Female flowers first: QTL mapping in eastern black walnut (Juglans nigra L.) identifies a dominant locus for heterodichogamy syntenic with that in Persian walnut (J. regia L.)

Authors: Chatwin, Warren; SHIRLEY, DEVIN - Utah Division Of Wildlife Resources; LOPEZ, JACQUELINE - University Of Notre Dame; SARRO, JOSEPH - University Of Notre Dame; CARLSON, JOHN - Pennsylvania State University; DEVAULT, ALISON - Arbor Biosciences; PFRENDER, MICHAEL - University Of Notre Dame; COGGESHALL, MARK - University Of Missouri; ROMERO-SEVERSON, JEANNE - University Of Notre Dame

Submitted to: Tree Genetics and Genomes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/30/2022
Publication Date: 12/17/2022
Citation: Chatwin, W.B., Shirley, D.L., Lopez, J.A., Sarro, J., Carlson, J.E., Devault, A.M., Pfrender, M.E., Coggeshall, M.V., Romero-Severson, J. 2022. Female flowers first: QTL mapping in eastern black walnut (Juglans nigra L.) identifies a dominant locus for heterodichogamy syntenic with that in Persian walnut (J. regia L.). Tree Genetics and Genomes. 19. Article 4. https://doi.org/10.1007/s11295-022-01580-9.
DOI: https://doi.org/10.1007/s11295-022-01580-9

Interpretive Summary: Eastern black walnut is important for its high-quality timber and nut production. Most black walnut trees to date are selected from or are recent descendants of wild-native trees. In order to have a foundation for genetically improving these trees, in 2008 we established an orchard of 326 trees that are all progeny of two wild-origin black walnut cultivars. We utilized multiple genetic sequencing techniques to obtain a wide variety of genetic markers situated across all 16 walnut chromosomes and constructed a genetic map that tells us which order these markers naturally occur on the chromosomes in our population. Many of these genetic markers are also usable in other walnut species and other closely related nut trees like pecan. This is important because it is how we confidently determine genes that are involved in controlling important physical characteristics like shell thickness, percent kernel yield, and other important traits. Once we know what genes are linked to these important physical characteristics, we can begin to utilize genetics to identify young trees with genes that control or are associated with these traits without needing to wait decades to observe these characteristics for ourselves. There is still much work to do before we are at that point, but the establishment of a genetic map for this population of eastern black walnuts is a significant scientific development on that path.

Technical Abstract: Eastern black walnut (Juglans nigra L.) is one of the most valuable timber and veneer trees in North America and is also cultivated for its nuts that have unique culinary attributes. In 2008, an F1full-sib mapping population comprised of 326 offspring of two eastern black walnut cultivars ('Sparrow' x 'Schessler') was established. A total of 301 of these trees still survive, and many are now bearing nuts. Our goal in this study was to create the first genetic map for J. nigra utilizing a mixed approach of single nucleotide polymorphisms (SNP) markers from reduced representation sequencing (RRS) and expressed sequence tag single sequence repeats (EST-SSRs) from a bait-capture and bulk sequencing method. Our map covers all 16 expected linkage groups and contains 356 SNP markers and 59 EST-SSR markers. When the EST-SSR markers were mapped to the J. regia 'Chandler' V2.0 genome sequence, a striking amount of collinearity existed with some possible structural changes on two of the sixteen chromosomes. EST-SSR markers are well known for their transferability across species and even other nut trees within the same family like pecan, due to the EST-SSR's conserved flanking coding sequence surrounding the SSR repeat. The establishment of these markers in our map provides a basis for comparison with other Juglans genetic maps if our markers are surveyed in those populations. Future plans for our mapping population include mapping QTL for nut quality, budbreak, heterodichogamy and other phenotypes measured over the last decade, as well as further sequencing to continue to improve the density of our genetic map.