Kentucky bluegrass (Poa pratensis L.) is an important facultative apomictic temperate perennial grass species used for both forage and cultivated turf. Through apomixis, this species is able to propagate diverse and odd ploidy levels, resulting in many genetically distinct phenotypes. A wide range of diverse cultivars and accessions of kentucky bluegrass have been previously characterized based on common turf performance or morphological characteristics as well as by random amplified polymorphic DNA (RAPD) markers. Although previous characterization efforts have provided valuable information, the use of both morphological characteristics and RAPD markers for genetic diversity analysis has limitations. In the current report, we developed and characterized 88 novel microsatellite markers for kentucky bluegrass. Polymorphism for each marker was assessed in 265 kentucky bluegrass cultivars, experimental selections, collections, and hybrids. The number of alleles for individual microsatellites ranged from four to 81 with an average of 38.3 alleles per simple sequence repeat. These polymorphic microsatellite markers would be useful tools for investigating genetic diversity, creation of genetic linkage maps, assessment of levels of apomixis in cultivars and experimental varieties, and identification of aberrant progeny in apomictic kentucky bluegrass breeding programs.
Josh A. Honig, Stacy A. Bonos, and William A. Meyer
Josh A. Honig, Vincenzo Averello, Stacy A. Bonos, and William A. Meyer
Kentucky bluegrass (Poa pratensis L.) is an important facultative apomictic temperate perennial grass species used for both forage and cultivated turf. Through apomixis, this species is able to propagate diverse and odd ploidy levels, resulting in many genetically distinct phenotypes. A wide range of diverse cultivars and accessions of kentucky bluegrass have been previously characterized based on pedigree, common turf performance, and morphological characteristics to create a kentucky bluegrass cultivar classification system. The objectives of the current study were to assess the amount of genetic divergence among kentucky bluegrass cultivars, experimental selections, and plant collections and revise/update the original pedigree, turf performance, and morphological characteristics kentucky bluegrass classification system using recently described kentucky bluegrass microsatellite [simple sequence repeat (SSR)] markers. In this study, 247 kentucky bluegrass cultivars, experimental selections, and collections were genotyped using 25 SSR markers. SSR markers showed a strong correlation between genetic relatedness as assessed by molecular markers and the original kentucky bluegrass classification system and also provided justification for a revision/update of the classification system. Traditional classification types that were supported by the current SSR analysis include BVMG, Compact, Compact-America, Julia, Mid-Atlantic, Midnight, and Shamrock types. Newly proposed classification types included Cynthia, Jefferson/Washington, Limousine, P-105, Sydsport, and three Eurasian types. The majority of cultivars, experimental selections, and collections were uniquely identified with the current set of SSR markers. Genetic relationships of individuals as assessed by SSR markers closely matched known pedigrees. The current set of SSR markers can be used to rapidly genotype and assign new cultivars/accessions to kentucky bluegrass classification types and assess genetic relatedness among individuals and should be considered for use in a kentucky bluegrass plant variety protection program.
Jennifer Johnson-Cicalese, James J. Polashock, Josh A. Honig, Jennifer Vaiciunas, Daniel L. Ward, and Nicholi Vorsa
Fruit rot is the primary threat to cranberry (Vaccinium macrocarpon) production in the northeastern United States, and increasingly in other growing regions. Efficacy of chemical control is variable because the disease is caused by a complex of pathogenic fungi. In addition, cranberries are often grown in environmentally sensitive areas, placing restrictions on chemical control measures. Thus, a major focus of the cranberry breeding program is to develop cultivars with improved fruit rot resistance (FRR). Several genetically diverse sources of FRR have been identified in our germplasm collection. However, the most resistant accessions lack one or more attributes; e.g., productivity, required for commercial acceptance. These resistant accessions were used in crosses with elite high-yielding selections and in 2009, 1624 progeny from 50 crosses were planted in 2.3-m2 field plots. During 2011–13, under field conditions with very limited fungicide management, disease pressure was severe, allowing evaluation for FRR. Plots were rated on a 1–5 scale for incidence of fruit rot (where 1 = 0% to 20% rot and 5 = 81% to 100% rotted fruit), and rotted fruit counts were made from selected plots to validate the ratings. There was a good correlation in the ratings between years (2011 vs. 2012: r = 0.59, P < 0.0001; 2011 vs. 2013: r = 0.50, P < 0.0001; 2012 vs. 2013: r = 0.62, P < 0.0001), and between rot ratings and percent rotted fruit (r = 0.90, P < 0.0001). Significant differences were found between and within families for FRR. High heritability estimates (h 2 = 0.81) were obtained with midparent-offspring regression of mean fruit rot ratings, indicating additive genetic variance for FRR. Introgression of FRR into higher yielding genetic backgrounds was also accomplished, as some progeny exhibiting high FRR also had commercially viable yield (>300 g/0.09 m2), as well as good berry size and color. Selections are being further evaluated for potential cultivar release.
