp40) was polymorphic among Texas walnuts. Samples of J. nigra from populations in the eastern U.S. revealed additional alleles for ntcp40, but remained monomorphic at the other loci. Samples of J. regia and J. mandshurica had unique alleles at all three plastid loci. Juglans microcarpa and J. major samples from Texas populations shared three alleles, two of which were the only alleles at the ntcp40 locus found in J. nigra from Texas. Observations are consistent with interspecific hybridization between Texas walnuts. Shell texture was a convincing indication of hybridity between J. major and J. nigra. Discriminant analysis was conducting using nut and leaf morphological descriptors plus one nuclear and one plastid molecular marker developed for Carya (ga38 and ntcp40, respectively). The analysis confirmed intermediacy of the hybrids between J. major and J. nigra, consistent with hybridity. In the course of this work, herbarium vouchers for Texas Juglans species at Texas A&M University and The University of Texas were annotated. The distribution map of Juglans species in Texas was revised.