Megan F. Muehlbauer, Josh A. Honig, John M. Capik, Jennifer N. Vaiciunas, and Thomas J. Molnar
The development of new cultivars resistant to the disease eastern filbert blight (EFB), caused by Anisogramma anomala, is of primary importance to hazelnut (Corylus sp.) breeders in North America. Recently, a large number of EFB-resistant cultivars, grower selections, and seedlings from foreign germplasm collections were identified. However, for a significant number of these, little is known of their origin, relationships, or genetic background. In this study, 17 microsatellite markers were used to investigate the genetic diversity and population structure of 323 unique accessions, including EFB-resistant and tolerant germplasm of uncertain origins, in comparison with a panel of known reference accessions representing a wide diversity of Corylus cultivars, breeding selections, and interspecific hybrids. The resulting allelic data were used to construct an unweighted pair group method using arithmetic averages (UPGMA) dendrogram and STRUCTURE diagram to elucidate relationships among the accessions. Results showed 11 consensus groups with EFB-resistant or tolerant accessions in all, providing strong evidence that EFB resistance is relatively widespread across the genus Corylus. Furthermore, open-pollinated seedlings tended to group together with reference accessions of similar geographic origins, providing insight into their genetic backgrounds. The results of this study add to the growing body of knowledge of hazelnut genetic resources and highlight recently introduced EFB-resistant seedling germplasm as new, unrelated genetic pools of resistance.
John M. Capik, Megan Muehlbauer, Ari Novy, Josh A. Honig, and Thomas J. Molnar
Stable genetic resistance to the fungal disease eastern filbert blight (EFB), caused by Anisogramma anomala, is vital for sustainable production of European hazelnut (Corylus avellana) in eastern North America. In this study, new hazelnut germplasm from the Russian Federation, Ukraine, and Poland (a total of 1844 trees from 66 seed lots) was subjected to A. anomala under field conditions over at least five years in New Jersey. Plants were then rated for the presence of EFB using an index of 0 (no disease) through 5 (all stems containing cankers). Nuts of the resistant trees were evaluated to identify plants with improved kernel characteristics. Genomic DNA of these trees was also screened with sequence-characterized amplified region (SCAR) markers generated by the primers BE-03, BE-33, and BE-68, which are closely linked to the single dominant R-gene of ‘Gasaway’, to assess the resistant seedlings for the presence of this well-known source of resistance. At final evaluation, 76 trees remained free of disease with nine expressing only minor symptoms (rating 1 or 2). The resistant trees spanned 24 different seed lots representing all three countries. The remaining trees ranged from moderately to severely infected with 81% of the total collection rating 5. Several of the resistant trees were found to produce commercial-sized (≈12 mm diameter), round kernels that blanched well. Although the results of the ‘Gasaway’ SCAR primers were inconclusive, the diverse collection origins and disease phenotypes provide evidence that novel sources of resistance were likely identified in this study. These new plants should broaden the genetic base of EFB-resistant C. avellana hazelnut germplasm available for breeding.
Josh A. Honig, Megan F. Muehlbauer, John M. Capik, Christine Kubik, Jennifer N. Vaiciunas, Shawn A. Mehlenbacher, and Thomas J. Molnar
European hazelnut (Corylus avellana L.) is an economically important edible nut producing species, which ranked sixth in world tree nut production in 2016. European hazelnut production in the United States is primarily limited to the Willamette Valley of Oregon, and currently nonexistent in the eastern United States because of the presence of a devastating endemic disease, eastern filbert blight (EFB) caused by Anisogramma anomala (Peck) E. Muller. The primary commercial means of control of EFB to date is through the development and planting of genetically resistant european hazelnut cultivars, with an R-gene introduced from the obsolete, late-shedding pollinizer ‘Gasaway’. Although the ‘Gasaway’ resistance source provides protection against EFB in the Pacific northwestern United States (PNW), recent reports have shown that it is not effective in parts of the eastern United States. This may be in part because the identification and selection of ‘Gasaway’ and ‘Gasaway’-derived cultivars occurred in an environment (PNW) with limited genetic diversity of A. anomala. The objectives of the current research were to develop a genetic linkage map using double digestion restriction site associated DNA sequencing (ddRADseq) and identify quantitative trait loci (QTL) markers associated with EFB resistance from the resistant selection Rutgers H3R07P25 from southern Russia. A mapping population composed of 119 seedling trees was evaluated in a geographic location (New Jersey) where the EFB fungus is endemic, exhibits high disease pressure, and has a high level of genetic diversity. The completed genetic linkage map included a total of 2217 markers and spanned a total genetic distance of 1383.4 cM, with an average marker spacing of 0.65 cM. A single QTL region associated with EFB resistance from H3R07P25 was located on european hazelnut linkage group (LG) 2 and was responsible for 72.8% of the phenotypic variation observed in the study. Based on its LG placement, origin, and disease response in the field, this resistance source is different from the ‘Gasaway’ source located on LG6. The current results, in combination with results from previous research, indicate that the H3R07P25 source is likely exhibiting resistance to a broader range of naturally occurring A. anomala isolates. As such, H3R07P25 will be important for the development of new european hazelnut germplasm that combines EFB resistance from multiple sources in a gene pyramiding approach. Identification of EFB resistance in high disease pressure environments representing a diversity of A. anomala populations is likely a requirement for identifying plants expressing durable EFB resistance, which is a precursor to the development of a commercially viable european hazelnut industry in the eastern United States.